In a April 17 news release NASA has announced that astronomers using the Kepler Space Telescope have discovered the first Earth-size planet orbiting a star in the circumstellar habitable zone, or the range of distance from a star where liquid water might pool on the surface of an orbiting planet, aka the Goldilocks Zone.
Though planets have previously been found in the habitable zone, they have all been at least 40% larger in size than Earth. The discovery of planet Kepler-186f confirms that planets the size of Earth exist in the habitable zone of stars other than our Sun. Although the size of Kepler-186f is known, its mass and composition are not, however previous research suggests that a planet the size of Kepler-186f is likely to be rocky.
Kepler-186f resides in the Kepler-186 system, located in the constellation Cygnus, about 500 light-years from Earth. The system is also home to four known inner planets, all orbiting a M dwarf, or red dwarf, star which is half the size and mass of our Sun. Making up to, by some estimates, 70% of the stars in the Milky Way, M dwarfs are the most common type of star in our galaxy – or at least in our neighborhood.
Kepler-186f orbits its star once every 130 days and receives about 1/3 of the energy from its star that Earth gets from the Sun, placing it nearer the outer edge of the habitable zone. From the surface of Kepler-186f, the brightness of its star at high noon is as bright as our Sun appears to us about an hour before sunset. The inner four companion planets each measure less than fifty percent the size of Earth. Kepler-186b, Kepler-186c, Kepler-186d and Kepler-186, orbit every 4, 7, 13 and 22 days, respectively, making them very hot and inhospitable for life as we know it.
“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 in the NASA news release. Quintana is a research scientist at the SETI Institute at NASA’s Ames Research Center and lead author of the paper published today in the journal Science. “Finding a habitable zone planet comparable to Earth in size is a major step forward.”
Animation of Kepler186 published on The SETI Institute’s YouTube channel.