06/11/98
BERKELEY -- Space scientists at the University of California, Berkeley, have found two nearby stars that appear to be continually bombarded by comets, thought by many to be the building blocks of planets.
The two stars, each about 450 light years from Earth, bring to four the number of known solar systems so young that their inner regions are still peppered with comets.
Because planets are thought to coalesce from the collision of comets and asteroids, it is likely that planets are forming within the gas and dust surrounding these stars.
"Observations of comet-like bodies, or 'planetesimals,' outside of our solar system are of great importance in understanding the role of comets in the formation of all planetary systems," said Barry Y. Welsh, a researcher in interstellar gas studies at UC Berkeley's Space Sciences Laboratory. "Our observations indicate a high level of cometary activity in these disk systems suggesting that there is potentially plenty of raw building material for new planetary bodies."
Welsh and colleagues Nahide Craig of UC Berkeley and Ian Crawford of University College, London, England, reported data on the two stars -- HD85905 and HR10 -- at the June 7-11 meeting of the American Astronomical Society in San Diego, Calif.
Both stars show clear evidence of surrounding disks of gas and dust, Welsh said. Until recently, only one star -- Beta Pictoris -- was known to possess both proto-planetary dust and gas clouds, although dust disks have been observed around perhaps a dozen other stars.
It seems likely that the stars' gravity is pulling swarms of kilometer-sized solid bodies out of the surrounding dust disk into highly elliptical orbits, which bring the comets within about 100 million miles of the stars' surfaces, where they are destroyed.
The discovery was made during two observing runs in 1997 from the 1.5-meter telescope at the Cerro Tololo Observatory in Chile. The telescope's high resolution spectrograph showed highly variable features that could only be attributed to the evaporation of large blobs of gas, presumably originating in the nuclei of comets as they approached close to the central stars.
"Our observations show large changes in the amount of gaseous calcium and sodium from night to night," Craig said. "The amounts seen are consistent with the evaporation of gas from comet-like objects, similar to the huge tails of glowing gas that many people in North America saw last year from Comet Hale-Bopp. These observations show identical behavior to that routinely seen towards the star Beta Pictoris, a well-known candidate proto-planetary system."
Beta Pictoris is the preeminent example of a solar system whose central star is being hit by continual swarms of gaseous bodies, probably comets. Since 1984, when Beta Pictoris was found to be surrounded by a disk of dust -- presumably a swirling planetary nursery -- astronomers have trained telescopes and radio dishes on the star in search of actual planets.
This intense study turned up a surprise in 1985, when a doughnut of gas was found around the star. Even more surprising, bursts of gas seemed to pop up almost daily, a phenomenon attributed to comet-like bodies, or planetesimals, falling into the center of the system and spewing out gas as they are heated by the star. Presumably the comets are pulled in from the dust cloud surrounding the star, just as comets in our solar system today are pulled from a distant concentration of comets called the Oort Cloud.
Despite intense searches, no similar star systems with cometary activity were found until last year, when variable ultraviolet gas cloud emissions indicated the presence of newly forming proto-planets around HD100546, a star also known to have a dusty disk. The UV emissions were detected by Carol Grady of Eureka Scientific, Inc., and coworkers using the International Ultraviolet Explorer satellite.
Welsh and his colleagues thought of a simpler way to look for systems like Beta Pictoris: search out stars that resemble Beta Pictoris as closely as possible.
"I made a list of everything we knew about Beta Pictoris from an astronomical viewpoint, and then I looked for stars with these characteristics," Welsh said. "How hot it is, how old it is, is there dust, how fast is it rotating?"
Welsh and his colleagues identified some 40 candidates and were able to look at six of them for five or more separate nights last year. Two showed clear evidence of gas bursts.
"I've discovered a cheap way to find them," Welsh said. "The only difficulty is that you have to look for many nights at these objects, and it is hard to get sufficient telescope time for that."
Gas can be detected around a star because it absorbs starlight. In looking at Beta Pictoris, astronomers noticed not only absorption from a doughnut of gas around the star, but also additional absorption that seemed to appear and disappear on a daily basis. The interpretation was that large comets or swarms of comets evaporated periodically as they approached the star, causing a brief decrease in the light coming from the star. The UC Berkeley team found the same signature with HD85905 and HR10.
This past April three other stars -- Vega, Fomalhaut and HR4796A -- made headlines when two separate groups of astronomers produced new Hubble Space Telescope photos that seemed to show evidence of proto-planets in the disks of dust around the stars.
"Much attention has recently been focused on detecting the infrared signatures from disks of dust surrounding proto-planetary systems such as Vega and HR4796," Crawford said. "Our new observations show the presence of gas disks and comet-like bodies in these planetary building sites. Although the inner planets in our own Solar System are made mostly of rock, the outer planets like Jupiter and Saturn are gas giants. Our new data show a potential reservoir of gaseous building materials for such planets."
While Welsh acknowledges that the evidence for planets in formation around the two stars is indirect, "there is no other mechanism that can explain what we're seeing -- the data tell us there are numerous asteroids and comets around these stars," he said.
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