UC Berkeley, NASA Astronomers Find Smallest Exoplanet to Date

Photo: UC Berkeley astronomers participated in the Kepler mission, which recently announced the discovery of a planet of similar size and composition as the Earth.
Geoff Marcy/Courtesy
UC Berkeley astronomers participated in the Kepler mission, which recently announced the discovery of a planet of similar size and composition as the Earth.

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Claire Perlman talks about Kepler-10b.

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Aristotle speculated about the existence of another Earth somewhere in the universe. Astronomers have not yet found such a planet, but they have discovered the first definitively rocky planet - the same composition as Earth - outside of this solar system.

The planet - dubbed Kepler-10b and first announced last week at the American Astronomical Society meeting in Seattle - orbits a star very similar to the sun. It is the smallest exoplanet - a planet outside of this solar system - found to date, with a diameter that is 40 percent larger than Earth's.

But unlike Earth or any other planet in this solar system, Kepler-10b orbits its star in about 20 hours, whereas the Earth orbits the sun in 365 days. Therefore, because the exoplanet is 30 times closer to its star than Earth is to the sun, it is entirely too hot to harbor life.

Astronomers from NASA, UC Berkeley and other institutions around the world launched the Kepler mission in March 2009 with the goal of finding an inhabitable planet similar to Earth.

"We don't know whether Earth-like planets are common or rare in the universe," said Geoff Marcy, a campus professor of astronomy. "The reason is when you point your telescope at a nearby star, any Earth-like planets would get lost in the glare of the host star."

Astronomers, including Marcy, teamed up with NASA to develop a telescope that could find the elusive Earth-like planet if it exists - a mission Marcy called "historic."

"Great philosophers starting from the Greeks all throughout the last 2,000 years asked, 'Is the Earth unique or are there lots of other planets like Earth?'" Marcy said.

The telescope aboard the Kepler rocket uses basic logic to determine the existence of a planet, according to Marcy, who is a co-investigator of the mission and has himself found more than 200 of the 421 planets so far discovered.

The telescope points at stars in Cygnus, a northern constellation, and does a seemingly simple task: It measures the individual brightness of the stars.

"Every time a planet passes in front (of a star), that star will dim just a little bit, just like if a fly landed on a light bulb," Marcy said.

With these data, astronomers can calculate the radius of the planet by measuring how much the star dims when a planet passes in front of it. In Kepler-10b's case, the light decreased a little more than one part per 10,000 as it orbited its star, signifying that its size is much closer to Earth's than that of any other exoplanet yet discovered.

However, in order to find the density, researchers needed the mass of the planet. So Marcy and his team spent 40 nights in a darkened room in Campbell Hall observing the gravitational pull the planet and sun have on each other. The resulting density calculation gave clues as to the planet's composition - it was rock, just like Earth.

The Kepler telescope is still sending data back to Earth every month. With each new set of data, researchers are able to get more and more precise in their search for a smaller, Earth-sized planet with longer orbital periods and therefore in the habitable zone.

"If we progress with the mission and if we receive more data from the spacecraft, we are able to push down to smaller and smaller planets, teasing out those tiny shallow signals from the data," said Natalie Batalha a co-investigator of the mission and lead author of the paper.


Claire Perlman is the lead research and ideas reporter. Contact her at [email protected]

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