Kepler-186f
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Kepler-186f (also known by its Kepler object of interest designation KOI-571.05) is an
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. 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, init ...
orbiting the
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 ...
Kepler-186 Kepler-186 is a main-sequence M1-type dwarf star, located 178.5 parsecs (582 light years) away in the constellation of Cygnus. The star is slightly cooler than the sun, with roughly half its metallicity. It is known to have five plane ...
, about 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 ...
. Distance to Kepler 186, after taking into account light extinction
/ref> It was the first planet with a radius similar to Earth's to be discovered 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 ...
of another star.
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
's
Kepler space telescope 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 ...
detected it using 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 ...
, along with four additional planets orbiting much closer to the star (all modestly larger than Earth). Analysis of three years of data was required to find its signal. The results were presented initially at a conference on 19 March 2014 See session 19 March 2014 – Wednesday 11:50–12:10 – Thomas Barclay: The first Earth-sized habitable zone exoplanets. and some details were reported in the media at the time. The public announcement was on 17 April 2014, followed by publication in ''
Science Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earliest archeological evidence for ...
''.


Physical characteristics


Mass, radius and temperature

The only physical property directly derivable from the observations (besides orbit) is the size of the planet relative to the central star, which follows from the amount of occultation of stellar light during a transit. This ratio was measured to be 0.021, giving a planetary radius of 1.11±0.14 times that of
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 ...
. The planet is about 11% larger in radius than Earth (between 4.5% smaller and 26.5% larger), giving a volume about 1.37 times that of Earth (between 0.87 and 2.03 times as large). A very wide range of possible
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
es can be calculated by combining the
radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
with
densities Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek language, Greek letter Rho (letter), rho), although the Latin letter ''D'' ca ...
derived from the possible types of
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...
from which planets can be made. For example, it could be a rocky
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Ve ...
or a lower density
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 ...
with a thick atmosphere. A massive
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. ...
(H/He) atmosphere is thought to be unlikely in a planet with a radius below 1.5 . Planets with a radius of more than 1.5 times that of Earth tend to accumulate the thick atmospheres which make them less likely to be habitable. Red dwarfs emit a much stronger
extreme ultraviolet Extreme ultraviolet radiation (EUV or XUV) or high-energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths from 124  nm down to 10 nm, and therefore (by the Planck– ...
(XUV) flux when young than later in life. The planet's primordial atmosphere would have been subjected to elevated
photoevaporation Photoevaporation denotes the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. This typically refers to an astrophysical context where ultraviolet electromagnetic radiation, radiation from hot st ...
during that period, which would probably have largely removed any H/He-rich envelope through hydrodynamic mass loss. Mass estimates range from 0.32 for a pure
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
/ice composition to 3.77 if made up entirely of
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
(both implausible extremes). For a body with radius 1.11 , a composition similar to that of Earth (i.e., 1/3 iron, 2/3 silicate rock) yields a mass of 1.44 , taking into account the higher density due to the higher average
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 ...
compared to Earth. That would make the force of gravity on the surface 17% higher than on Earth. The estimated
equilibrium temperature The planetary equilibrium temperature is a theoretical temperature that a planet would be if it were a black body being heated only by its parent star. In this model, the presence or absence of an atmosphere (and therefore any greenhouse effect) is ...
for Kepler-186f, which is the surface temperature without an atmosphere, is said to be around , somewhat colder than the equilibrium temperature of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
.


Host star

The planet orbits
Kepler-186 Kepler-186 is a main-sequence M1-type dwarf star, located 178.5 parsecs (582 light years) away in the constellation of Cygnus. The star is slightly cooler than the sun, with roughly half its metallicity. It is known to have five plane ...
, an M-type
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 ...
which has a total of five known planets. The star has a mass of 0.54 and a radius of 0.52 . It has a temperature of 3755 K and is about 4 billion years old, about 600 million years younger than 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 ...
, which is 4.6 billion years old and has a temperature of . The star's
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 ...
, or how bright it appears from Earth's perspective, is 14.62. This is too dim to be seen with the naked eye, which can only see objects with a
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
up to at least 6.5 – 7 or lower.


Orbit

Kepler-186f orbits its star with about 5% of the Sun's luminosity with an orbital period of 129.9 days and an orbital radius of about 0.40 times that of Earth's (compared to for
Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
). The habitable zone for this system is estimated conservatively to extend over distances receiving from 88% to 25% of Earth's illumination (from ). Kepler-186f receives about 32%, placing it within the conservative zone but near the outer edge, similar to the position of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
in our planetary system.


