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Astronomers Find Comets Around Two Nearby Stars

University of California-Berkeley

NEWS RELEASE: 06/11/98

UC Berkeley astronomers find comets around two nearby stars, indicating the
likelihood of planets forming

By Robert Sanders, Public Affairs

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.