Exoplanet host stars
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Planet-hosting stars are
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s which host
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 ...
s, therefore forming
planetary system A planetary system is a set of gravitationally 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 interacti ...
s. This article describes the correlations between stars' characteristics and the characteristics of the planets that orbit them, and other connections between stars and their planets.


Proportion of stars with planets

Most stars have planets but exactly what proportion of stars have planets is uncertain because not all planets can yet be detected. That said it has been calculated that there is at least one planet on average per star. One in five Sun-like stars are expected to 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 ...
. The radial-velocity method and the transit method (which between them are responsible for the vast majority of detections) are most sensitive to large planets in small orbits. Thus many known exoplanets are "hot Jupiters": planets of
Jovian Jovian is the adjectival form of Jupiter and may refer to: * Jovian (emperor) (Flavius Iovianus Augustus), Roman emperor (363–364 AD) * Jovians and Herculians, Roman imperial guard corps * Jovian (lemur), a Coquerel's sifaka known for ''Zoboomafo ...
mass or larger in very small orbits with periods of only a few days. A 2005 survey of radial-velocity-detected planets found that about 1.2% of Sun-like stars have a hot Jupiter, where "Sun-like star" refers to any main-sequence star of
spectral classes In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
late- F, G, or early- K without a close stellar companion. This 1.2% is more than double the frequency of hot jupiters detected by the Kepler spacecraft, which may be because the Kepler field of view covers a different region of the Milky Way where the metallicity of stars is different. It is further estimated that 3% to 4.5% of Sun-like stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least 30 Earth masses. It is known that small planets (of roughly Earth-like mass or somewhat larger) are more common than giant planets. It also appears that there are more planets in large orbits than in small orbits. Based on this, it is estimated that perhaps 20% of Sun-like stars have at least one giant planet whereas at least 40% may have planets of lower mass. A 2012 study of
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 ...
data collected between 2002 and 2007 concludes the proportion of stars with planets is much higher and estimates an average of 1.6 planets orbiting between 0.5 and 10 AU per star in 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 ...
, the authors of this study conclude that "stars are orbited by planets as a rule, rather than the exception". In November 2013 it was announced that 22±8% of Sun-likeFor 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 wasn't available so this statistic is an extrapolation from data about
K-type star In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
s
stars have an Earth-sizedFor the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii planet in the habitableFor 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). zone. Whatever the proportion of stars with planets, the total number of exoplanets must be very large. Because the Milky Way has at least 200 billion stars, it must also contain tens or hundreds of billions of planets.


Type of star, spectral classification

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 ...
, that is,
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. One reason is that planet-search programs have tended to concentrate on such stars. In addition, statistical analyses indicate that 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 ...
. Nevertheless, many planets around red dwarfs have been discovered by the
Kepler spacecraft The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...
by 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 ...
, which can detect smaller planets. Stars of spectral category A typically rotate very quickly, which makes it very difficult to measure the small Doppler shifts induced by orbiting planets because the spectral lines are very broad.Retired A Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants
John A. Johnson, Debra A. Fischer, Geoffrey W. Marcy, Jason T. Wright, Peter Driscoll, R. P. Butler, Saskia Hekker, Sabine Reffert, Steven S. Vogt, 19 Apr 2007
However, this type of massive star eventually evolves into a cooler
red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
that rotates more slowly and thus can be measured using the radial-velocity method. A few tens of planets have been found around red giants. Observations using the
Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, f ...
indicate that extremely massive stars of spectral category O, which are much hotter than the Sun, produce a photo-evaporation effect that inhibits
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 ...
. When the O-type star goes
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 ...
any planets that had formed would become free-floating due to the loss of stellar mass unless the natal kick of the resulting remnant pushes it in the same direction as an escaping planet.Limits on Planets Orbiting Massive Stars from Radio Pulsar Timing
, Thorsett, S.E. Dewey, R.J. 16-Sep-1993
Fallback disks of matter that failed to escape orbit during a supernova may form planets around
neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
s and
black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
s.The fate of fallback matter around newly born compact objects
Rosalba Perna Rosalba Perna is an Italian and American theoretical astrophysicist whose research concerns high-energy cosmic sources including gamma-ray bursts and neutron star mergers. She has also studied exoplanets and the growth of supermassive black hol ...
, Paul Duffell, Matteo Cantiello, Andrew MacFadyen, (Submitted on 17 Dec 2013)
Doppler surveys around a wide variety of stars indicate about 1 in 6 stars having twice the mass of the Sun are orbited by one or more Jupiter-sized planets, vs. 1 in 16 for Sun-like stars and only 1 in 50 for
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. On the other hand,
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 ...
surveys indicate that long-period
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
-mass planets are found around 1 in 3 red dwarfs. Kepler Space Telescope observations of planets with up to one year periods show that occurrence rates of Earth- to Neptune-sized planets (1 to 4 Earth radii) around M, K, G, and F stars are successively higher towards cooler, less massive stars. At the low-mass end of star-formation are sub-stellar objects that don't fuse hydrogen: the
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 and
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, of spectral classification L,T and Y. Planets and protoplanetary disks have been discovered around brown dwarfs, and disks have been found around sub-brown dwarfs (e.g.
OTS 44 OTS 44 is a free-floating planetary-mass object or brown dwarf located at in the constellation Chamaeleon near the reflection nebula IC 2631. It is among the lowest-mass free-floating substellar objects, with approximately 11.5 times ...
).
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 ejected from their system could retain a system of satellites.


