![Hr8799 orbit hd](https://upload.wikimedia.org/wikipedia/commons/4/48/Hr8799_orbit_hd.gif)
An exoplanet or extrasolar planet is a
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
outside the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such.
The first confirmation of detection occurred in 1992. A different planet, initially detected in 1988, was confirmed in 2003.
There are many
methods of detecting exoplanets
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of ...
.
Transit photometry
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty ...
and
Doppler spectroscopy
Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in t ...
have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the
tidal locking
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked b ...
zone. In several cases,
multiple planets have been observed around a star.
About 1 in 5
Sun-like stars[For the purpose of this 1 in 5 statistic, "Sun-like" means ]G-type star
A G-type main-sequence star (Spectral type: G-V), also often, and imprecisely called a yellow dwarf, or G star, is a main-sequence star (luminosity class V) of spectral type G. Such a star has about 0.9 to 1.1 solar masses and an effective tempe ...
. Data for Sun-like stars was not available so this statistic is an extrapolation from data about K-type stars K-type may refer to:
*AEC K-type, a bus chassis
*K-type star, a stellar spectral classification
*K-type filter, a type of electronic filter
*K-type asteroid K-type asteroids are relatively uncommon asteroids with a moderately reddish spectrum shor ...
. have an "
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
-sized"
[For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii.] planet in the
habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
.
[For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).] Assuming there are 200 billion stars in the Milky Way,
[About 1/4 of stars are GK Sun-like stars. The number of stars in the galaxy is not accurately known, but assuming 200 billion stars in total, the ]Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
would have about 50 billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11 billion would have Earth-sized planets in the habitable zone. Including red dwarfs would increase this to 40 billion. it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous
red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
s are included.
The
least massive exoplanet known is
Draugr
The draugr or draug ( non, draugr, plural ; modern is, draugur, fo, dreygur and Danish language, Danish, Swedish language, Swedish, and no, draug) is an undead creature from the Scandinavian saga literature and folktale.
Commentators extend t ...
(also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. The
most massive exoplanet listed on the
NASA Exoplanet Archive
The NASA Exoplanet Archive is an online astronomical exoplanet catalog and data service that collects and serves public data that support the search for and characterization of extra-solar planets (exoplanets) and their host stars. It is part of ...
is
HR 2562 b
HR 2562 b is a brown dwarf or gas giant exoplanet. It is a substellar companion of the debris disk host star HR 2562. HR 2562 is a sixth-magnitude F-type main-sequence star located away. HR 2562 is about 37% more massive than the Sun.
Initially ...
,
about 30 times the mass of
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
. However, according to some definitions of a planet (based on the nuclear fusion of
deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
), it is too massive to be a planet and might be a
brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
instead. Known orbital times for exoplanets vary from
less than an hour (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it.
Almost all of the planets detected so far are within the Milky Way. However, there is evidence that
extragalactic planet
An extragalactic planet, also known as an extragalactic exoplanet or an extroplanet, is a star-bound planet or rogue planet located outside of the Milky Way Galaxy. Due to the immense distances to such worlds, they would be very hard to detect dire ...
s, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.
The
nearest exoplanets are located 4.2
light-year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s (1.3
parsec
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
s) from Earth and orbit
Proxima Centauri
Proxima Centauri is a small, low-mass star located away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest tarof Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-kno ...
, the closest star to the Sun.
The discovery of exoplanets has intensified interest in the search for
extraterrestrial life
Extraterrestrial life, colloquially referred to as alien life, is life that may occur outside Earth and which did not originate on Earth. No extraterrestrial life has yet been conclusively detected, although efforts are underway. Such life might ...
. There is special interest in planets that orbit in a star's
habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
(or sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for
life
Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energ ...
as we know it, to exist on the surface. However, the study of
planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from ...
also considers a wide range of other factors in determining the suitability of a planet for hosting life.
Rogue planets
A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors ( stars and brown dwarfs) and without a ...
are those that do not orbit any star. Such objects are considered a separate category of planets, especially if they are
gas giant
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s, often counted as
sub-brown dwarf
A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a planetary mass, therefore by definition below the limi ...
s. The rogue planets in the Milky Way possibly number in the billions or more.
Definition
IAU
The official
definition of the term ''planet'' used by the
International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(IAU) only covers the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
and thus does not apply to exoplanets. The IAU Working Group on Extrasolar Planets issued a position statement containing a working definition of "planet" in 2001 and which was modified in 2003. An ''exoplanet'' was defined by the following criteria:
This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018. The official working definition of an ''exoplanet'' is now as follows:
The IAU noted that this definition could be expected to evolve as knowledge improves.
Alternatives
The
IAU
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
's working definition is not always used. One alternate suggestion is that planets should be distinguished from
brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
s on the basis of formation. It is widely thought that giant planets form through core
accretion
Accretion may refer to:
Science
* Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity
* Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...
, which may sometimes produce planets with masses above the deuterium fusion threshold;
massive planets of that sort may have already been observed. Brown dwarfs form like stars from the direct gravitational collapse of clouds of gas and this formation mechanism also produces objects that are below the limit and can be as low as .
Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of AU and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have a composition more similar to their host star than accretion-formed planets which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent the low-mass end of brown dwarf formation.
One study suggests that objects above formed through gravitational instability and should not be thought of as planets.
Also, the 13-Jupiter-mass cutoff does not have precise physical significance. Deuterium fusion can occur in some objects with a mass below that cutoff.
