Kepler-186f

Template:Planetbox begin Template:Planetbox image Template:Planetbox star Template:Planetbox character Template:Planetbox orbit Template:Planetbox discovery Template:Planetbox catalog Template:Planetbox end Kepler-186f is an exoplanet orbiting the red dwarf Kepler-186, about 500 light-years from the Earth. It is the first planet with a radius similar to Earth's to be discovered in the habitable zone of another star. NASA's Kepler spacecraft detected it using the transit method, 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 and some details were reported in the media at the time. The full public announcement was on 17 April 2014, followed by publication in Science.

Orbital parameters relative to habitable zone

Comparison of the Kepler-186 system and the Solar System (17 April 2014)

Kepler-186f orbits a star with about 4% of the Sun's luminosity with an orbital period of 129.9 days and an orbital radius of about 0.36 or 0.40 times that of Earth's (compared to 0.39 AU for Mercury). The habitable zone for this system is estimated conservatively to extend over distances receiving from 88% to 25% of Earth's illumination (from 0.22 to 0.40 AU). Kepler-186f receives about 32%, placing it within the conservative zone but near the outer edge, similar to the position of Mars in our Solar System. The stellar flux received by Kepler-186f is similar to that of Gliese 581d.

Mass, density and composition

The only physical property directly derivable from the observations (besides the orbital period) is the ratio of the radius of the planet to that of the central star, which follows from the amount of occultation of stellar light during a transit. This ratio was measured to be 0.021. This yields a planetary radius of 1.11±0.14 times that of Earth, taking into account uncertainty in the star's diameter and the degree of occultation. Thus, 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).

There is a very wide range of possible masses that can be calculated by combining the radius with densities derived from possible planetary compositions; it could be a rocky terrestrial planet or a lower density ocean planet with a thick atmosphere. However, a massive hydrogen/helium (H/He) atmosphere is thought to be unlikely in a planet with a radius below 1.5 R_⊕. Planets with a radius of more than 1.5 times that of Earth tend to accumulate the thick atmospheres that would make them less likely to be habitable. Red dwarfs emit a much stronger extreme ultraviolet (XUV) flux when young than later in life; the planet's primordial atmosphere would have been subjected to elevated photoevaporation during that period, which would probably have largely removed any H/He-rich envelope through hydrodynamic mass loss. Mass estimates range from 0.32 M_⊕ for a pure water/ice composition to 3.77 M_⊕ if made up entirely of iron (both implausible extremes). For a body with radius 1.11 R_⊕, a composition similar to Earth’s (1/3 iron, 2/3 silicate rock) yields a mass of 1.44 M_⊕, taking into account the higher density due to the higher average pressure compared to Earth.

Formation, tidal evolution and habitability

The star hosts four other planets discovered so far, though Kepler-186 b, c, d, and e (in order of increasing orbital radius) are too close to the star, and so too hot, to have liquid water. The four innermost planets are probably tidally locked but Kepler-186f is further out, where the star's tidal effects are much weaker, so there may not have been enough time for its spin to slow down that much. Because of the very slow evolution of red dwarfs, the age of the Kepler-186 system is poorly constrained, although it is likely to be greater than a few billion years. There is a roughly 50-50 chance it is tidally locked. 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).

Artist's concept of a rocky Earth-sized exoplanet in the habitable zone of its host star, possibly compatible with Kepler-186f’s known data (NASA/SETI/JPL)

Kepler-186f's axial tilt (obliquity) is likely very small, in which case it wouldn't have tilt-induced seasons as Earth and Mars do. Its orbit is probably close to circular, so it will also lack eccentricity-induced seasonal changes like those of Mars. However, the axial tilt could be larger (about 23 degrees) if another undetected nontransiting 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.

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. Kepler-186f is too distant, however, for its atmosphere to be analyzed by existing telescopes (e.g., NESSI) or next-generation instruments such as the James Webb Space Telescope. 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 273 K (0 deg C) 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.

Target of SETI investigation

As part of the SETI Institute's search for extraterrestrial intelligence, the Allen Telescope Array 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, however, such transmissions, if radiated isotropically, would need to be at least 10 times as strong as those from Arecibo Observatory. Another search, undertaken at the crowdsourcing project SETILive, reports inconclusive but optimistic-looking signs in the radio noise from the Allen Array observations. Given the interstellar distance of 492 light-years the signals would have left the planet that many years ago.

Follow-up studies

At nearly 500 light years distant, Kepler-186f is too remote 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 and CHEOPS, will examine nearby stars throughout the sky. Nearby stars with planets can then be studied by the upcoming James Webb Space Telescope and future large ground-based telescopes to analyze atmospheres, determine masses and infer compositions.

Comparison with previously discovered low-mass planets in the habitable zone

The nearest-to-Earth-size planet in a habitable zone previously known was Kepler-62f with 1.4 Earth radii; however, such a planet is possibly very different from Earth, as the volume of a planet grows with the cube of the radius and, assuming the same composition, the mass grows even more. The volume of a 1.1 R_⊕ planets is about 1.3 Earth-volume; a 1.4 R_⊕ planet has nearly three times Earth's volume, which due to compression could fit proportionally even more mass.

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.

Candidate Earth-size planets in the habitable zone of Sun-like stars

Kepler-186f orbits a red dwarf but there are some Kepler candidate planets yet to be confirmed that could be Earth-size and orbiting in the habitable zones of Sun-like stars.

Table comparing Kepler-186f (KIC 8120608 = KOI-571.05) to candidate Earth-size HZ planets:

Kepler Input Catalog	Stellar Luminosity (LSun)	Stellar radius (RSun)	Metallicity (dex)	Kepler Object of Interest	Planet radius (REarth)	Planet semi-major axis (AU)	Orbital period (days)	Insolation (% of Earth's)
KIC 5091808	0.69	0.83	−0.82	KOI 5123.01	1.09	0.77	289	116%
KIC 11654039	1.09	0.97	−0.14	KOI 5927.01	1.24	1.13	436	85%
KIC 10663976	0.44	0.73	−0.46	KOI 5819.01	1.29	0.95	381	49%
KIC 11465869	0.53	0.74	−0.28	KOI 5904.01	0.77	0.86	323	72%
KIC 8570210	0.67	0.80	−0.42	KOI 5545.01	1.05	1.25	541	43%
KIC 8120608	0.04	0.47	−0.28	KOI-571.05	1.11	0.36	130	31%

Insolation is calculated by taking the luminosity and dividing it by the semimajor axis squared.

Previous names

As the Kepler telescope observational campaign proceeded, an initially identified system was entered in the Kepler Input Catalog (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 also

• Habitability of red dwarf systems

External links

• NASA – Kepler Discoveries – Summary Table.
• NASA – Kepler Mission.
• NASA – NASA Press kit
• SETI Institute – Google+Hangout – Kepler 186f – A Planet in the Habitable Zone (video) 17 April 2014
• Kepler-186f at The NASA Exoplanet Archive
• Kepler-186f at The Extrasolar Planets Encyclopaedia