NASA Discovers First Earth-Size Planets Beyond Our Solar
System, But Not In Habitable Zone
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, 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, 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. 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 continue to reveal new insights about
the diversity of planets and planetary systems within our
galaxy."
Kepler Prepared For Launch
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, 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.
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." (Images provided by
NASA)