The amount of deuterium fused depends to some extent on the composition of the object. As of 2011 the
Extrasolar Planets Encyclopaedia
The Extrasolar Planets Encyclopaedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with indi ...
included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around in the observed mass spectrum reinforces the choice to forget this mass limit".
As of 2016 this limit was increased to 60 Jupiter masses based on a study of mass–density relationships.
The
includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the
sin i ambiguity."
The
NASA Exoplanet Archive
The NASA Exoplanet Archive is an online astronomical exoplanet catalog and data service that collects and serves public data that support the search for and characterization of extra-solar planets (exoplanets) and their host stars. It is part of ...
includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.
Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
is dominated by
coulomb pressure or
electron degeneracy pressure
Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure. The Pauli exclusion principle disallows two identical half-integer spin (physics), spin particles (electrons and all other fe ...
with the dividing line at around 5 Jupiter masses.
Nomenclature
The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the
International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter. Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of
circumbinary planet
A circumbinary planet is a planet that orbits two stars instead of one. The two stars orbit each other in a binary system, while the planet typically orbits farther from the center of the system than either of the two stars. In contrast, circums ...
s. A limited number of exoplanets have
IAU-sanctioned proper names. Other naming systems exist.
History of detection
For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they were real in fact, how common they were, or how similar they might be to the planets of the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. Various detection claims made in the nineteenth century were rejected by astronomers.
The first evidence of a possible exoplanet, orbiting
Van Maanen 2
Van Maanen 2, or van Maanen's Star, is the closest known solitary white dwarf to the solar system. It is a dense, compact stellar remnant no longer generating energy and has equivalent to about 68% of the Sun's mass but only 1% of its ...
, was noted in 1917, but was not recognized as such. The astronomer
Walter Sydney Adams
Walter Sydney Adams (December 20, 1876 – May 11, 1956) was an American astronomer.
Life and work
Adams was born in Antioch, Turkey, to Lucien Harper Adams and Nancy Dorrance Francis Adams, missionary parents, and was brought to the U.S. in ...
, who later became director of the
Mount Wilson Observatory
The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles.
The observat ...
, produced a spectrum of the star using
Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an
F-type main-sequence star
An F-type main-sequence star (F V) is a main-sequence, hydrogen-fusing star of spectral type F and luminosity class V. These stars have from 1.0 to 1.4 times the mass of the Sun and surface temperatures between 6,000 and 7,600 K.Tables VII ...
, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.
The
first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the
pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
PSR B1257+12
PSR B1257+12, previously designated PSR 1257+12, alternatively designated PSR J1300+1240, is a millisecond pulsar located 2,300 light-years from the Sun in the constellation of Virgo, rotating at about 161 times per second (faster than a ...
.
The first confirmation of an exoplanet orbiting a
main-sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...
star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star
51 Pegasi
51 Pegasi (abbreviated 51 Peg), formally named Helvetios , is a Sun-like star located from Earth in the constellation of Pegasus. It was the first main-sequence star found to have an exoplanet (designated 51 Pegasi b, officially named ...
. Some exoplanets have been
imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the
transit method
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty o ...
and the
radial-velocity method
Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in t ...
. In February 2018, researchers using the
Chandra X-ray Observatory
The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 1 ...
, combined with a planet detection technique called
microlensing
Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers ...
, found evidence of planets in a distant galaxy, stating, "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this
arawaygalaxy is more than a trillion."
On 21st March 2022, the 5000th exoplanet beyond our solar system was confirmed.
Early speculations
In the sixteenth century, the Italian philosopher
Giordano Bruno
Giordano Bruno (; ; la, Iordanus Brunus Nolanus; born Filippo Bruno, January or February 1548 – 17 February 1600) was an Italian philosopher, mathematician, poet, cosmological theorist, and Hermetic occultist. He is known for his cosmologic ...
, an early supporter of the
Copernican theory that Earth and other planets orbit the Sun (
heliocentrism
Heliocentrism (also known as the Heliocentric model) is the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed the Earth at ...
), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
In the eighteenth century, the same possibility was mentioned by
Isaac Newton
Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a "natural philosopher"), widely recognised as one of the grea ...
in the "
General Scholium
The "General Scholium" (''Scholium Generale'' in the original Latin) is an essay written by Isaac Newton, appended to his work of ''Philosophiæ Naturalis Principia Mathematica'', known as the ''Principia''. It was first published with the secon ...
" that concludes his ''
Principia''. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of ''One''."
In 1952, more than 40 years before the first
hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...
was discovered,
Otto Struve
Otto Struve (August 12, 1897 – April 6, 1963) was a Russian-American astronomer of Baltic German origins. In Russian, his name is sometimes given as Otto Lyudvigovich Struve (Отто Людвигович Струве); however, he spent most o ...
wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that
Doppler spectroscopy
Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in t ...
and the
transit method
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty o ...
could detect
super-Jupiter
A super-Jupiter is a gas giant exoplanet that is more massive than the planet Jupiter. For example, companions at the planet–brown dwarf borderline have been called super-Jupiters, such as around the star Kappa Andromedae.
By 2011 there were ...
s in short orbits.
Discredited claims
Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the
binary star
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...
70 Ophiuchi
70 Ophiuchi is a binary star system located 16.6 light years away from the Earth. It is in the constellation Ophiuchus. At magnitude 4 it appears as a dim star visible to the unaided eye away from city lights.
History
In the Ptolemy's ...
. In 1855
William Stephen Jacob
William Stephen Jacob (1813–1862) was an English immigrant astronomer in India, who acted as the director of the Madras Observatory from 1848 to 1859. His early claim of 1855 to have detected an exoplanet, in orbit around 70 Ophiuchi, is now th ...
at the
East India Company
The East India Company (EIC) was an English, and later British, joint-stock company founded in 1600 and dissolved in 1874. It was formed to trade in the Indian Ocean region, initially with the East Indies (the Indian subcontinent and Southea ...
