On April 17, astronomers announced the discovery of the first Earth-sized planet orbiting in the habitable zone of another star.
While planets have previously been found in the habitable zones of other stars, they are all at least 40 percent larger in diameter than Earth, raising questions of whether they are more like terrestrial planets or more like Uranus and Neptune.
The planet is known as Kepler-186f, being the fifth planet discovered in orbit of Kepler-186, a star that is 151 parsecs away from Earth. Some planets in the Kepler-186 system were part of a 715-planet mother-lode revealed in February.
“The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth,” said Paul Hertz, NASA's Astrophysics Division director at the agency's headquarters in Washington. “Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind's quest to find truly Earth-like worlds.”
Kepler-186 is a star in the constellation Cygnus that has 0.48 times the mass of the Sun and 0.47 times the radius of the Sun. Its mean surface temperature is estimated to be 3,788 K, while the mean surface temperature of the Sun is 5,778 K. The spectral classification of Kepler-186 is M0V, which is significantly dimmer and redder than the G2V spectral classification of the Sun. Such M-class dwarfs make up 70 percent of the stars in the Milky Way galaxy
The five planets that have been discovered have been given the names Kepler-186b, Kepler-186c, Kepler-186d, Kepler-186e, and Kepler-186f.
Kepler-186b has an estimated radius of 1.070 Earth radii. It is 0.0343 AU away from its star, and completes an orbit of Kepler-186 once every 3.8867907 Earth days. For comparison, Mercury is 0.387 AU away from the Sun on average, and orbits the Sun once every 87.969 Earth days. The black-body temperature of Kepler-186b is 643 K or 370°C, which is close to the surface temperature on Venus. Complex life as we know it would be impossible on this planet, as liquid water can only exist at temperatures below 647 K, and the actual temperature of any planet will be warmer than its black-body temperature due to albedo, atmosphere, and internal heating effects.
Kepler-186c has an estimated radius of 1.250 Earth radii. It is 0.0451 AU away from its star, and completes an orbit of Kepler-186 once every 7.267302 Earth days. The black-body temperature of Kepler-186c is 525 K or 252°C, which is consistent with an oven set to broil. Extreme pressure from gravity and the planet's atmosphere may make liquid water possible despite the high temperature, which would boil liquid water at atmospheric pressure on Earth.
Kepler-186d has an estimated radius of 1.400 Earth radii. It is 0.0781 AU away from its star, and completes an orbit of Kepler-186 once every 13.342996 Earth days. The black-body temperature of Kepler-186d is 429 K or 156°C, which is consistent with moderate heat inside an oven.
Kepler-186e has an estimated radius of 1.270 Earth radii. It is 0.11 AU away from its star, and completes an orbit of Kepler-186 once every 22.407704 Earth days. The black-body temperature of Kepler-186e is 361 K or 88°C, which leaves a slight possibility of liquid water on the surface, especially near the poles.
Finally, there is the specimen of interest. Kepler-186f has an estimated radius of 1.110 Earth radii. It is 0.356 AU away from its star, and completes an orbit of Kepler-186 once every 129.9459 Earth days. The black-body temperature of Kepler-186f is 180 K or -93°C, which is colder than Earth's black-body temperature of 254.3 K and Mars' black-body temperature of 210 K, but warmer than Jupiter's black-body temperature of 110 K.
There are several methods that can be used to detect extrasolar planets. The five planets detected around Kepler-186 were observed by the transit method. With this method, the observed visual brightness of the star drops a small amount when the planet passes between Earth and the planet's star. This method has two major disadvantages: planetary transits are only observable for planets whose orbits happen to be perfectly aligned from the astronomers' vantage point, and the method suffers from a high rate of false detections. A transit detection requires additional confirmation, typically from the radial velocity method. The main advantage of the transit method is that the size of the planet can be determined from the lightcurve. When combined with the radial velocity method (which determines the planet's mass) one can determine the density of the planet, and hence learn something about the planet's physical structure. As the radial velocity method has not been applied to the Kepler-186 planets, mass and composition information for them is not yet available.
“We know of just one planet where life exists -- Earth. When we search for life outside our solar system we focus on finding planets with characteristics that mimic that of Earth,” said Elisa Quintana, research scientist at the SETI Institute at NASA's Ames Research Center in Moffett Field, Calif., and lead author of the paper published in the journal Science. “Finding a habitable zone planet comparable to Earth in size is a major step forward.”
“Being in the habitable zone does not mean we know this planet is habitable. The temperature on the planet is strongly dependent on what kind of atmosphere the planet has,” said Thomas Barclay, research scientist at the Bay Area Environmental Research Institute at Ames, and co-author of the paper. “Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth.”