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Other Earths? A new estimate raises the odds of finding them.

Researchers predict finding many Earth-sized planets around sun-like stars, right in our own galaxy. But don't pack your bags just yet – you might not want to live on them.

By Staff writer / October 29, 2010

This artist's impression shows how the Arches star cluster appears from deep inside the hub of our Milky Way Galaxy. A new estimate predicts that finding Earth-sized planets around other sun-like stars in the Milky Way are improving.

Artist's Concept/ESA/STScI/NASA

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The prospects for finding Earth-sized planets around other sun-like stars in the Milky Way appear to be improving.

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A new estimate predicts that for every four sun-like stars, one should host at least one planet with anywhere from one-half to twice Earth's mass.

The estimate was among the results of a study by an international research team commissioned by NASA to identify potential targets for as-yet unbuilt space telescopes.

IN PICTURES: Where stars form

The study, which will appear in Friday's issue of the journal Science, doesn't directly address the Holy Grail for planet hunters – finding an Earth-size planet at a life-sustaining distance from its host star. The researchers' prediction involves Earth-size objects in orbits that would fall well inside the orbit of Mercury, the inhospitable first rock from the sun. Their estimate was based on careful observations of stars from a volcano-based telescope in Hawaii.

But if Earth-scale planets are present at the distance the study's researchers predict, it would imply a greater likelihood that such objects would be present in more-distant orbits as well – including within a star's so-called habitable zone, they say.

At the least, such close-in objects would be among the first Earth-size planets NASA's Kepler mission is expected to detect. The Kepler spacecraft, which was launched in March 2009 and trails Earth in its orbit around the sun, is designed to detect Earth-size planets in "life-friendly orbits" around distant stars.

If the international research team's prediction holds up, Kepler, in addition to the planets it is already looking for, would also find as many as 260 of the star-hugging Earths the researchers predict.

"This is a first estimate," cautions Andrew Howard, a University of California at Berkeley astronomer who, along with Berkeley colleague Geoffery Marcy, led the effort, which NASA commissioned five years ago as it contemplated a new generation of space telescopes for studying planets in other solar systems. "The real number could be one in eight instead of one in four. But it's not one in 100, which is glorious news."

Plenty of targets

The results suggest that if those proposed space telescopes get beyond the stage of Powerpoint slides, astronomers will have plenty of targets, relatively nearby, to observe.

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