08/06/1997 07:00:00

Possible Planet-Forming Disk imaged by Caltech Radio Astronomers

PASADENA—A giant disk of gas and dust over 10 times the size of our own solar system has been detected rotating around a young star in the constellation of Auriga. The star is both more massive and brighter than our sun, and appears to be a young version of another star called Beta Pictoris, where astronomers have long suspected the presence of planets.

The new discovery was made by radio astronomers at the California Institute of Technology using the millimeter-wave array at Caltech's Owens Valley Radio Observatory in central California. The results appear in the current issue of the journal Nature, and concern a relatively massive star known as MWC480, which is about 450 light-years from Earth.

How prevalent is planet formation around young stars? Past work had shown that stars similar to our own sun possess protoplanetary disks in their youth, disks we believe will form planets, perhaps as our own solar system did. However, little was known about the propensity of disks to form planets around stars that are more massive than our sun.

According to Vince Mannings, the paper's first author, the new results provide unprecedentedly clear evidence for the presence of a rotating disk of gas surrounding MWC480, and support earlier indications of rotating disks encircling some less massive and young sunlike stars. Not only is the gas around MWC480 clearly discernible at radio wavelengths, he says, but the orbital rotation of the entire disklike cloud is also unambiguously observed.

The presence of rotation suggests that, as for the disks around the young sunlike stars, the disk structure around MWC480 is long-lived. Indeed, this massive reservoir of orbiting material could last long enough to form new planets. "Families of planets, perhaps resembling our own solar system, are thought to originate in such disks," says Mannings. "Our sun, when very young, possibly had a disk similar to that around MWC480."

The star in the middle of the MWC480 disk resembles a much older star called Beta Pictoris, which is surrounded by a comparatively lightweight "debris disk," probably composed in part of dust-grain remnants from processes connected with an earlier phase of planet building. The new results imply that, in its youth, Beta Pictoris may have possessed a massive disk comparable to that now identified around MWC480. Beta Pictoris might have been, effectively, a "planetary construction site," says Mannings.

Other members of the research team are David Koerner, an astronomer at the Caltech/NASA Jet Propulsion Lab, and Anneila Sargent, who is executive director of Caltech's Owens Valley Radio Observatory.

Mannings says, "We believe that the amount of material in this disk is sufficient to produce a system of planets. We detect enough gas and dust to build planets with the same total mass as that of the nine planets in our own solar system. But we emphasize that the possibility of planet building within this particular disk is speculation only."

The radio image is sufficiently detailed to show that the large disk of gas and dust is tilted about 30 degrees from face-on. A tantalizing aspect of the image is that the rotation of the disk can be detected by measuring the velocities of the gas, most of which is in the form of molecular hydrogen. About 1 percent of the disk is dust grains, and just a trace amount of the material is carbon monoxide. The hydrogen is not detected directly, but the gas velocities can be probed using spectral-line radio waves emitted by the carbon monoxide. The Caltech measurements demonstrate that gas south of the star travels approximately toward us, and away from us when north of the star. From our vantage point, the disk is inferred to be rotating roughly from south to north.

For the first time, astronomers have identified clearly a young massive disk that could gradually evolve into a debris disk such as that surrounding the older star Beta Pictoris, perhaps building planets along the way. By studying stars like MWC480, say Mannings, Koerner and Sargent, we can hope to learn not only about the origins of the Beta Pictoris debris disk, but perhaps about the beginnings of our own solar system too. Astronomers have targeted nearby sunlike stars for searches for new planets, but this discovery shows that brighter stars should also be included.