Metallicity

Ordinary stars are composed mainly of the light elements
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 ...
and
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. ...
. They also contain a small proportion of heavier elements, and this fraction is referred to as a star's
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 ...
(even if the elements are not
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
s in the traditional sense), denoted /Hand expressed on a
logarithmic scale A logarithmic scale (or log scale) is a way of displaying numerical data over a very wide range of values in a compact way—typically the largest numbers in the data are hundreds or even thousands of times larger than the smallest numbers. Such a ...
where zero is the Sun's metallicity. Stars with a higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity. A 2012 study of the
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 ...
spacecraft data found that smaller planets, with radii smaller than Neptune's were found around stars with metallicities in the range −0.6 < /H< +0.5 (about four times less than that of the Sun to three times more),Converting log scale /Hto multiple of solar metallicity: 10−0.6 ≈ 1/4), (100.5 ≈ 3)/ref> whereas larger planets were found mostly around stars with metallicities at the higher end of this range (at solar metallicity and above). In this study small planets occurred about three times as frequently as large planets around stars of metallicity greater than that of the Sun, but they occurred around six times as frequently for stars of metallicity less than that of the Sun. The lack 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 around low-metallicity stars could be because the metallicity of
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 ...
s affects how quickly
planetary core A planetary core consists of the innermost layers of a planet. Cores may be entirely solid or entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. In the Solar System, core sizes range from about 20% (the Moon ...
s can form and whether they accrete a gaseous envelope before the gas dissipates. However, Kepler can only observe planets very close to their star and the detected gas giants probably migrated from further out, so a decreased efficiency of migration in low-metallicity disks could also partly explain these findings. A 2014 study found that not only giant planets, but planets of all sizes have an increased occurrence rate around metal-rich stars compared to metal-poor stars, although the larger the planet, the greater this increase as the metallicity increases. The study divided planets into three groups based on radius: gas giants, gas dwarfs, and terrestrial planets with the dividing lines at 1.7 and 3.9 Earth radii. For these three groups the planet occurrence rates are 9.30, 2.03, and 1.72 times higher for metal-rich stars than for metal-poor stars, respectively. There is a bias against detecting smaller planets because metal-rich stars tend to be larger, making it more difficult to detect smaller planets, which means that these increases in occurrence rates are lower limits. It has also been shown that Sun-like stars with planets are much more likely to be deficient in
lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...
, although this correlation is not seen at all in other types of stars. However, this claimed relationship has become a point of contention in the planetary astrophysics community, being frequently denied but also supported.


Multiple stars

Stellar multiplicity increases with stellar mass: the likelihood of stars being in multiple systems is about 25% for red dwarfs, about 45% for Sun-like stars, and rises to about 80% for the most massive stars. Of the multiple stars about 75% are binaries and the rest are higher-order multiplicities. More than one hundred planets have been discovered orbiting 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 (e.g.
55 Cancri 55 Cancri is a binary star system located 41 light-years away from the Sun in the zodiac constellation of Cancer. It has the Bayer designation Rho1 Cancri (ρ1 Cancri); ''55 Cancri'' is the Flamsteed designation (abbrevia ...
, possibly
Alpha Centauri Bb Alpha Centauri Bb (α Cen B b) was a proposed exoplanet orbiting the K-type main-sequence star Alpha Centauri B, located 4.37 light-years from Earth in the southern constellation of Centaurus, but there has not been enough evidence to support ...
), 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 a binary star (e.g.
PSR B1620-26 b 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) ( ...
,
Kepler-16b Kepler-16b (formally Kepler-16 (AB)-b) is an extrasolar planet. It is a Saturn-mass planet consisting of half gas and half rock and ice, and it orbits a binary star, Kepler-16, with a period of 229 days. " tis the first confirmed, unambiguous e ...
). A few dozen 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 (e.g. 16 Cygni Bb) and two in quadruple systems Kepler 64 and 30 Arietis. The
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 ...
results indicate circumbinary planetary systems are relatively common (as of October 2013 the spacecraft had found seven circumbinary planets out of roughly 1000
eclipsing binaries 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 ...
searched). One puzzling finding is that although half of the binaries have an orbital period of 2.7 days or less, none of the binaries with circumbinary planets have a period less than 7.4 days. Another surprising Kepler finding is circumbinary planets tend to orbit their stars close to the critical instability radius (theoretical calculations indicate the minimum stable separation is roughly two to three times the size of the stars' separation). In 2014, from statistical studies of searches for companion stars, it was inferred that around half of exoplanet host stars have a companion star, usually within 100AU. This means that many exoplanet host stars that were thought to be single are binaries, so in many cases it is not known which of the stars a planet actually orbits, and the published parameters of transiting planets could be significantly incorrect because the ''planet radius'' and ''distance from star'' are derived from the stellar parameters. Follow-up studies with imaging (such as
speckle imaging Speckle imaging describes a range of high-resolution astronomical imaging techniques based on the analysis of large numbers of short exposures that freeze the variation of atmospheric turbulence. They can be divided into the shift-and-add ("'' ...
) are needed to find or rule out companions (and
radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...
techniques would be required to detect binaries really close together) and this has not yet been done for most exoplanet host stars. Examples of known binary stars where it is not known which of the stars a planet orbits are Kepler-132 and Kepler-296, although a 2015 study found that the Kepler-296 planets were likely orbiting the brighter star.