's
Madras Observatory
The Madras Observatory was an astronomical observatory which had its origins in a private observatory set up by William Petrie in 1786 and later moved and managed by the British East India Company from 1792 in Madras (now known as Chennai). The ...
reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s,
Thomas J. J. See of the
University of Chicago
The University of Chicago (UChicago, Chicago, U of C, or UChi) is a private research university in Chicago, Illinois. Its main campus is located in Chicago's Hyde Park neighborhood. The University of Chicago is consistently ranked among the b ...
and the
United States Naval Observatory
United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the Depo ...
stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year
period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
* Period (music), a concept in musical composition
* Periodic sentence (or rhetorical period), a concept ...
around one of the stars. However,
Forest Ray Moulton
Forest Ray Moulton (April 29, 1872 – December 7, 1952) was an American astronomer.
Biography
He was born in Le Roy, Michigan, and was educated at Albion College. After graduating in 1894 (Bachelor of Arts, A.B.), he performed his graduate s ...
published a paper proving that a three-body system with those orbital parameters would be highly unstable. During the 1950s and 1960s,
Peter van de Kamp
Piet van de Kamp (December 26, 1901 in Kampen (Overijssel), KampenLaurence W. Fredrick, Peter van de Kamp (1901–1995)', Publications of the Astronomical Socitiey of the Pacific 108:556–559, July 1996 – May 18, 1995 in Amsterdam), known a ...
of
Swarthmore College
Swarthmore College ( , ) is a Private college, private Liberal arts colleges in the United States, liberal arts college in Swarthmore, Pennsylvania. Founded in 1864, with its first classes held in 1869, Swarthmore is one of the earliest coeduca ...
made another prominent series of detection claims, this time for planets orbiting
Barnard's Star
Barnard's Star is a red dwarf about six light-years from Earth in the constellation of Ophiuchus. It is the fourth-nearest-known individual star to the Sun after the three components of the Alpha Centauri system, and the closest star in the ...
. Astronomers now generally regard all the early reports of detection as erroneous.
In 1991
Andrew Lyne
Andrew Geoffrey Lyne (born 13 July 1942) is a British physicist. Lyne is Langworthy Professor of Physics in the School of Physics and Astronomy, University of Manchester, as well as an ex-director of the Jodrell Bank Observatory. Despite retiring ...
,
M. Bailes and S. L. Shemar claimed to have discovered a
pulsar planet
Pulsar planets are planets that are found orbiting pulsars, or rapidly rotating neutron stars. The first such planets to be discovered were around a millisecond pulsar and were the first extrasolar planets to be confirmed as discovered.
History
...
in orbit around
PSR 1829-10
PSR may refer to:
Organizations
* Pacific School of Religion, Berkeley, California, US
* Palestinian Center for Policy and Survey Research
* Physicians for Social Responsibility, US
;Political parties:
* Revolutionary Socialist Party (Portugal) ( ...
, using
pulsar timing
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of ...
variations.
The claim briefly received intense attention, but Lyne and his team soon retracted it.
Confirmed discoveries
As of , a total of confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s. The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the
University of Victoria
The University of Victoria (UVic or Victoria) is a public research university located in the municipalities of Oak Bay and Saanich, British Columbia, Canada. The university traces its roots to Victoria College, the first post-secondary instit ...
and the
University of British Columbia
The University of British Columbia (UBC) is a public university, public research university with campuses near Vancouver and in Kelowna, British Columbia. Established in 1908, it is British Columbia's oldest university. The university ranks a ...
.
Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star
Gamma Cephei
Gamma Cephei (γ Cephei, abbreviated Gamma Cep, γ Cep) is a binary star system approximately 45 light-years away in the constellation of Cepheus. The primary (designated Gamma Cephei A, officially named Errai , the traditional name of ...
. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been
brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
s, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts.
Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.
![The Star AB Pictoris and its Companion - Phot-14d-05-normal](https://upload.wikimedia.org/wikipedia/commons/1/11/The_Star_AB_Pictoris_and_its_Companion_-_Phot-14d-05-normal.jpg)
On 9 January 1992, radio astronomers
Aleksander Wolszczan
Aleksander Wolszczan (born 29 April 1946) is a Polish astronomer. He is the co-discoverer of the first confirmed extrasolar planets and pulsar planets.
Early life and education
Wolszczan was born on 29 April 1946 in Szczecinek located in pre ...
and
Dale Frail
Dale A. Frail is a Canadian astronomer working at the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico.
Early life
He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the ...
announced the discovery of two planets orbiting the
pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
PSR 1257+12.
This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press. These pulsar planets are thought to have formed from the unusual remnants of the
supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
that produced the pulsar, in a second round of planet formation, or else to be the
remaining rocky cores of
gas giant
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s that somehow survived the supernova and then decayed into their current orbits. As pulsars are aggressive stars, it was considered unlikely at the time that a planet may be able to be formed in their orbit.
In the early 1990s, a group of astronomers led by
Donald Backer
Donald C. Backer (November 9, 1943 – July 25, 2010) was an American astrophysicist who primarily worked in radio astronomy. Backer made important contributions to the understanding and study of pulsars (including the discovery of the first mil ...
, who were studying what they thought was a binary pulsar (
PSR B1620−26 b
PSR B1620−26 b is an exoplanet located approximately 12,400 light-years from Earth in the constellation of Scorpius. It bears the unofficial nicknames "Methuselah" and "the Genesis planet" (named after the Biblical character Methuselah, who, ...
