NASA's Kepler mission has
discovered the first Earth-size planets orbiting a sun-like star
outside our solar system, the U.S. space agency announce Tuesday.
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, according to
NASA.
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, 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.