Open clusters

Most stars form in
open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...
s, but very few planets have been found in open clusters and this led to the hypothesis that the open-cluster environment hinders
planet 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 orbiting t ...
. However, a 2011 study concluded that there have been an insufficient number of surveys of clusters to make such a hypothesis. The lack of surveys was because there are relatively few suitable open clusters in the Milky Way. Recent discoveries of both giant planets and low-mass planetsThe same frequency of planets inside and outside open clusters of stars, Søren Meibom, Guillermo Torres, Francois Fressin, David W. Latham, Jason F. Rowe, David R. Ciardi, Steven T. Bryson, Leslie A. Rogers, Christopher E. Henze, Kenneth Janes, Sydney A. Barnes, Geoffrey W. Marcy, Howard Isaacson, Debra A. Fischer, Steve B. Howell, Elliott P. Horch, Jon M. Jenkins, Simon C. Schuler & Justin Crepp Nature 499, 55–58 (04 July 2013) doi:10.1038/nature12279 Received 06 November 2012 Accepted 02 May 2013 Published online 26 June 2013 in open clusters are consistent with there being similar planet occurrence rates in open clusters as around field stars. The
open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...
NGC 6811 NGC 6811 is an open cluster in the constellation of Cygnus, near the constellation of Lyra. It has an angular size half that of the full Moon and includes about 1000 stars of roughly similar magnitude. It has also been called "The Hole in the C ...
contains two known planetary systems
Kepler-66 Kepler-66 is a star with slightly more mass than the Sun in the NGC 6811 open cluster in the Cygnus constellation. It has one confirmed planet, slightly smaller than Neptune, announced in 2013. Planetary system References External ...
and
Kepler-67 Kepler-67 is a star in the open cluster NGC 6811 in the constellation Cygnus. It has slightly less mass than the Sun and has one confirmed planet, slightly smaller than Neptune Neptune is the eighth planet from the Sun and the ...
.


Age


The Ages of Stars
David R. Soderblom, 31 Mar 2010
Towards asteroseismically calibrated age-rotation-activity relations for Kepler solar-like stars
R.A. Garcia et al. 27 Mar 2014
Accurate parameters of the oldest known rocky-exoplanet hosting system: Kepler-10 revisited
Alexandra Fogtmann-Schulz et al. 5 Dec 2013


Asteroseismology


The importance of asteroseismology in exoplanetary science
F Borsa, E Poretti - sait.oat.ts.astro.it
What asteroseismology can do for exoplanets: Kepler-410A b is a Small Neptune around a bright star, in an eccentric orbit consistent with low obliquity
Vincent Van Eylen et al. 17 Dec 2013
Pulsations and planets: the asteroseismology-extrasolar-planet connection
Sonja Schuh, 19 May 2010


Stellar activity


How stellar activity affects the size estimates of extrasolar planets
S. Czesla, K. F. Huber, U. Wolter, S. Schröter, J. H. M. M. Schmitt, 19 Jun 2009
Hot Jupiters and stellar magnetic activity
A. F. Lanza, 20 May 2008
Extrasolar Giant Planets and X-ray Activity
Vinay L. Kashyap, Jeremy J. Drake, Steven H. Saar, 21 Jul 2008
Mass loss of "Hot Jupiters"—Implications for CoRoT discoveries. Part I: The importance of magnetospheric protection of a planet against ion loss caused by coronal mass ejections
Khodachenko et al. April 2007


Further reading


Different types of star-planet interactions
A. A. Vidotto, 25 Nov 2019


References

{{exoplanet Planetary systems