Alien worlds, just like home

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Harvard astronomers, working as part of NASA’s Kepler mission, have detected the first Earth-sized planets orbiting a distant star, a milestone in the hunt for alien worlds that brings scientists a step closer to their ultimate goal of finding a twin to Earth.

Described in the Dec. 20 issue of the journal Nature, the two planets, dubbed Kepler-20e and Kepler-20f, are the smallest planets found by the Kepler mission to date. They have diameters of 6,900 miles and 8,200 miles — equivalent to 0.87 times Earth (slightly smaller than Venus) and 1.03 times Earth. These worlds are expected to have rocky compositions, so their masses should be, respectively, approximately 1.7 and 3 times Earth’s.

“The goal of Kepler is to find Earth-sized planets in the habitable zone. Proving the existence of Earth-sized exoplanets is a major step toward achieving that goal,” said Francois Fressin, a research associate at the Harvard-Smithsonian Center for Astrophysics (CfA).

Both worlds circle Kepler-20, a star located 950 light-years from Earth. Though that star is slightly cooler than the Sun, the planets’ tight orbits — Kepler-20e orbits every 6.1 days at a distance of 4.7 million miles, while Kepler-20f orbits every 19.6 days at a distance of 10.3 million miles — they are heated to temperatures of 1,400 and 800 degrees Fahrenheit, respectively.

In addition to the two newfound worlds, the Kepler-20 system contains three previously identified planets. All five have orbits closer than Mercury in our solar system, and show an unexpected arrangement. In our solar system, small, rocky worlds orbit close to the Sun, while large, gas, giant ones orbit farther out. In contrast, the planets of Kepler-20 are organized in alternating size: big, little, big, little, big.

“We were surprised to find this system of flip-flopping planets,” said David Charbonneau, professor of astronomy and a co-author of the Nature paper. “It’s very different than our solar system.”

The three largest planets are designated Kepler-20b, Kepler-20c, and Kepler-20d. They have diameters of 15,000, 24,600, and 22,000 miles, and orbit once every 3.7, 10.9, and 77.6 days, respectively.

In the hunt for planets, Kepler identifies “objects of interest” by looking for stars that dim slightly, which can occur when a planet crosses the star’s face. Such dimming, however, could be caused by a variety of phenomena. To determine whether an object truly is a planet, astronomers use Blender, a software package developed by Fressin and Willie Torres of CfA. After running millions of possible simulations, the team concluded that the odds are strongly in favor of Kepler-20e and Kepler-20f being planets.

Fressin and Torres also used Blender to confirm the existence of Kepler-22b, a planet in the habitable zone of its star that was announced by NASA earlier this month. However, that world was much larger than Earth.

“These new planets are significantly smaller than anything found up till now orbiting a sunlike star,” added Fressin.

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