By DENNIS OVERBYE, NY Times
The roll call of planets beyond the solar system swelled significantly with the announcement of a trio of newly discovered worlds much smaller than any previously discovered around other stars. The masses of these new planets are comparable to those of Neptune or Uranus in our solar system, ranging from about 14 to 20 times the mass of Earth.
The previous planets found around living stars other than the Sun have been giants like Jupiter or Saturn, at least 50 times the mass of Earth, composed of gas at crushing pressures and scorchingly high temperatures and unlikely abodes for life. Astronomers said the new planets might be "ice giants" like Uranus and Neptune, or even giant hunks of iron and rock dubbed "super-Earths."
Like those previously discovered planets, the new ones are circling too close to their stars to be viable for life. But their discovery, astronomers said, is an encouraging sign that planets are plentiful and varied in the galaxy and that a new generation of planet-hunting space missions planned for the next decade will find planets as small as Earth.
"We're getting closer to answering the golden question of whether there is life out there," said Dr Geoff Marcy, an astronomer and longtime planet hunter at the University of California, Berkeley. "We're trying to find our own roots, chemically and biologically, in the stars."
Dr Paul Butler of the Carnegie Institution of Washington said, "We are prepared unexpectedly for the next step in planetary science, finding truly Earth-mass planets."
One of the new planets is part of a system around a star 55 Cancri, already known to harbor three larger planets, making it the first quadruple-planet system to be found beyond the solar system, and a likely target for research. Dr Barbara McArthur of the University of Texas said, "We're on the way to finding the first extra-solar planet Earth, and it's an exciting road to be on."
Dr Butler and Dr McArthur were the leaders of two overlapping teams who announced the discovery of two of the planets at a news conference at NASA headquarters in Washington yesterday.
A third team, composed of European astronomers led by Dr Michel Mayor of the Geneva Observatory in Sauverny, Switzerland, announced the discovery of a third small planet in a news release issued last week from the European Southern Observatory, a consortium based in Garching, Germany, which operates telescopes in Chile.
A pair of papers by the American teams have been approved for publication in the Astrophysical Journal in December. The European group has submitted a paper to Astronomy and Astrophysics.
Other astronomers hailed the results as an example of how fast research is progressing. Dr David Spergel, an astronomer from Princeton who is involved in a NASA project to find and study terrestrial planets, said: "This is an exciting result. Given the existence of these super-Earths, I am willing to bet that there are Earth-like planets around nearby stars."
Dr Marcy said that as a result of the new work, he and his longtime collaborator Dr Butler were revamping their planet searching strategy with the goal of finding planets as small as 10 Earth masses or less, before the space missions to find planets put them out of business.
Dr Butler's team discovered a planet about roughly 20 times the mass of Earth, orbiting a star called Gliese 436, a reddish dwarf about 41 light years away in the constellation Leo, every 2.64 days. It was found as part of a survey of nearby stars using the giant 400-inch diameter Keck I telescope on Mauna Kea in Hawaii.
The second "Neptune" discovered, by Dr McArthur's team, is about 18 Earth masses, the astronomers said. It was discovered by combining data obtained with the new 360-inch diameter Hobby-Eberly Telescope at the University of Texas with observations from the Hubble Space Telescope and others. It circles 55 Cancri every 2.8 days, far inside the orbits of the other three Jupiter-like stars.
The European team asserted that they had found the lightest planet yet on record. It orbits the star mu Arae, in the constellation Ara, about 50 light years from here. It completes a revolution every 9.5 days.
The new planets join more than 100 others that have been detected around other stars in the past decade. Like a vast majority of their predecessors, they were discovered by the gravitational tugs they exert on their parent stars.
The gravitational wobble technique is most sensitive to giant planets orbiting lethally close to their stars - they give the biggest kicks - so it is not surprising that the first planets discovered were in such orbits, astronomers say. Longer observations are required to discern the effects of smaller planets in more comfortable orbits. Indeed, two of the new planets were discovered by continuing to refine the data from stars where giant Jupiter-like planets had already been detected.
Both the mu Arae systems and the 55 Cancri system were already known to have Jupiter-like planets. The 55 Cancri system has drawn interest because one of its "Jupiters" has an orbit, about 13 years in duration, similar to that of Jupiter in our own solar system.
Dr McArthur, who has organized a campaign to study the 55 Cancri system, said: "This is the closest analogue we have to our solar system. All these things make it the premier laboratory for studying planetary systems."
The Gliese 436 discovery is important, astronomers said, because so-called red dwarfs are the most abundant stars in the universe. About 70 percent of the 200 billion stars in the galaxy are red dwarfs, known technically as M dwarfs, but because they are so dim, only a few percent as bright as the Sun, they are hard to study. The new planet is only the second one found around a red dwarf.
Dr Spergel said, "The detection of these planets is definitely good news for T.P.F. The worry has been that we build this exquisitely powerful telescope and then find that there are no Earth-like planets for it to observe."
That worry now seems unnecessary. As Dr Marcy said, "We can't see Earth-like planets yet, but we can see their big brothers."
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