Kepler-186 is a M1-type dwarf star located in the constellation Cygnus, about 500 light years away from Earth.
The star hosts at least five planets, four of which – Kepler-186b, Kepler-186c, Kepler-186d, and Kepler-186e – are in very short-period orbits and are very hot, while the fifth, named Kepler-186f, is in the center of the habitable zone.
Kepler-186f was discovered by the transit method, which detects potential planets as their orbits cross in front of their star and cause a very tiny but periodic dimming of the star’s brightness.
After the team was able to confirm that Kepler-186f was a planet, they used the transit information to calculate the planet’s size.
Kepler-186f is about 1.1 times the radius of the Earth and orbits the star once every 130-days. The astronomers estimate that the planet receives about 1/3 the heat energy that our planet does from the Sun.
“What makes this finding particularly compelling is that this Earth-sized planet, one of five orbiting this star, which is cooler than the Sun, resides in a temperate region where water could exist in liquid form,” said Dr Elisa Quintana of the SETI Institute and NASA Ames Research Center, the lead author of the paper published in the journal Science.
Co-author Dr Justin Crepp from the University of Notre Dame added: “the host star, Kepler-186, is an M1-type dwarf star which means it will burn hydrogen forever, so there is ample opportunity to develop life around this particular star and because it has just the right orbital period water may exist in a liquid phase on this planet.”
“Some people call these habitable planets, which of course we have no idea if they are. We simply know that they are in the habitable zone, and that is the best place to start looking for habitable planets,” said co-author Dr Stephen Kane of San Francisco State University.
Although Kepler-186f shows exciting signs of being Earth-like, Dr Kane said its differences are also fascinating.
“We’re always trying to look for Earth analogs, and that is an Earth-like planet in the habitable zone around a star very much the same as our Sun. This situation is a little bit different, because the star is quite different from our Sun.”
Kepler-186 is much smaller and cooler than the Sun. These stars are numerous in our Milky Way Galaxy, and have some features that make them promising places to look for life.
“For example, small stars live a lot longer than larger stars and so that means there is a much longer period of time for biological evolution and biochemical reactions on the surface to take place,” Dr Kane said.
On the other hand, small stars tend to be more active than stars the size of our Sun, sending out more solar flares and potentially more radiation toward a planet’s surface.
“The diversity of these exoplanets is one of the most exciting things about the field. We’re trying to understand how common our Solar System is, and the more diversity we see, the more it helps us to understand what the answer to that question really is,” Dr Kane concluded.
Source: Elisa V. Quintana et al. 2014. An Earth-Sized Planet in the Habitable Zone of a Cool Star. Science, vol. 344, no. 6181, pp. 277-280; doi: 10.1126/science.1249403
