Seeking new worlds: more from 'Beta Pic.' (possible planetary system forming around star Beta Pictoris)

Seeking new worlds: More from 'Beta Pic'

The star Beta Pictoris is a leading figure in what has become one of the most tantalizing quests in astronomy: the search for planets orbiting stars other than our own. None have been seen, but a disk of material discovered around "Beta Pic" in 1984 has drawn increasing interest as a possible planetary breeding ground.

The disk appears to consist primarily of tiny, dust-sized grains, but some astronomers now suggest far larger chunks are immersed within it. The disk seems oriented nearly edge-on to earth, so that earth-bound astronomers see it as long features extending from the star in opposite directions. On one side, notes Bradford Smith of the University of Arizona in Tucson, it resembles "a long, thin spike," about 1150 astronomical units (nearly 107 billion miles) in length. In the other direction, however, besides being shorter -- about 900 AU, according to Smith -- it is also variable in thickness, "suggesting the presence of a perturbing body."

That is not the same, however, as asserting some large object is there, Smith adds.

It remains unclear what materials make up the disk. Smith and colleague Richard J. Terrile of the Jet Propulsion Laboratory in Pasadena, Calif., report the disk to be a "very neutral" color, except for a "downturn" in the violet end of the spectrum. This, Terrile noted this week in a presentation to a group of scientists meeting at the Space Telescope Institute on the Johns Hopkins University campus in Baltimore, Md., is consistent with the color of dark, carbon-rich material like that found in some places in our own solar system, including some meteorites. Furthermore, the researchers find, light reflected from the disk shows a large amount of polarization, a characteristic not usually associated with bright, shiny particles such as ice.

On the other hand, three European researchers on leave at the Institute (Pawel Artymowicz of the Copernicus Astronomical Center in Warsaw, and Christopher Burrows and Francesco Paresce of the European Space Agency's Astrophysics Division) reported indications that particles in the disk are quite bright and icy indeed. The group did not have the polarization measurements to go by, but they were able to compare their earth-based visible-light observations with others made in the far infrared portion of the spectrum by the Infrared Astronomy Satellie (IRAS).

Furthermore, Artymowicz concludes, the disk's particles may indeed include more than just little grains. "If there were no large bodies, of the order of asteroid mass, the dust particles would collide with themselves, and the result would be the quick flattening of the disk into one even thinner than we see, like Saturn's rings. I conclude that sub-planetary masses are present in the Beta Pic disk." No such bodies in the portion of the disk covered by the European observations are likely to be as large as, say, Jupiter, says Artymowicz, because planets of such size would leave visible evidence by creating gaps in the disk. But, he maintains, "I infer from indirect dynamic analysis that there may be hundreds of lunar-sized objects." Is Beta Pic thus a planetary system in formation? It may well be, Artymowicz says, "that there is a planetary-like system already formed," with the smaller particles perhaps producing tiny "microcraters" on the larger ones.