Habitability

Kepler-186f's location within the habitable zone does not ensure it is habitable; this is also dependent on its atmospheric characteristics, which are unknown. However, Kepler-186f is too distant for its atmosphere to be analyzed by existing telescopes (e.g.,
NESSI The New Mexico Exoplanet Spectroscopic Survey Instrument (NESSI) is a ground-based near-infrared spectrographic system specifically designed to study the atmospheres of exoplanets. The NESSI instrument was mounted in 2014 on a 2.4 meter telescope ...
) or next-generation instruments such as the
James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Spa ...
. A simple climate model – in which the planet's inventory of volatiles is restricted to nitrogen, carbon dioxide and water, and clouds are not accounted for – suggests that the planet's surface temperature would be above if at least 0.5 to 5 bars of CO2 is present in its atmosphere, for assumed N2 partial pressures ranging from 10 bar to zero, respectively. The star hosts four other planets discovered so far, although Kepler-186 b, c, d, and e (in order of increasing orbital radius), being too close to their star, are considered too hot to have liquid water. The four innermost planets are probably
tidally locked Tidal locking between a pair of co-orbiting astronomical body, 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 ...
, but Kepler-186f is in a higher orbit, where the star's tidal effects are much weaker, so the time could have been insufficient for its spin to slow down significantly. Because of the very slow evolution of red dwarfs, the age of the Kepler-186 system was poorly constrained, although it is likely to be greater than a few billion years. Recent results have placed the age at around 4 billion years. The chance that it is tidally locked is approximately 50%. Since it is closer to its star than Earth is to the Sun, it will probably rotate much more slowly than Earth; its day could be weeks or months long (see Tidal effects on rotation rate, axial tilt and orbit). Kepler-186f's
axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbi ...
(obliquity) is likely very small, in which case it would not have tilt-induced seasons like Earth's. Its orbit is probably close to circular, so it will also lack eccentricity-induced seasonal changes like those of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
. However, the axial tilt could be larger (about 23 degrees) if another undetected non-transiting planet orbits between it and Kepler-186e; planetary formation simulations have shown that the presence of at least one additional planet in this region is likely. If such a planet exists, it cannot be much more massive than Earth as it would then cause orbital instabilities. One review essay in 2015 concluded that Kepler-186f, along with the exoplanets
Kepler-442b Kepler-442b (also known by its Kepler object of interest designation ''KOI-4742.01'') is a confirmed near-Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the K-type main-sequence star Kepler-442, about from Earth in ...
and
Kepler-62f Kepler-62f (also known by its Kepler Object of Interest designation ''KOI-701.04'') is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's ''Ke ...
, were likely the best candidates for being potentially habitable planets. In June 2018, studies suggest that Kepler-186f may have seasons and a climate similar to those on Earth.


Follow-up studies


Target of SETI investigation

As part of the
SETI Institute The SETI Institute is a not-for-profit research organization incorporated in 1984 whose mission is to explore, understand, and explain the origin and nature of life in the universe, and to use this knowledge to inspire and guide present and futu ...
's
search for extraterrestrial intelligence The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other pl ...
, the
Allen Telescope Array The Allen Telescope Array (ATA), formerly known as the One Hectare Telescope (1hT), is a radio telescope array dedicated to astronomical observations and a simultaneous search for extraterrestrial intelligence (SETI). The array is situated at th ...
had listened for radio emissions from the Kepler-186 system for about a month as of 17 April 2014. No signals attributable to extraterrestrial technology were found in that interval; however, to be detectable, such transmissions, if radiated in all directions equally and thus not preferentially towards the Earth, would need to be at least 10 times as strong as those from
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
. Another search, undertaken at the
crowdsourcing Crowdsourcing involves a large group of dispersed participants contributing or producing goods or services—including ideas, votes, micro-tasks, and finances—for payment or as volunteers. Contemporary crowdsourcing often involves digita ...
project SETI-Live, reports inconclusive but optimistic-looking signs in the radio noise from the Allen Array observations. The more well known SETI @ Home search does not cover any object in the Kepler field of view.See the fourth question at Another follow-up survey using the
Green Bank Telescope The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Radio ...
has not reviewed Kepler 186f. Given the interstellar distance of , the signals would have left the planet many years ago.


