Kepler-186f: First potentially habitable Earth-sized planet confirmed - water

The artist's concept depicts Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone, a range of distances from a star where liquid water might pool on the surface of an orbiting planet.

The discovery of Kepler-186f confirms that Earth-size planets exist in the habitable zone of other stars and signals a significant step closer to finding a world similar to Earth.

The artistic concept of Kepler-186f is the result of scientists and artists collaborating to help imagine the appearance of these distant worlds. 

Credit: Danielle Futselaar.

The first Earth-sized exoplanet orbiting within the habitable zone of another star has been confirmed by observations with both the W. M. Keck Observatory and the Gemini Observatory.

The initial discovery, made by NASA's Kepler Space Telescope, is one of a handful of smaller planets found by Kepler and verified using large ground-based telescopes.

It also confirms that Earth-sized planets do exist in the habitable zone of other stars.

"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," says Elisa Quintana of the SETI Institute and NASA Ames Research Center who led the paper published in the current issue of the journal Science.

The region in which this planet orbits its star is called the habitable zone, as it is thought that life would most likely form on planets with liquid water.

Steve Howell, Kepler's Project Scientist and a co-author on the paper, adds that neither Kepler (nor any telescope) is currently 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."

With such a small host star, the team employed a technique that eliminated the possibility that either a background star or a stellar companion could be mimicking what Kepler detected.

To do this, the team obtained extremely high spatial resolution observations from the eight-meter Gemini North telescope on Mauna Kea in Hawai`i using a technique called speckle imaging, as well as adaptive optics (AO) observations from the ten-meter Keck II telescope, Gemini's neighbour on Mauna Kea.

Together, these data allowed the team to rule out sources close enough to the star's line-of-sight to confound the Kepler evidence, and conclude that Kepler's detected signal has to be from a small planet transiting its host star.

The diagram compares the planets of the inner solar system to Kepler-186, a five-planet system about 500 light-years from Earth in the constellation Cygnus. 

The five planets of Kepler-186 orbit a star classified as a M1 dwarf, measuring half the size and mass of the sun. 

The Kepler-186 system is home to Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone—a range of distances from a star where liquid water might pool on the surface of an orbiting planet. 

The discovery of Kepler-186f confirms that Earth-size planets exist in the habitable zone of other stars and signals a significant step closer to finding a world similar to Earth. 

Kepler-186f is less than ten percent larger than Earth in size, but its mass and composition are not known. 

Kepler-186f orbits its star once every 130-days and receives one-third the heat energy that Earth does from the sun, placing it near the outer edge of the habitable zone. 

The inner four companion planets all measure less than fifty percent the size of Earth. Kepler-186b, Kepler-186c, Kepler-186d, and Kepler-186e, orbit every three, seven, 13, and 22 days, respectively, making them very hot and inhospitable for life as we know it. 

The Kepler space telescope, which simultaneously and continuously measured the brightness of more than 150,000 stars, is NASA's first mission capable of detecting Earth-size planets around stars like our sun. 

Kepler does not directly image the planets it detects. The space telescope infers their existence by the amount of starlight blocked when the orbiting planet passes in front of a distant star from the vantage point of the observer. 

The artistic concept of Kepler-186f is the result of scientists and artists collaborating to help imagine the appearance of these distant Credit: Credit: NASA Ames/SETI Institute/JPL-CalTech.

More information: "An Earth-Sized Planet in the Habitable Zone of a Cool Star," by E.V. Quintana et al. Science, 2014.