Kepler-186f is the First Earth-Sized Exoplanet Found in Habitable Zone

April 17, 2014

The best chance for finding life outside our solar system is on extrasolar planets that resemble Earth in both size and distance from its host star. By using data from Kepler Space Telescope, astronomers have discovered the first exoplanet that meets both requirements – Kepler-186f.

Comparison between Earth and Kepler-186f in size and position in their systems.

Comparison between Earth and Kepler-186f in size and position in their systems. Credit: NASA Ames/SETI Institute/JPL-Caltech

At the moment, detecting extrasolar planets similar in size and mass to Earth is more difficult than detecting gas giants like Jupiter, but it can be done. Detecting these planets in a habitable zone – a zone around its host star where water can stay in its liquid form – has been even more difficult. After over 1700 detected exoplanets, astronomers have finally found the first one to be both Earth-like and at the right distance from its star.

"The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth," said Paul Hertz, NASA's Astrophysics Division director. "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."

Guessing that the Earth is not one of its kind was simple, but actually finding a planet identical in size has proven to be more difficult. Previously, there were findings of rocky planets in the habitable zone, but they were at least 40 percent larger than Earth. Although its not impossible for larger planets to support life, based on what we know, we have better chance of finding life on exoplanets similar to Earth in every way.

"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. She is research scientist at the SETI Institute at NASA's Ames Research Center in Moffett Field, USA, 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."

The planet Kepler-186f orbits a red dwarf star, Kepler-186, located 500 light-years from Earth in constellation Cygnus. The system has four other known planets, all closer to the star than Kepler-186f and all less than 1.5 times the size of Earth. Their proximity to the star makes them inhabitable, but Kepler-186f could be suitable for life, depending on its atmosphere.

"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 Research Center. He is the co-author of the paper.

Kepler-186f isn't exactly a true Earth twin, since its host star is only half the size of the Sun and thus less brighter, but it is the next best thing. The planet, which orbits the red dwarf every 130 days, receives only a third of the energy that Earth absorbs from the Sun.

"Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth," added Barclay.

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