Earth-Size Planets Found Beyond Solar System Says NASA

Earth-size planets orbiting a sun-like star outside our solar system have been discovered by NASA's Kepler mission. According to the space agency, 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.

NASA says the discovery marks an 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.

A Hot Spot
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, is similar to an
average day on the planet Mercury. The surface temperature of
Kepler-20e, at more than 1,400 degrees Fahrenheit, 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, Massachusetts, 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. 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.

This solar system has an unexpected arrangement, says NASA. In Earth's 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, California.
"The analysis of Kepler data continue 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.

NASA says 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, 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
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, explained the space agency.

On 5 December 2011 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."

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