NASA Discovers First Earth-Size Planets Beyond Our Solar System

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Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
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Trent Perrotto 202-358-0321
NASA Headquarters, Washington
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Michele Johnson 650-604-6982
Ames Research Center, Moffett Field, Calif.
michele.johnson@nasa.gov

News release: 2011-390 December 20, 2011

NASA Discovers First Earth-Size Planets Beyond Our Solar System

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-390&cid=release_2011-390

PASADENA, Calif. -- NASA's Kepler mission has discovered the first Earth-size planets orbiting a
sun-like star outside our solar system. The planets, called Kepler-20e and Kepler-20f, are too close
to their star to be in the so-called habitable zone where liquid water could exist on a planet's surface,
but they are the smallest exoplanets ever confirmed around a star like our sun.

The discovery marks the next important milestone in the ultimate search for planets like Earth. The
new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87
times the radius of Earth. Kepler-20f is slightly larger than Earth, measuring 1.03 times its radius.
Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years away
in the constellation Lyra.

Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days. These short orbital
periods mean very hot, inhospitable worlds. Kepler-20f, at 800 degrees Fahrenheit (427 degrees
Celsius), is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e,
at more than 1,400 degrees Fahrenheit (760 degrees Celsius), would melt glass.

"The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," said
Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead
author of a new study published in the journal Nature. "This discovery demonstrates for the first
time that Earth-size planets exist around other stars, and that we are able to detect them."

The Kepler-20 system includes three other planets that are larger than Earth but smaller than
Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth
planet, orbit their star every 3.7, 10.9 and 77.6 days, respectively. All five planets have orbits lying
roughly within Mercury's orbit in our solar system. The host star belongs to the same G-type class as
our sun, although it is slightly smaller and cooler.

The system has an unexpected arrangement. In our solar system, small, rocky worlds orbit close to
the sun and large, gaseous worlds orbit farther out. In comparison, the planets of Kepler-20 are
organized in alternating size: large, small, large, small and large.

"The Kepler data are showing us some planetary systems have arrangements of planets very
different from that seen in our solar system," said Jack Lissauer, planetary scientist and Kepler
science team member at NASA's Ames Research Center in Moffett Field, Calif. "The analysis of
Kepler data continues to reveal new insights about the diversity of planets and planetary systems
within our galaxy."

Scientists are not certain how the system evolved, but they do not think the planets formed in their
existing locations. They theorize the planets formed farther from their star and then migrated inward,
likely through interactions with the disk of material from which they originated. This allowed the
worlds to maintain their regular spacing despite alternating sizes.

The Kepler space telescope detects planets and planet candidates by measuring dips in the brightness
of more than 150,000 stars to search for planets crossing in front of, or transiting, their stars. The
Kepler science team requires at least three transits to verify a signal as a planet.

The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review
observations on planet candidates the Kepler spacecraft finds. The star field Kepler observes in the
constellations Cygnus and Lyra can be seen only from ground-based observatories in spring through
early fall. The data from these other observations help determine which candidates can be validated
as planets.

To validate Kepler-20e and Kepler-20f, astronomers used a computer program called Blender, which
runs simulations to help rule out other astrophysical phenomena masquerading as a planet.

On Dec. 5, the team announced the discovery of Kepler-22b in the habitable zone of its parent star.
It is likely to be too large to have a rocky surface. While Kepler-20e and Kepler-20f are Earth-size,
they are too close to their parent star to have liquid water on the surface.

"In the cosmic game of hide and seek, finding planets with just the right size and just the right
temperature seems only a matter of time," said Natalie Batalha, Kepler deputy science team lead and
professor of astronomy and physics at San Jose State University. "We are on the edge of our seats
knowing that Kepler's most anticipated discoveries are still to come."

NASA's Ames Research Center in Moffett Field, Calif., manages Kepler's ground system
development, mission operations and science data analysis. JPL managed the Kepler mission's
development.

For more information about the Kepler mission and to view the digital press kit, visit:
http://www.nasa.gov/kepler .

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