December 20, 2011
This artist's animation flies through the Kepler-20 star system, where NASA's Kepler mission discovered the first Earth-size planets around a star beyond our own. The system is jam-packed with five planets, all circling within a distance roughly equivalent to Mercury's orbit in our solar system. credit: NASA

NASA announced on Tuesday that its Kepler mission has discovered the first Earth-size planets orbiting a star like our sun outside our solar system.

Planets Kepler-20e and Kepler-20f are too close to their star to be in the habitable zone where liquid water could exist, but NASA said they are the smallest exoplanets ever confirmed around a star like our sun.

The space agency said that the discovery marks the next important milestone in the search for Earth-like planets.

Kepler-20e is thought to be smaller than Venus, measuring 0.87 times the radius of Earth, while Kepler-20f measures 1.03 times the radius of Earth.

Both the newly discovered planets reside in a five-planet system known as Kepler-20, which is about 1,000 light-years away from Earth in the constellation Lyra.

Kepler-20e orbits its parent star every 61 days and Kepler-20f every 19.6 days. This short orbit means the planets are very hot and inhospitable.

Kepler-20f has a surface temperature of 800 degrees Fahrenheit, which is similar to the planet Mercury.

The surface temperature of Kepler-20e is over 1,400 degrees Fahrenheit, which is hot enough to melt glass.

"The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a new study published in the journal Nature, said in a press release. "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 is the closest planet to its star, followed by Kepler-20c and Kepler-20d. 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, but is slightly smaller and cooler.

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

Scientists theorize that the planets formed farther from their star and then migrated inward.

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 their stars.

The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planet candidates the spacecraft finds.

Another Earth-like planet known as Kepler-22b was announced on December 5. This planet is believed to be too large to have a rocky surface, but it lies in the habitable zone of its parent star.

"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," Natalie Batalha, Kepler deputy science team lead and professor of astronomy and physics at San Jose State University, said in a press release. "We are on the edge of our seats knowing that Kepler's most anticipated discoveries are still to come."

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