), determined that a third object was needed to explain the observed
Doppler shift
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
s. Within a few years, the gravitational effects of the planet on the orbit of the pulsar and
white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
had been measured, giving an estimate of the mass of the third object that was too small for it to be a star. The conclusion that the third object was a planet was announced by
Stephen Thorsett
Stephen Erik Thorsett (born December 3, 1964) is an American academic and astronomer serving as the president of Willamette University. His research interests include radio pulsars and gamma ray bursts. He is known for measurements of the masses o ...
and his collaborators in 1993.
On 6 October 1995,
Michel Mayor
Michel Gustave Édouard Mayor (; born 12 January 1942) is a Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the Observatory ...
and
Didier Queloz
Didier Patrick Queloz (; born 23 February 1966) is a Swiss astronomer. He is the Jacksonian Professor of Natural Philosophy at the University of Cambridge, where he is also a fellow of Trinity College, Cambridge, as well as a professor at the ...
of the
University of Geneva
The University of Geneva (French: ''Université de Genève'') is a public research university located in Geneva, Switzerland. It was founded in 1559 by John Calvin as a theological seminary. It remained focused on theology until the 17th centu ...
announced the first definitive detection of
an exoplanet orbiting a
main-sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...
star, nearby
G-type star
A G-type main-sequence star (Spectral type: G-V), also often, and imprecisely called a yellow dwarf, or G star, is a main-sequence star (luminosity class V) of spectral type G. Such a star has about 0.9 to 1.1 solar masses and an effective tempe ...
51 Pegasi
51 Pegasi (abbreviated 51 Peg), formally named Helvetios , is a Sun-like star located from Earth in the constellation of Pegasus. It was the first main-sequence star found to have an exoplanet (designated 51 Pegasi b, officially named ...
.
This discovery, made at the
Observatoire de Haute-Provence
The Haute-Provence Observatory (OHP, french: Observatoire de Haute-Provence) is an astronomical observatory in the southeast of France, about 90 km east of Avignon and 100 km north of Marseille. It was established in 1937 as a national ...
, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019
Nobel Prize in Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
. Technological advances, most notably in high-resolution
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their
gravitational
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "
hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...
s", because theories of
planetary formation
The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbitin ...
had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets.
In 1999,
Upsilon Andromedae
Upsilon Andromedae (υ Andromedae, abbreviated Upsilon And, υ And) is a binary star located 44 light-years from Earth in the constellation of Andromeda (constellation), Andromeda. The system consists of an F-type main-sequence star (desi ...
became the first main-sequence star known to have multiple planets.
Kepler-16
Kepler-16 is an eclipsing binary star system in the constellation of Cygnus that was targeted by the Kepler spacecraft. Both stars are smaller than the Sun; the primary, Kepler-16A, is a K-type main-sequence star and the secondary, ...
contains the first discovered planet that orbits around a binary main-sequence star system.
On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the
''Kepler'' Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".
Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the ''Kepler'' planets are mostly between the size of Neptune and the size of Earth.
On 23 July 2015, NASA announced
Kepler-452b
Kepler-452b (sometimes quoted to be an ''Earth 2.0'' or ''Earth's Cousin'' based on its characteristics; also known by its Kepler Object of Interest designation ''KOI-7016.01'') is a super-Earth exoplanet orbiting within the inner edge of the ha ...
, a near-Earth-size planet orbiting the habitable zone of a G2-type star.
On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.
[ This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called ]Fulton gap
The small planet radius gap (also called the Fulton gap, photoevaporation valley, or Sub-Neptune Desert) is an observed scarcity of planets with radii between 1.5 and 2 times Earth's radius, likely due to photoevaporation-driven mass loss. A bimod ...
. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range. Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[
In January 2020, scientists announced the discovery of ]TOI 700 d
TOI 700 d is a near-Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the red dwarf TOI 700, the outermost planet within the system. It is located roughly away from Earth in the constellation of Dorado. The exoplanet is ...
, the first Earth-sized planet in the habitable zone detected by TESS.
Candidate discoveries
As of January 2020, NASA's ''Kepler'' and TESS Tess or TESS may refer to:
Music
* Tess (band), a Spanish pop band active from 2000 to 2005
* TESS (musician), a UK musician
Film and theatre
* ''Tess'' (1979 film), a 1979 film adaptation of '' Tess of the d'Urbervilles''
* ''Tess'' (2016 film) ...
missions had identified 4374 planetary candidates yet to be confirmed, several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.
In September 2020, astronomers reported evidence, for the first time, of an extragalactic planet
An extragalactic planet, also known as an extragalactic exoplanet or an extroplanet, is a star-bound planet or rogue planet located outside of the Milky Way Galaxy. Due to the immense distances to such worlds, they would be very hard to detect dire ...
, M51-ULS-1b
The Whirlpool Galaxy, also known as Messier 51a, M51a, and NGC 5194, is an interacting grand-design spiral galaxy with a Seyfert 2 active galactic nucleus.
It lies in the constellation Canes Venatici, and was the first galaxy to be classified ...
, detected by eclipsing a bright X-ray source (XRS), in the Whirlpool Galaxy
The Whirlpool Galaxy, also known as Messier 51a, M51a, and NGC 5194, is an interacting grand-design spiral galaxy with a Seyfert 2 active galactic nucleus.
It lies in the constellation Canes Venatici, and was the first galaxy to be classifie ...
(M51a).
