Beyond the Starry Skies Lies More Planets

November 2, 2010

(Ivanhoe Newswire)– NASA won’t have to look too far to find Earth-size planets.

UC Berkeley astronomers Andrew Howard and Geoffrey Marcy observed 166 stars within 80 light years of Earth with the powerful Keck telescope for five years and found increasing numbers of smaller planets, down to the smallest size detectable today ““ planets called super-Earths, about three times the mass of Earth.

“Of about 100 typical sun-like stars, one or two have planets the size of Jupiter, roughly six have a planet the size of Neptune, and about 12 have super-Earths between three and 10 Earth masses,” Andrew Howards, research astronomer in UC Berkeley’s Department of Astronomy and at the Space Sciences Laboratory, was quoted as saying. “If we extrapolate down to Earth-size planets ““ between one-half and two times the mass of Earth ““ we predict that you’d find about 23 for every 100 stars.”

Only 22 of the stars had detectable planets ““ 33 planets in all ““ within this range of masses and orbital distances. After accounting statistically for the fact that some stars were observed more often than others, the researchers estimated that about 1.6 percent of the sun-like stars in their sample had Jupiter-size planets and 12 percent had super-Earths (3-10 Earth masses). If the trend of increasing numbers of smaller planets continues, they concluded, 23 percent of the stars would have Earth-size planets.

“This is the first estimate based on actual measurements of the fraction of stars that have Earth-size planets,” Geoffrey Marcy, UC Berkeley professor of astronomy, was quoted as saying.

Previous studies have estimated the proportion of Jupiter and Saturn-size exoplanets, but never down to Neptunes and super-Earths, enabling an extrapolation to Earth-size planets.

Because the researchers detected only close-in planets, there could be even more Earth-size planets at greater distances, including within the habitable zone located at about the same distance as the earth is from our sun. The habitable, or “Goldilocks,” zone is the distance from a star neither two hot nor too cold to allow the presence of liquid water.

The researchers’ results conflict with current models of planet formation and migration, Marcy stated.

After their birth in a protoplanetary disk, planets had been thought to spiral inward because of interactions with the gas in the disk. Such models predict a “planet desert” in the inner region of solar systems.

“Just where we see the most planets, models predict we would find no cacti at all,” Marcy said. “These results will transform astronomers’ views of how planets form.”

“One of astronomy’s goals is to find eta-Earth (?Earth), the fraction of sun-like stars that have an earth,” Howard said. “This is a first estimate, and the real number could be one in eight instead of one in four. But it’s not one in 100, which is glorious news.”

Twelve possible planets also were detected, but they need further confirmation, Marcy said. If these candidate planets are included in the count, the team detected a total of 45 planets around 32 stars.

SOURCE: Science, October 2010

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