The discovery of a giant planet amid a cluster of primitive stars is challenging one of astronomers' fundamental notions about how planets are made. The work suggests that some planetary systems were born billions of years before most astronomers thought the universe had spawned the raw materials needed to make them.
According to current theories, planet formation requires a healthy dollop of "metals"--elements heavier than hydrogen and helium--swirling in the gas and dust around a baby star. Metals arise in the nuclear furnaces of stars, whose death throes spew them into space. New stars incorporate this debris, and over several generations, enough metals build up to form the rocky grains thought to assemble the cores of all planets. By that logic, globular clusters--swarms of metal-poor stars as old as our galaxy--are the last place one would expect to find planets.
But now, astronomers think they have clinched the case for a planet in M4, a 13-billion-year-old globular cluster with just 1/30th the metal content of our sun. A team led by astrophysicist Steinn Sigurdsson of Pennsylvania State University, University Park, studied the stars where a white dwarf and a more distant partner tug a rapidly spinning pulsar to and fro. When Sigurdsson and colleagues analyzed images of the white dwarf from the Hubble Space Telescope, they concluded that the distant, unseen companion is not a low-mass star, as many researchers had thought, but a planet with about 2.5 times the mass of Jupiter. "It's a big shock" that such a planet could form, says Sigurdsson, whose team reports its results in the 11 July issue of Science.
Theorists are delighted with the inferred ancient planet. "If you find one, there must be large numbers of them," because globular clusters contain hundreds of thousands of stars that formed at about the same time under the same conditions, says astrophysicist Frederic Rasio of Northwestern University in Evanston, Illinois. "Clearly this would suggest that planet formation does not require high-metal environments." One controversial theory posits that giant planets might not need rocky cores if they form directly from unstable whorls of gas in the nebula around a young star. The globular cluster M4 is so metal-poor that theorists may have to swallow hard and take that model seriously, Sigurdsson notes. What's more, he says, the finding adds 5 or 6 billion years to the amount of time during which life might have appeared elsewhere in the universe.