Also in September 2020, astronomers using microlensing techniques reported the detection {{Unreferenced, date=March 2018
In general, detection is the action of accessing information without specific cooperation from with the sender.
In the history of radio communications, the term "detector" was first used for a device that detected t ...
, for the first time, of an earth-mass rogue planet
A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a h ...
unbounded by any star, and free floating in the Milky Way galaxy
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
.
Detection methods
Direct imaging
Planets are extremely faint compared with their parent stars. For example, a Sun-like star is about a billion times brighter than the reflected light from any exoplanet orbiting it. It is difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star in order to reduce the glare while leaving the light from the planet detectable; doing so is a major technical challenge which requires extreme optothermal stability Optothermal stability describes the rate at which an optical element distorts due to a changing thermal environment. A changing thermal environment can cause an optic to bend due to either 1) changing thermal gradients on the optic and a non-zero co ...
. All exoplanets that have been directly imaged are both large (more massive than Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
) and widely separated from their parent star.
Specially designed direct-imaging instruments such as Gemini Planet Imager
The Gemini Planet Imager (GPI) is a high contrast imaging instrument that was built for the Gemini South Telescope in Chile. The instrument achieves high contrast at small angular separations, allowing for the direct imaging and integral field spec ...
, VLT-SPHERE
Spectro-Polarimetric High-contrast Exoplanet REsearch (VLT-SPHERE) is an adaptive optics system and coronagraphic facility at the Very Large Telescope (VLT). It provides direct imaging as well as spectroscopic and polarimetric characterization o ...
, and SCExAO will image dozens of gas giants, but the vast majority of known extrasolar planets have only been detected through indirect methods.
Indirect methods
* Transit method
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty o ...
:
If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. Because the transit method requires that the planet's orbit intersect a line-of-sight between the host star and Earth, the probability that an exoplanet in a randomly oriented orbit will be observed to transit the star is somewhat small. The ''Kepler'' telescope used this method.
* Radial velocity or Doppler method
:As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity—that is, the speed with which it moves towards or away from Earth—can be detected from displacements in the star's spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
s due to the Doppler effect
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
. Extremely small radial-velocity variations can be observed, of 1 m/s or even somewhat less.
* Transit timing variation
Transit-timing variation is a method for detecting exoplanets by observing variations in the timing of a transit. This provides an extremely sensitive method capable of detecting additional planets in the system with masses potentially as small as ...
(TTV)
:When multiple planets are present, each one slightly perturbs the others' orbits. Small variations in the times of transit for one planet can thus indicate the presence of another planet, which itself may or may not transit. For example, variations in the transits of the planet Kepler-19b
Kepler-19b is a planet orbiting around the star Kepler-19. The planet has an orbital period of 9.3 days, with an estimated radius of roughly 2.2 times that of the Earth
Earth is the third planet from the Sun and the only astronomical ...
suggest the existence of a second planet in the system, the non-transiting Kepler-19c
Kepler-19c is an extra-solar planet orbiting the star Kepler-19 approximately 717 light years from Earth.
Discovery
The planet was discovered as a result of examinations of data from the previously discovered exoplanet, Kepler-19b. Timing va ...
.[Planet Hunting: Finding Earth-like Planets]
. Scientific Computing. 19 July 2010
* Transit duration variation (TDV)
:When a planet orbits multiple stars or if the planet has moons, its transit time can significantly vary per transit. Although no new planets or moons have been discovered with this method, it is used to successfully confirm many transiting circumbinary planets.
* Gravitational microlensing
Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers ...
:Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in the magnification as it varies over time. Unlike most other methods which have detection bias towards planets with small (or for resolved imaging, large) orbits, microlensing method is most sensitive to detecting planets around 1–10 AU away from Sun-like stars.
* Astrometry
Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way.
His ...
:Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because the motion is so small, however, this method has not yet been very productive. It has produced only a few disputed detections, though it has been successfully used to investigate the properties of planets found in other ways.
* Pulsar timing
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of ...
:A pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
(the small, ultradense remnant of a star that has exploded as a supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
) emits radio waves extremely regularly as it rotates. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. The first confirmed discovery of an extrasolar planet was made using this method. But as of 2011, it has not been very productive; five planets have been detected in this way, around three different pulsars.
* Variable star timing (pulsation frequency)
:Like pulsars, there are some other types of stars which exhibit periodic activity. Deviations from the periodicity can sometimes be caused by a planet orbiting it. As of 2013, a few planets have been discovered with this method.
* Reflection/emission modulations
:When a planet orbits very close to the star, it catches a considerable amount of starlight. As the planet orbits around the star, the amount of light changes due to planets having phases from Earth's viewpoint or planet glowing more from one side than the other due to temperature differences.
* Relativistic beaming
Relativistic beaming (also known as Doppler beaming, Doppler boosting, or the headlight effect) is the process by which relativistic effects modify the apparent luminosity of emitting matter that is moving at speeds close to the speed of ligh ...
:Relativistic beaming measures the observed flux from the star due to its motion. The brightness of the star changes as the planet moves closer or further away from its host star.
* Ellipsoidal variations
:Massive planets close to their host stars can slightly deform the shape of the star. This causes the brightness of the star to slightly deviate depending how it is rotated relative to Earth.
* Polarimetry
Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves. Typically polarimetry is done on electromagnetic waves that have traveled through or hav ...
:With polarimetry method, a polarized light reflected off the planet is separated from unpolarized light emitted from the star. No new planets have been discovered with this method although a few already discovered planets have been detected with this method.
* Circumstellar disks
A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the re ...
