EMBARGOED FOR Common stars like our Sun rich in heavy elements are more likely to harbor planets, according to a new study.
The finding suggests extrasolar planets might be more common than thought, and it should help planet hunters pick their targets more effectively.
Theorists had long said that planet formation would be more common around stars rich in "metals," which in astronomers' parlance means anything heavier than hydrogen and helium, the two most common elements in the universe. The new study confirms this beyond expectations.
The survey of 754 relatively nearby stars showed that the more metal a star contains the more likely it is to harbor one or more planets. The results represent a stark shift in the percentage of metal-rich stars now said to be potential homes to orbiting worlds.
"Astronomers have been saying that only 5 percent of stars have planets," said
Debra Fischer, an astronomer at the University of California, Berkeley. "If you look at the metal-rich stars, 20 percent have planets. That's stunning." Elements are forged in stars. The first stars in the universe were made almost entirely hydrogen. Subsequent generations of stars contain more and heavier elements. Our Sun, at about 4.6 billion years old, is young compared to the universe, which is 13.7 billion years old. The Sun is fairly metal-rich.
What does this have to do with planets? A star is born from a cloud of material -- hydrogen and other elements. Planets form from the leftovers. If the cloud is rich in heavy material -- metals and rock -- then it has more of the stuff that theorists think planets require to begin their development.
"The metals are the seeds from which planets form," said Fischer's colleague, Jeff Valenti of the Space Telescope Science Institute.
Very metal poor stars can form planets, however. One example, announced recently, involves what is presumed to be the
oldest planet in the universe. It must have been born around one of the earliest stars -- a star with very little metal, a separate research team said.Almost all of the 121 known extrasolar planets have been found nearby and around stars similar to the Sun, though. Most are larger than Jupiter and made mostly of gas. But scientists suspect they might have rocky cores. At the least, metals are needed for the gas to properly organize into a planetary ball, theorists say.
Stars abundant in heavy metals are five times more likely to have a planetary companion than metal-poor stars, Fischer said. The survey included 61 stars known to have planets and 693 that do not, though further investigation could show that some of these other stars do in fact harbor planets that are very far away from their host star and thus hard to detect.
It is the first large-scale survey comparing stellar metalicity to planet formation. The results were presented Monday at a meeting of the International Astronomical Union meeting in Sydney, Australia.
Stars like the Sun, a typical star in this part of the Milky Way, have a 5 to 10 percent chance of hosting planets, the analysis showed. Stars three times more metal-rich than the Sun have a 20 percent chance of hosting planets, while stars with only one-third the amount of solar metalicity have just a 3 percent chance.
Fischer also is part of the world's
most prolific planet-hunting team, lead by Geoffrey Marcy at UC Berkeley and Paul Butler of the Carnegie Institution of Washington. "These results tell us why some of the stars in our Milky Way galaxy have planets while others do not," Marcy said. "The heavy metals must clump together to form rocks which themselves clump into the solid cores of planets."
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