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In a boon for those anticipating future discovery of alien life and/or human colonisation of other worlds, NASA boffins say that their latest analysis indicates that almost one in four stars may be orbited by planets as small as Earth.

"We studied planets of many masses - like counting boulders, rocks and pebbles in a canyon - and found more rocks than boulders, and more pebbles than rocks. Our ground-based technology can't see the grains of sand, the Earth-size planets, but we can estimate their numbers," says Andrew Howard, boss scientist of the team conducting the study.

"Earth-size planets in our galaxy are like grains of sand sprinkled on a beach - they are everywhere," says Howard.

The astronomer and his colleagues used the Keck telescope in Hawaii for a five-year sky census involving 166 sun-like stars in the general vicinity of our solar system. A NASA statement released yesterday specifies that they employed the "the radial velocity, or 'wobble' technique" to assay just how many planets of various sizes each sun possessed.

According to NASA:

A distinct trend jumped out of the data: smaller planets outnumber larger ones. Only 1.6 percent of stars were found to host giant planets orbiting close in. That includes the three highest-mass planet groups in the study, or planets comparable to Saturn and Jupiter. About 6.5 percent of stars were found to have intermediate-mass planets, with 10 to 30 times the mass of Earth – planets the size of Neptune and Uranus. And 11.8 per cent had the so-called "super-Earths," weighing in at only three to 10 times the mass of Earth.

"The data tell us that our galaxy, with its roughly 200 billion stars, has at least 46 billion Earth-size planets, and that's not counting Earth-size planets that orbit farther away from their stars in the habitable zone," says top planet-hunter boffin Geoff Marcy, part of the Keck team.

Some models of planetary formation suggest that there would be a "desert" of planets in the hot zones close to stars, implying that Earth-size worlds would normally be found in the cooler outer regions.

Though the new study refers only to Earth-sized worlds orbiting close in to their parent stars - ones which would be too hot for liquid water to exist, and thus unable to develop or sustain life along Earthly lines - it means that there are in fact many, many such planets to be found orbiting nearby suns. This should also mean that there are a decent number within their stars' habitable zones, perhaps home to alien life now or the colonists of humanity one day in the far future once we've cracked the problems of star travel.

The planet-sniffing study is published here by major boffinry mag Science (subscription required for full article). ®

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