:Disks of space dust surround many stars, thought to originate from collisions among asteroids and comets. The dust can be detected because it absorbs starlight and re-emits it as infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
radiation. Features in the disks may suggest the presence of planets, though this is not considered a definitive detection method.
Formation and evolution
Planets may form within a few to tens (or more) of millions of years of their star forming.
The planets of the Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming to planetary systems of over 10 Gyr old. When planets form in a gaseous protoplanetary disk
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...
, they accrete hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
/helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
envelopes. These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space. This means that even terrestrial planets may start off with large radii if they form early enough. An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.
Planet-hosting stars
There is at least one planet on average per star.
About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
.
Most known exoplanets orbit stars roughly similar to the Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, i.e. main-sequence stars
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color index, color versus absolute magnitude, brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams afte ...
of spectral categories F, G, or K. Lower-mass stars (red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
s, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method
Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in t ...
. Despite this, several tens of planets around red dwarfs have been discovered by the ''Kepler'' spacecraft, which uses the transit method
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty o ...
to detect smaller planets.
Using data from ''Kepler
Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...
'', a correlation has been found between the metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...
of a star and the probability that the star hosts a giant planet, similar to the size of Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.
Some planets orbit one member of a binary star
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...
system, and several circumbinary planet
A circumbinary planet is a planet that orbits two stars instead of one. The two stars orbit each other in a binary system, while the planet typically orbits farther from the center of the system than either of the two stars. In contrast, circums ...
s have been discovered which orbit around both members of binary star. A few planets in triple star
A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speak ...
systems are known and one in the quadruple system Kepler-64
PH1b (standing for "Planet Hunters 1"), or by its NASA designation Kepler-64b, is an extrasolar planet found in a circumbinary planet, circumbinary orbit in the Star system#Quaternary, quadruple star system Kepler-64. The planet was discovered b ...
.
Orbital and physical parameters
General features
Color and brightness
In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo
Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
measurements of HD 189733b
HD 189733 b is an exoplanet approximately away from the Solar System in the constellation of Vulpecula. Astronomers in France discovered the planet orbiting the star HD 189733 on October 5, 2005, by observing its transit across the star's face. W ...
suggest that it is deep dark blue. Later that same year, the colors of several other exoplanets were determined, including GJ 504 b
Gliese 504 b (often shortened to GJ 504 b) is considered by NASA to be a Jovian planet and it is located in the system of the solar analog 59 Virginis (GJ 504),In spite of names of some exoplanets, derived from theirs host stars Flamsteed designa ...
which visually has a magenta color, and Kappa Andromedae b
}
Kappa Andromedae b is a directly imaged substellar object and likely superjovian-mass planet orbiting Kappa Andromedae, a young B9IV star in the Andromeda constellation, about 170 light-years away. The companion's mass is roughly 13 times the m ...
, which if seen up close would appear reddish in color. Helium planet
A helium planet is a planet with a helium-dominated atmosphere. This contrasts with ordinary gas giants such as Jupiter and Saturn, whose atmospheres consist primarily of hydrogen, with helium as a secondary component only. Helium planets might fo ...
s are expected to be white or grey in appearance.
The apparent brightness (apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.
The darkest known planet in terms of geometric albedo
In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source (i.e. at zero phase angle) to that of an ''idealized'' flat, fully reflecting, diffusively scattering ( Lambertian) disk w ...
is TrES-2b
TrES-2b (TrES-2 or Kepler-1b) is an extrasolar planet orbiting the star GSC 03549-02811 located 750 light years away from the Solar System. The planet was identified in 2011 as the darkest known exoplanet, reflecting less than 1% of any light that ...
, a hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...
that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.
For gas giant
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.
There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
-dependent, this is not always the case in the near infrared.
Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.
Magnetic field
In 2014, a magnetic field around HD 209458 b
HD 209458 b, which is also nicknamed Osiris after the Egyptian god, is an exoplanet that orbits the solar analog HD 209458 in the constellation Pegasus, some from the Solar System. The radius of the planet's orbit is , or one-eighth the radius o ...
was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.
The magnetic fields of exoplanets may be detectable by their auroral
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of br ...
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
emissions with sensitive enough radio telescopes such as LOFAR. The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...
results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.
Hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...
s have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind
A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.
D ...
and the planet's magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
creating an electric current through the planet that heats it up (Joule heating
Joule heating, also known as resistive, resistance, or Ohmic heating, is the process by which the passage of an electric current through a conductor (material), conductor produces heat.
Joule's first law (also just Joule's law), also known in c ...
) causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.
In August 2018, scientists announced the transformation of gaseous deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
into a liquid metallic hydrogen
Metallic hydrogen is a phase of hydrogen in which it behaves like an electrical conductor. This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington.
At high pressure and temperatures, metallic hydrogen c ...
form. This may help researchers better understand giant gas planets, such as Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
, Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s.
Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flare
The asterisk ( ), from Late Latin , from Ancient Greek , ''asteriskos'', "little star", is a typographical symbol. It is so called because it resembles a conventional image of a heraldic star.
Computer scientists and mathematicians often voc ...
s and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733
HD 189733, also catalogued as V452 Vulpeculae, is a binary star system approximately 64.5 light-years away in the constellation of Vulpecula (the Fox). The primary star is suspected to be an orange dwarf star, while the secondary star i ...
system. The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.
In 2019 the strength of the surface magnetic fields of 4 hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...
s were estimated and ranged between 20 and 120 gauss
Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes refer ...
compared to Jupiter's surface magnetic field of 4.3 gauss.
Plate tectonics
In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
on larger super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively.
The term "super-Earth" refers only to ...
s with one team saying that plate tectonics would be episodic or stagnant and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.
