Intruder in the Stardust

WASHINGTON, D.C.--Astronomers may have caught a glimpse of a newborn planet in its dusty nursery around a nearby star. New images from the Hubble Space Telescope reveal pronounced warps in a dramatic disk of dust circling Beta Pictoris, a young star 63 light-years away. Researchers who unveiled the images here today at a meeting of the American Astronomical Society said the bulges may betray the existence of at least one planet. But others suggested that gravitational nudges from a passing star or a large dim companion, such as a brown dwarf, might account for the distortions.

Astronomers first spotted a dusty veil girdling Beta Pictoris in 1984 in images from IRAS, an infrared satellite. The disk, seen from Earth nearly edge-on, may have provided the ingredients for planet formation. In September 1997, Hubble's new STIS spectrograph blocked the harsh light of Beta Pictoris to probe the disk in great detail--spying features as close to the star as the orbit of Uranus around the sun, some 60% closer than previous work.

Asymmetric bulges within the innermost part of the disk strongly suggest the perturbing influence of a planet circling Beta Pictoris in an orbit slightly inclined to the plane of the disk, says astronomer Sally Heap of NASA's Goddard Space Flight Center in Greenbelt, Maryland. The planet could range in size from 10 times the mass of Earth to 17 times the mass of Jupiter, depending on its distance from the star.

Astronomer Fred Bruhweiler of the Catholic University of America in Washington, D.C., is not yet convinced, however. "I don't think it's quite that tied down yet," he says. Bruhweiler points to warpings of the disk at much greater distances from the star--as far out as 20 times Pluto's distance from the sun. Those could not possibly arise from a planet in a tight orbit, he claims. Bruhweiler favors a disturbance on a much grander scale, from either a star that swung by Beta Pictoris but has long since disappeared, or an invisible companion dwarf star.

Astronomer Sergio Fajardo-Acosta of the University of Denver observes that Heap's and Bruhweiler's deductions are not mutually exclusive. Different phenomena could distort different parts of the disk, he says. But Fajardo-Acosta also urges his colleagues not to rule out unevenly distributed clumps of matter within the disk itself or other sources of asymmetry, such as dense clouds of young comets. Theoretical models of the various proposals should eventually help astronomers decide whether a planet does indeed lurk in Beta Pictoris's dusty disk.