Astronomers have discovered the
first Earth-size planet orbiting a star in the "habitable zone" -- the range of
distance from a star where liquid water might pool on the surface of an orbiting
planet. The discovery of Kepler-186f confirms that planets the size of Earth
exist in the habitable zone of stars other than our sun.
While planets have previously been found in the habitable zone, they are all at
least 40 percent larger in size than Earth and understanding their makeup is
challenging. Kepler-186f is more reminiscent of Earth.
"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, research scientist at the SETI
Institute at Ames Research Center in Moffett Field, Calif., 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."Kepler-186f resides
in the Kepler-186 system, about 500 light-years from Earth in the constellation
Cygnus. The system is also home to four companion planets, which orbit a star
half the size and mass of our sun. The star is classified as an M dwarf, or red
dwarf, a class of stars that makes up 70 percent of the stars in the Milky Way
galaxy.
"M dwarfs are the most numerous stars," said Quintana. "The first signs of other
life in the galaxy may well come from planets orbiting an M dwarf."
Kepler-186f orbits its star once every 130-days and receives one-third the
energy from its star that Earth gets from the sun, placing it nearer the outer
edge of the habitable zone. On the surface of Kepler-186f, the brightness of its
star at high noon is only as bright as our sun appears to us about an hour
before sunset.
"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, and co-author of the paper.
"Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It
has many properties that resemble Earth."
The four companion planets, Kepler-186b, Kepler-186c, Kepler-186d, and
Kepler-186e, whiz around their sun every four, seven, 13, and 22 days,
respectively, making them too hot for life as we know it. These four inner
planets all measure less than 1.5 times the size of Earth.
The next steps in the search for distant life include looking for true
Earth-twins -- Earth-size planets orbiting within the habitable zone of a
sun-like star -- and measuring the their chemical compositions. 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.
Ames is responsible for Kepler's ground system development, mission operations,
and science data analysis. NASA's Jet Propulsion Laboratory in Pasadena, Calif.,
managed Kepler mission development. Ball Aerospace & Technologies Corp. in
Boulder, Colo., developed the Kepler flight system and supports mission
operations with the Laboratory for Atmospheric and Space Physics at the
University of Colorado in Boulder. The Space Telescope Science Institute in
Baltimore archives, hosts and distributes Kepler science data. Kepler is NASA's
10th Discovery Mission and was funded by the agency's Science Mission
Directorate.
The SETI Institute is a private, nonprofit organization dedicated to scientific
research, education and public outreach. The mission of the SETI Institute is to
explore, understand and explain the origin, nature and prevalence of life in the
universe.