WASHINGTON – Rival teams of European and American astronomers have detected the three smallest planets ever seen outside the solar system – possible “super-Earths” about the size of Neptune and Uranus that are orbiting nearby stars.

Two to three times bigger and 14 to 21 times heavier than our own Earth, the planets are major milestones on the quest to find Earthlike bodies that might support life.

“These discoveries are going to bring us closer to answering the question: Are we alone in the universe?” Anne McKinney, the director of NASA’s Universe Division, said at a Washington news conference Tuesday.

In addition, a fourth new planet – a so-called “gas giant” about the size of Jupiter – was reported Aug. 24 by another American group, making the week an extraordinarily successful one for planet hunters.

Until now, all 125 extrasolar planets discovered in the last nine years have been blazing hot balls of gas, jocularly known as “roasters,” where life couldn’t possibly exist. The new smaller, lighter planets are a long-hoped-for advance.

“We are now poised for the next step: finding truly Earth-mass planets,” said Geoffrey Marcy, an astronomer at the University of California, Berkeley.

“We can’t quite see Earth-like planets yet, but we are seeing their big brothers,” added Marcy’s partner, astronomer Paul Butler of the Carnegie Institution of Washington.

The competition between the European and American astronomers has been going on since 1995, when Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland reported the first planet orbiting a sunlike star.

Within months, Marcy and Butler followed with more discoveries, and now hold the world title for bagging the greatest number of distant planets.

Last Wednesday, however, the European team announced it had found what it called the first “super-Earth,” a planet 14 times as massive as Earth. Named mu Arae, it orbits a star 50 light-years away in the southern constellation Ara (the Altar).

(One light-year is about 6 trillion miles. The Milky Way is about 50,000 light-years across.)

“The object is likely to be a planet with a rocky core surrounded by a small gaseous envelope and would therefore qualify as a “super earth,”‘ the Europeans declared in a news release.

The American team hastily scheduled Tuesday’s news conference to announce that it had discovered not one but two possible super-Earths, one-upping the Europeans.

Marcy and Butler’s latest planet is 21 times heavier than Earth, a bit bulkier than Neptune. It orbits the star Gliese 436, 30 light-years away in the constellation Leo (the Lion).

The other new American find, 18 times more massive than Earth, was made by Barbara McArthur, an astronomer at the University of Texas in Austin. It’s the fourth planet to be found in a miniature solar system circling the star 55 Cancri, 41 light-years away in the constellation Cancer (the Crab). The other three are Jupiter-sized gas giants.

McArthur’s planets make up the first quadruple planetary system yet found, outside our own. It will be “a premier laboratory for the study of the formation and evolution of planetary systems,” she said.

Because the three new super-Earths are 10 or so times smaller than Jupiter, astronomers think they’re more likely to be composed of rock, like Earth, than of gas.

“These may be the first of a new class of super-Earths,” said Alan Boss, a theoretical astrophysicist at the Carnegie Institution of Washington.

These three planets, however, are so hot and so close to their mother stars that life on them is extremely unlikely. The only remote possibility would be in a narrow band between the bright side and the dark side of the planet around Gliese 436, where the temperature might be “lukewarm,” Marcy said.


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