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Earth-size Planet in Life Zone

18 April 2014

NASA's Kepler Space Telescope has confirmed that a planet the size of Earth exists in the habitable zone of another star than our sun. Kepler-186f orbits its star at a distance where liquid water might pool on its surface. It is less than 10 per cent larger than Earth and is the outermost planet detected at Kepler 186, a red dwarf 500 light-years from the sun and half its radius. There are probably more than 750 billion of these class M stars in the Galaxy, with masses between 10 and 50 per cent that of the sun, so Earth-like worlds probably circle stars closer to Earth, within reach of future telescopes able to detect signs of life.

Because M stars are small and radiate less strongly than the sun, their habitable zones are closer in. Their planets also appear relatively large, so their transits are easier to measure. And M stars evolve very slowly in luminosity, so their habitable zones are stable over billions of years. Class M stars have been shunned in the past as potential abodes of life, because the closeness to their star poses radiation hazards. The closeness also threatens to lock the planet's spin to its orbit,  causing one hemisphere to always face the heat of the star while the other faces the frigidity of deep space. More recently, greater optimism about the prospects for life on red dwarfs has been expressed.


This diagram compares the planets of the inner solar system with those of Kepler-186, 500 light-years from our sun. The five-planet system orbits an M dwarf, a star half the size and mass of the sun. Kepler-186f is the first validated Earth-size planet orbiting in the habitable zone (shown green) of a distant star  The size of Kepler-186f is known to be less ten percent larger than Earth, but its mass and composition are not known. The Kepler space telescope infers the existence of a planet by the amount of starlight blocked when it passes in front of its star. The artistic concept of Kepler-186f is the result of scientists and artists collaborating to imagine the appearance of this distant world. (Credit: NASA JPL)

Kepler-186f orbits its star once every 130 days. On its surface, its star's brightness at high noon is like that of our sun one hour before sunset. As a result, it receives one-third the energy from its star that Earth gets from the sun, placing it nearer the outer edge of the star's habitable zone. Close in, four companion planets orbit Kepler-186 every 22, 13, 7, and 4 days, making them too hot for life as we know it. The small planets do not perturb the star or each other to a significant extent, so the interpretation of the transits as indicating true planets has to be carried out statistically. The process is described in a paper published in Science, authored by Elisa Quintana and others of the Kepler team. The team obtained high-contrast images Kepler-186 from the Keck II and Gemini-North telescopes. No nearby sources that would mimic planets were detected, and the probability that the planets were orbiting an undetected companion star was very low.

The authors write, "The probability of finding an interlopiing star with the parameters needed to masquerade as a transiting planet is very small relative to the a priori probability of the planets orbiting Kepler-186 (<0.02%). Therefore we are confident that all five planets orbit Kepler-186." Planetary thermal evolution models indicate that Kepler-186f can range from 0.32 the mass of the Earth, if composed of pure water/ice, to 3.77 times the mass of the Earth, if composed of pure iron. An earth-like composition of one-third iron to two-thirds silicate rock would give a mass 1.44 times that of the Earth.

The authors note that it is difficult for an Earth-sized planet in the habitable zone of an M star to collect and retain water, but being at the outer perimeter of the habitable zone reduces the difficulty. And while the four inner planets of Kepler-186 are likely to be tidally locked, the distance of Kepler-186f from its star precludes determining its rotation rate. Even if it is, calculations show that a planet in a pseudo-synchronous rotation around its star is not precluded from having liquid water on its surface. On this basis, the team suggests that Kepler-186f is likely to retain liquid water.

NASA Ames is responsible for Kepler's ground system development, mission operations, and science data analysis. NASA's Jet Propulsion Laboratory in Pasadena, Calif., managed Kepler mission development. Ball Aerospace & Technologies Corp. in Boulder CO, developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.


An Earth-Sized Planet in the Habitable Zone of a Cool Star, by Elisa V. Quintana and others. Science, 344, 18 April 2014, p. 277.