Future technology

At approximately distant, Kepler-186f is too far and its star too faint for current telescopes or the next generation of planned telescopes to determine its mass or whether it has an atmosphere. However, the discovery of Kepler-186f demonstrates conclusively that there are other Earth-sized planets in habitable zones. The Kepler spacecraft focused on a single small region of the sky but next-generation planet-hunting space telescopes, such as
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) ...
and
CHEOPS Khufu or Cheops was an ancient Egyptian monarch who was the second pharaoh of the Fourth Dynasty, in the first half of the Old Kingdom period (26th century BC). Khufu succeeded his father Sneferu as king. He is generally accepted as having com ...
, will examine nearby stars throughout the sky. Nearby stars with planets can then be studied by the
James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Spa ...
and future large ground-based telescopes to analyze atmospheres, determine masses and infer compositions. Additionally the
Square Kilometer Array The Square Kilometre Array (SKA) is an intergovernmental international radio telescope project being built in Australia (low-frequency) and South Africa (mid-frequency). The combining infrastructure, the Square Kilometre Array Observatory (SKA ...
would significantly improve radio observations over the
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
and
Green Bank Telescope The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Radio ...
.


Previous names

As the
Kepler telescope 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 orbit ...
observational campaign proceeded, an initially identified system was entered in the
Kepler Input Catalog The Kepler Input Catalog (or KIC) is a publicly searchable database of roughly 13.2 million targets used for the Kepler Spectral Classification Program (SCP) and ''Kepler''. Overview The Kepler SCP targets were observed by the 2MASS project ...
(KIC), and then progressed as a candidate host of planets to a Kepler Object of Interest (KOI). Thus, Kepler 186 started as ''KIC 8120608'' and then was identified as ''KOI 571''. Kepler 186f was mentioned when known as KOI-571-05 or KOI-571.05 or using similar nomenclatures in 2013 in various discussions and publications before its full confirmation. See comment by "Holger 16 November 2013 at 14:21".
^
^
^


Comparison

The nearest-to-Earth-size planet in a habitable zone previously known was
Kepler-62f Kepler-62f (also known by its Kepler Object of Interest designation ''KOI-701.04'') is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's ''Ke ...
with 1.4 Earth radii. Kepler-186f orbits an M-dwarf star, while Kepler-62f orbits a K-type star. A study of atmospheric evolution in Earth-size planets in habitable zones of G-Stars (a class containing the Sun, but not Kepler-186) suggested that 0.8–1.15 R🜨 is the size range for planets small enough to lose their initial accreted hydrogen envelope but large enough to retain an outgassed secondary atmosphere such as Earth's.


In popular culture

* Along with five other exoplanets, Kepler-186f was included in '' Civilization: Beyond Earth''s exoplanet DLC as a playable map. * Dutch rock band named their 2017 album ''Kepler-186f'' after this exoplanet.; *Kepler-186f is the location of a future earth colony in the short story "Stars" by
Drew Hayden Taylor Drew Hayden Taylor (born 1 July 1962) is a Canadian playwright, author and journalist. Life and career Born in Curve Lake, Ontario, Taylor is part Ojibwe and part Caucasian. About his background Taylor says: "I plan to start my own nation. Bec ...
.


See also

*
Habitability of red dwarf systems The habitability of red dwarf systems is presumed to be determined by a large number of factors from a variety of sources. Modern evidence indicates that planets in red dwarf systems are unlikely to be habitable, due to their low stellar flux, ...
*
List of potentially habitable exoplanets This is a list of potentially habitable exoplanets. The list is mostly based on estimates of habitability by the Habitable Exoplanets Catalog (HEC), and data from the NASA Exoplanet Archive. The HEC is maintained by the Planetary Habitability Labor ...
*
Lists of astronomical objects This is a list of lists, grouped by type of astronomical object. Solar System * List of Solar System objects * List of gravitationally rounded objects of the Solar System * List of Solar System objects most distant from the Sun * List of So ...


Notes


References


External links


NASA – Mission overview

NASA – Kepler Discoveries – Summary Table

NASA – Kepler-186f
at The NASA Exoplanet Archive.
NASA – Kepler-186f
at The Exoplanet Data Explorer.
NASA – Kepler-186f
at
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 ...
.
Habitable Exolanets Catalog
at UPR-Arecibo.
NASA – Kepler 186f – SETI Institute – A Planet in the Habitable Zone (video)
2014.
NASA – NASA Press kit
{{Sky, 19, 54, 36.651, +, 43, 57, 18.06, 492 186f Exoplanets discovered in 2014 Exoplanets in the habitable zone Kepler-186 Transiting exoplanets Near-Earth-sized exoplanets in the habitable zone Cygnus (constellation)