One of the key objectives in the search for planets orbiting other stars has been to locate planets in the habitable zones of such stars, i.e. planets on which liquid water could potentially exist. A number of planets with habitable zone orbits have been described in recent years, though the majority have been large, gas giant type planets, unlikely to be capable of supporting life. The few potentially rocky worlds discovered, such as Kepler 62f or Gliese 581d, have also been considerably larger than the Earth.
In a paper published in the journal Science on 18 April 2014, a team of Scientists led by Elisa Quintana of the SETI Institute and NASA’s Ames Research Center, describe the discovery of an Earth-sized planet in the habitable zone of another star, based upon observations by the Kepler Space Telescope, and additional follow up observations by the ground-based Keck-II and Gemini-North telescopes.
The star, named Kepler 186 (i.e. the 186th star in the Kepler survey discovered to have planets) is an M-type Red Dwarf star (151 parsecs – translate to light years) from the Earth, with a solar radius 47% of that of the Sun and a mass 0.478 times that of the Sun. This star is calculated to have an effective surface temperature of 3788 K, compared to 5778 K for the Sun, and an iron abundance roughly half the Sun’s. Such stars are extremely long lived and slow evolving, so their habitable zones are likely to remain stable for billions of years. In the case of the Kepler 186 system this habitable zone is estimated to be between 0.22 and 0.40 AU from the star (i.e. between 22% and 40% of the average distance at which the Earth orbits the Sun).
The Kepler 186 system was found to have at least five planets, detected as they passed in front of the star rather than by any tidal wobble they created on the stars movement (a common way of detecting large exoplanets). The presence of additional planets which did not pass in front of the star during the period of the survey, either because their orbital period was too long or because their orbit was inclined to the rest of the system, could not be ruled out. All of these planets are thought to be small rocky bodies, rather than large gas giant type planets, none of them having a radius larger than 1.5 times that of the Earth.
Four of these planets orbit considerably inside the system’s habitable zone (i.e. they are too close to the star for liquid water to exist), at between 0.0343 and 0.110 AU from the star (between 3.43% and 11.0% of the average distance between the Earth and the Sun). However the fifth planet, named Kepler 186f, orbits at a distance of 0.356 AU from the star, comfortably within the system’s habitable zone.
A schematic diagram of the Kepler- 186 system. A top-down view of the system during a transit of planet f. The relative planet sizes are correct but are not on the same scale as the orbits (shown as black curves). Quintana et al. (2014).
Kepler 186f has a radius of 1.11 times that of the Earth, and takes 130 days to orbit its parent star. Its mass is thought to be between 0.32 times that of the Earth (for a body comprised of pure ice) and 3.77 times that of the Earth (for a body comprised of pure iron); for a body with a similar composition to the Earth this would give a mass 1.44 times that of the Earth.
Being within a star’s habitable zone does not necessarily mean a planet is habitable. Mars is considered to lie within the habitable zone of our Solar System, however its small size means that it cannot retain a dense atmosphere, and therefore water cannot form a liquid on its surface (it is thought that if Mars were Earth-sized then it would have liquid water, and possibly life, on its surface).
Kepler 186f is clearly large enough to retain liquid water at its surface, but the ability of a planet in a Red Dwarf system to acquire a large volume of water is unclear. The Earth is thought to have formed to close to the Sun for much water to have accumulated during its formation; current models suggest that it acquired the water that forms its oceans by bombardment by icy comets early in its history. These comets formed further out in the protoplanetary disk from which the planets formed, beyond the ‘snow line’, where the system was cool enough for water ice to form. The planets of the Kepler 186 system are all in a common orbital plain, suggesting that they formed from a protoplanetary disk similar to that hypothesized for the Solar System, but it is not clear if a young Red Dwarf system would form a large body of comets beyond its snow line, with which to bombard a young, Earth-like planet.
A side-on view comparing Kepler-186 with the solar system (with Earth and Mars in the habitable zone) and the Gliese 581 planets. The stars are located at the left edge of the plot. The dark grey regions represent conservative estimates of the habitable zone while the lighter grey regions are more optimistic extensions of the habitable region around each star. Quintana et al. (2014).
The remaining planets of the Kepler 186 system are all thought to be too hot for liquid water to exist.
Kepler 186b is thought to orbit at a distance of 0.0343 AU, and to complete one orbit every 3.89 days. This planet has a radius 1.07 times that of the Earth, and is calculated to have a mass of between 0.90 times that of the Earth, for a pure silicate body, and 3.23 times that of the Earth, for a pure iron body.
Kepler 186c is thought to orbit at a distance of 0.0520 AU, and to complete one orbit every 2.27 days. This planet has a radius 1.25 times that of the Earth, and is calculated to have a mass of between 1.56 times that of the Earth, for a pure silicate body, and 6.30 times that of the Earth, for a pure iron body.
Kepler 186d is thought to orbit at a distance of 0.0781 AU, and to complete one orbit every 13.3 days. This planet has a radius 1.40 times that of the Earth, and is calculated to have a mass of between 2.38 times that of the Earth, for a pure silicate body, and 10.0 times that of the Earth, for a pure iron body.
Kepler 186e is thought to orbit at a distance of 0.110 AU, and to complete one orbit every 22.4 days. This planet has a radius 1.27 times that of the Earth, and is calculated to have a mass of between 1.66 times that of the Earth, for a pure silicate body, and 6.76 times that of the Earth, for a pure iron body.
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