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.
--ROBERT IRION
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