Earth-Sized Planets Could Be Common

Artist's rendering of exoplanets around a star. (Credit: NASA)Reported for

The Earth may not be as unique as we think it is. That's according to findings announced today by UC Berkeley. Astronomers there believe that Earth-sized planets may be more abundant in the universe than previously thought.

For five years, a team of scientists lead by UC Berkeley watched 166 stars, similar in size to our Sun and all within 80 light years of Earth. In all, they discovered extra-solar planets or "exoplanets" orbiting 22 of the stars. Some are as large as Jupiter while others are about three times the size of Earth, the smallest planet they can detect. Smaller planets were found more frequently than the larger planets.

"We found smaller planets in spades," said astronomer Andrew Howard of UC Berkeley. Using the data, Howard and his team created a statistical model to predict what other planets might be present. "We extrapolated that trend down to Earth-sized planets."


Howard says the data shows that nearly one in four stars like our Sun could have Earth-sized planets. "This is really the first quantitative estimate of the fraction of sun-like stars that have Earth-like planets. Before, the guesses were all over the map. Some people thought it was 100%. Some people thought it was one in a million."

The 33 planets found in the study orbit very close to their stars, meaning temperatures there are most likely too high to support life. The discoveries were made with the Keck Observatory in Hawaii using 10-meter ground telescopes. The planets were found using the "wobble" of the stars - the subtle movement that occurs when a star is pulled by the gravity of its orbiting planets.

The announcement joins a number of exoplanet discoveries in recent months, including NASA's finding of two exoplanets in August. Today's findings were published in the journal Science.

Howard says while the ultimate goal is to find Earth-like planets that could have liquid water, this finding is an important first step. "People have wondered for millennia: is the Earth common or is it rare? And we're starting to learn that the Earth is not a one-off in the universe. It may have cousins out there."

For more on how scientists find exoplanets, check out this QUEST story.

QUEST on KQED Public Media.

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