If super-Earths have more than 80 times as much water as Earth then they become ocean planet
An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the surfa ...
s with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.
Volcanism
Large surface temperature variations on 55 Cancri e
55 Cancri e (abbreviated 55 Cnc e, formally named Janssen and nicknamed "Hell on Earth") is an exoplanet in the orbit of its Sun-like host star 55 Cancri A. The mass of the exoplanet is about 8.63 Earth masses and its diameter is about twice ...
have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.
Rings
The star 1SWASP J140747.93-394542.6
1SWASP J140747.93−394542.6 (also known as 1SWASP J140747, J1407 and Mamajek's Object) is a star similar to the Sun in the constellation Centaurus at a distance of about 434 light-years from Earth. A relatively young star, its age is estimated t ...
is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.
The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.
The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a axial tilt, tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.
Moons
In December 2013 a candidate exomoon of a rogue planet
A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a h ...
was announced. On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.
Atmospheres
Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b
HD 209458 b, which is also nicknamed Osiris after the Egyptian god, is an exoplanet that orbits the solar analog HD 209458 in the constellation Pegasus, some from the Solar System. The radius of the planet's orbit is , or one-eighth the radius o ...
in 2001.
As of February 2014, more than fifty Transit method, transiting and five Direct imaging, directly imaged exoplanet atmospheres have been observed, resulting in detection of molecular spectral features; observation of day–night temperature gradients; and constraints on vertical atmospheric structure. Also, an atmosphere has been detected on the non-transiting hot Jupiter Tau Boötis b.
In May 2017, glints of light from Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, seen as twinkling from an orbiting satellite a million miles away, were found to be Reflection (physics), reflected light from ice crystals in the Atmosphere of Earth, atmosphere. The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
Comet-like tails
KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet. The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.
In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail long.
Insolation pattern
Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil. Planets with an Orbital eccentricity, eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres. Planets with both an eccentric orbit and a axial tilt, tilted axis of rotation would have more complicated insolation patterns.
Surface
Surface composition
Surface features can be distinguished from atmospheric features by comparing emission and reflection spectroscopy with transmission spectroscopy. Mid-infrared spectroscopy of exoplanets may detect rocky surfaces, and near-infrared may identify magma oceans or high-temperature lavas, hydrated silicate surfaces and water ice, giving an unambiguous method to distinguish between rocky and gaseous exoplanets.
Surface temperature
The temperature of an exoplanet can be estimated by measuring the intensity of the light it receives from its parent star. For example, the planet OGLE-2005-BLG-390Lb is estimated to have a surface temperature of roughly −220 °C (50 K). However, such estimates may be substantially in error because they depend on the planet's usually unknown albedo
Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
, and because factors such as the greenhouse effect may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b
HD 189733 b is an exoplanet approximately away from the Solar System in the constellation of Vulpecula. Astronomers in France discovered the planet orbiting the star HD 189733 on October 5, 2005, by observing its transit across the star's face. W ...
has been estimated to have an average temperature of 1,205 K (932 °C) on its dayside and 973 K (700 °C) on its nightside.
Habitability
As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of Astrobiology, life on planets beyond the Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. At cosmic distances, life
Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energ ...
can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes). For example, molecular oxygen () in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an Biomarker, indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means. Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.
Habitable zone
The habitable zone around a star is the region where the temperature is just right to allow liquid water to exist on the surface of planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star, so that the habitable zone can be at different distances for different stars. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out. Rocky planets with a thick hydrogen atmosphere could maintain surface water much further out than the Earth–Sun distance. Planets with larger mass have wider habitable zones because the gravity reduces the water cloud column depth which reduces the greenhouse effect of water vapor, thus moving the inner edge of the habitable zone closer to the star.
Planetary #Rotation and axial tilt, rotation rate is one of the major factors determining the #Atmospheric circulation, circulation of the atmosphere and hence the pattern of clouds: slowly rotating planets create thick clouds that albedo, reflect more and so can be habitable much closer to their star. Earth with its current atmosphere would be habitable in Venus's orbit, if it had Venus's slow rotation. If Venus lost its water ocean due to a runaway greenhouse effect#Venus, runaway greenhouse effect, it is likely to have had a higher rotation rate in the past. Alternatively, Venus never had an ocean because water vapor was lost to space during its formation and could have had its slow rotation throughout its history.
Tidal locking, Tidally locked planets (a.k.a. "eyeball" planets) can be habitable closer to their star than previously thought due to the effect of clouds: at high stellar flux, strong convection produces thick water clouds near the substellar point that greatly increase the planetary albedo and reduce surface temperatures.
Habitable zones have usually been defined in terms of surface temperature, however over half of Earth's biomass is from subsurface microbes, and the temperature increases with depth, so the subsurface can be conducive for microbial life when the surface is frozen and if this is considered, the habitable zone extends much further from the star, even rogue planet
A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a h ...
s could have liquid water at sufficient depths underground. In an earlier era of the universe the temperature of the cosmic microwave background would have allowed any rocky planets that existed to have liquid water on their surface regardless of their distance from a star. Jupiter-like planets might not be habitable, but they could have Habitability of natural satellites, habitable moons.
