Artist Danielle Futselaar's depiction
Astronomers have discovered the first Earth-sized planet that meets just the right qualities to potentially harbor life. With the help of the Kepler Space Telescope, they identified Kepler-186f, a planet roughly the size of Earth orbiting a star in a “habitable zone” ─ a range of distance meeting the conditions needed for liquid water to pool on its surface.
Because Earth is the only planet with known life, scientists focus on finding planets with characteristics that mimic those of Earth. Prior to the discovery of the Kepler-186f, all other planets found in habitable orbits were typically 40 percent smaller than the size of Earth; Kepler-186f is the first planet to simultaneously mirror the size of Earth and orbit a star at a distance capable of supporting life. “The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth,” said Paul Hertz, director of NASA's Astrophysics Division.
While the Kepler Space Telescope could successfully define Kepler-186f’s size, it cannot determine its mass and composition. Hertz adds: “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.”
How was it found?
Steve Howell ─ Kepler's Project Scientist and a co-author on the paper ─ explains that neither Kepler (nor any telescope) is able to directly spot an exoplanet of this size and proximity to its host star. “However, what we can do is eliminate essentially all other possibilities so that the validity of these planets is really the only viable option.” To eliminate the possibility of error caused by a background star or a stellar companion mimicking what Kepler detected, the astronomers used a combination of the speckle imaging technique from the 8-meter Geini North telescope on Mauna Kea in Hawaii, with adaptive optics (AO) observations from the 10-meter Keck II telescope to obtain extremely high spatial resolution observations.
Kepler-186f’s characteristics
Kepler-186f resides 500 light years away from Earth in the stellar constellation Cygnus, of the Kepler-186 system. It is one of five planets orbiting an M dwarf, a type of star half the size and mass of our sun that makes up 70% of the stars in the Milky Way galaxy. “M dwarfs are the most numerous stars,” says Elisa Quintana of NASA Ames Research Center, lead study author, “The first signs of other life in the galaxy may well come from planets orbiting an M dwarf.”
However, being in a habitable zone doesn’t automatically imply the planet is habitable. “The temperature on the planet is strongly dependent on what kind of atmosphere the planet has,” says Thomas Barclay, research scientist at the Bay Area Environmental Research Institute at Ames, and coauthor of the paper.” Barclay doesn’t necessarily want us to get our hopes, stating that Kepler-186f may be thought of as a cousin-planet to Earth, rather than a sibling.
Kepler-186f orbits its star once every 130 days and receives one-third the energy from its star that Earth gets from the sun, indicating that its brightness at high noon is only as bright as our sun appears an hour before sunset.
Overall, the discovery of habitable Earth-sized planets is extremely difficult to detect and confirm, says Elisa Quintana, “[but] now that we've found one, we want to search for more.” The next step in exploration involves looking for planets mirroring Earth’s atmospheric conditions, orbiting sunlike stars at similar distances. The Kepler Space Telescope’s ability to simultaneously measure brightness of more than 150,000 stars is key.
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