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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.

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Such observations also would add to a growing body of evidence that current theories about how solar systems form, particularly how planets wind up in their final orbits, are either incomplete or wrong, some astronomers say.

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As those theories stand now, they suggest that the close-in regions occupied by these Earths and other planets the team detected should be relatively planet-free.

Instead, they are populated with planets ranging from Jupiter- and Neptune-size objects to so-called super-Earths, with five to 30 times Earth's mass. Based on a statistical analysis of the masses of planets at a given orbital distance, the team calculated that 23 percent of sun-like stars should host an Earth-size planet.

Other astronomers have tried to develop similar estimates, notes Ray Jayawardana, an astronomer at the University of Toronto who was not involved with the project. But this team's attempt at a census of extrasolar planets and the relative abundance of planets of various sizes is the most rigorous to date, he adds.

The estimate is based on a five-year effort to detect planets around 166 sun-like stars within 80 light-years of Earth. The vast majority of extrasolar planets found so far have been so-called hot Jupiters – massive planets whose orbit times often are measured in a handful of days. Using the 10-meter Keck Telescope atop Hawaii's Mauna Kea volcano, the researchers detected the planets by watching for subtle back-and-forth shifts in a star's spectrum that an orbiting planet imparts.

Of the stars the team surveyed, 22 hosted a combined 33 detectable planets with orbital periods no longer than 50 days.

The team's analysis showed that out of each 100 sun-like stars, as few as two would host a Jupiter-mass planet, six would host planets comparable to Neptunes, and 12 would have super-Earths.

Planets defy theory

Even if the team's predictions for truly Earth-size planets fail to pan out, the detected planets pose a conundrum, Dr. Jayawardana says.

Planets form from a broad disk of dust, gas, and ice that encircles their star.

Computer simulations of this process suggest that if planets actually form in or close to the orbits in which the team found its samples, rocky planets will either continue to grow into monstrous gas giants, or they will quickly interact with any remaining disk in ways that send them slowly spiraling into their star.

"Either way, you shouldn't get a lot of intermediate-mass planets at these kinds of orbits," he says.

But, according to the team's observations, there they are.

IN PICTURES: Where stars form

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