Ice ages and snowball states
The outer edge of the habitable zone is where planets are completely frozen, but planets well inside the habitable zone can periodically become frozen. If orbital fluctuations or other causes produce cooling then this creates more ice, but ice reflects sunlight causing even more cooling, creating a feedback loop until the planet is completely or nearly completely frozen. When the surface is frozen, this stops Solution weathering, carbon dioxide weathering, resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a greenhouse effect which thaws the planet again. Planets with a large axial tilt are less likely to enter snowball states and can retain liquid water further from their star. Large fluctuations of axial tilt can have even more of a warming effect than a fixed large tilt. Paradoxically, planets orbiting cooler stars, such as red dwarfs, are less likely to enter snowball states because the infrared radiation emitted by cooler stars is mostly at wavelengths that are absorbed by ice which heats it up.
Tidal heating
If a planet has an eccentric orbit, then tidal heating can provide another source of energy besides stellar radiation. This means that eccentric planets in the radiative habitable zone can be too hot for liquid water. Tides also tidal circularization, circularize orbits over time so there could be planets in the habitable zone with circular orbits that have no water because they used to have eccentric orbits. Eccentric planets further out than the habitable zone would still have frozen surfaces but the tidal heating could create a subsurface ocean similar to Europa (moon), Europa's. In some planetary systems, such as in the Upsilon Andromedae
Upsilon Andromedae (υ Andromedae, abbreviated Upsilon And, υ And) is a binary star located 44 light-years from Earth in the constellation of Andromeda (constellation), Andromeda. The system consists of an F-type main-sequence star (desi ...
system, the eccentricity of orbits is maintained or even periodically varied by perturbations from other planets in the system. Tidal heating can cause outgassing from the mantle, contributing to the formation and replenishment of an atmosphere.
Potentially habitable planets
A review in 2015 identified exoplanets Kepler-62f, Kepler-186f and Kepler-442b as the best candidates for being potentially habitable. These are at a distance of 1200, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is in similar size to Earth with its 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
star.
When looking at the nearest terrestrial exoplanet candidates, Proxima Centauri b is about 4.2 light-years away. Its equilibrium temperature is estimated to be .
Earth-size planets
* In November 2013 it was estimated that 22±8% of Sun-like stars in the Milky Way galaxy may have an Earth-sized planet in the habitable[ zone.] Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earths, rising to 40 billion if red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
s are included.
* Kepler-186f, a 1.2-Earth-radius planet in the habitable zone of a red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
, reported in April 2014.
*Proxima Centauri b, a planet in the habitable zone of Proxima Centauri
Proxima Centauri is a small, low-mass star located away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest tarof Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-kno ...
, the nearest known star to the solar system with an estimated minimum mass of 1.27 times the mass of the Earth.
* In February 2013, researchers speculated that up to 6% of small red dwarfs may have Earth-size planets. This suggests that the closest one to the Solar System could be 13 light-years away. The estimated distance increases to 21 light-years when a 95% confidence interval is used. In March 2013 a revised estimate gave an occurrence rate of 50% for Earth-size planets in the habitable zone of red dwarfs.
* At 1.63 times Earth's radius Kepler-452b
Kepler-452b (sometimes quoted to be an ''Earth 2.0'' or ''Earth's Cousin'' based on its characteristics; also known by its Kepler Object of Interest designation ''KOI-7016.01'') is a super-Earth exoplanet orbiting within the inner edge of the ha ...
is the first discovered near-Earth-size planet in the Circumstellar habitable zone, "habitable zone" around a G star, G2-type Sun-like star (July 2015).
Search projects
*CoRoT - Mission to look for exoplanets using the transit method.
*Kepler space telescope, Kepler - Mission to look for large numbers of exoplanets using the transit method.
*TESS Tess or TESS may refer to:
Music
* Tess (band), a Spanish pop band active from 2000 to 2005
* TESS (musician), a UK musician
Film and theatre
* ''Tess'' (1979 film), a 1979 film adaptation of '' Tess of the d'Urbervilles''
* ''Tess'' (2016 film) ...
- To search for new exoplanets; rotating so by the end of its two-year mission it will have observed stars from all over the sky. It is expected to find at least 3,000 new exoplanets.
*High Accuracy Radial Velocity Planet Searcher, HARPS - High-precision Echelle grating, echelle planet-finding spectrograph installed on the ESO 3.6 m Telescope, ESO's 3.6m telescope at La Silla Observatory in Chile.
*ESPRESSO - A rocky planet-finding, and stable spectroscopic observing, spectrograph mounted on ESO's 4 by 8.2m Very Large Telescope, VLT telescope, sited on the levelled summit of Cerro Paranal in the Atacama Desert of northern Chile.
*Extremely Large Telescope#Instrumentation, ANDES - The ArmazoNes high Dispersion Echelle Spectrograph, a planet finding and planet characterisation spectrograph, is expected to be fitted onto Extremely Large Telescope, ESO's ELT 39.3m telescope. ANDES was formally known as HIRES, which itself was created after a merger of the consortia behind the earlier CODEX (optical high-resolution) and SIMPLE (near-infrared high-resolution) spectrograph concepts.
Notes
See also
* Lists of exoplanets
References
Further reading
* (Hardback); (Paperback).
* (Hardback); (Paperback).
* (Hardcover).
*
* .
* .
* (Paperback).
*
External links
What Is An Exoplanet? (SETI; 2020)
Earth Similarity Index Calculator
The Extrasolar Planets Encyclopaedia
(Paris Observatory)
NASA Exoplanet Archive
Open Exoplanet Catalogue
The Habitable Exoplanets Catalog
(PHL/University of Puerto Rico at Arecibo, UPR Arecibo)
Extrasolar Planets
nbsp;– D. Montes, UCM
Exoplanets
at Paris Observatory
Graphical Comparison of Extrasolar Planets
*
*
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Exoplanetology
Exoplanets,
Search for extraterrestrial intelligence
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Concepts in astronomy
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