This false-color Hubble Space Telescope image of the star Beta Pictoris shows a slightly warped dust disk. The warp hints at the presence of planet-like objects. Like planets, comets are thought to form from the accretion of material in such disks.Usually, scientists tend NOT to think of Earth and our solar system as special places. They reason that what goes on here probably occurs in other cosmic neighborhoods as well.
Take HD 163296, for instance. This poor star not only doesn't have a name, but it is young and may be in the process of giving birth to its own solar system (ouch!). It may also harbor thousands of comets.
In 1996, Karen Bjorkman, an astronomer now at the University of Toledo, and Carol Grady and Mario Pérez of the Applied Research Corp. in Landover, Md., reported evidence of as many as 35 comets a year falling toward the star, a hint that there might be many more since they were looking only across one small slice of space around the star. Situated about 500 light years from Earth in the constellation Sagittarius, HD 163296 is a good place to look for comets because it has a surrounding protoplanetary disk, a whorl of dust and gas that may be similar to the one that astronomers believe gave rise to our own solar system.
Clouds of comets everywhere?
"I think most astronomers would agree that it is very likely that other stars will have comet clouds," says Bjorkman. "Based on what we think we understand about the way planetary systems form, it seems likely that similar processes to those that occurred to form our solar system would also have happened around other stars, and the formation of a comet cloud is an integral part of that process."
HD 163296 is one of about 10 stars that show evidence of having orbiting hunks of ice and dust, AKA comets. And just how, you ask, do extrasolar comet hunters find clues to the existence of these slippery devils?
Where's the proof?
There are three lines of evidence, says Bjorkman. The first is found in the ultraviolet light sampled by such skyward looking satellites as the International Ultraviolet Explorer. The ultraviolet light collected by the satellite can show signs of material falling into other stars. The light, broken down into its constituent spectra, show, in the case of HD 163296, that some of that material is similar to what we find on comets that orbit our sun.
"The material is so cool," says Bjorkman, "that it could not have gotten as close to the star unless it were carried in by something, perhaps a comet." In other words, the cool stuff astronmers are seeing wouldn't be that close to the star -- at those temperatures at least -- unless it was hitching a ride on something like a speeding comet.
Sliding over to the infrared part of the electromagnetic spectrum, yields more clues to extrasolar fuzzy ice balls. New observations of stars made by the Infrared Space Observatory show the material in the disks around some stars has the exact same spectral signature as the comets that trip through our solar system.
Finally, astronomers using the Hubble Space Telescope have taken pictures of extended disks of material around other stars. These disks are comparable in size and look just like what we would see if we could step back -- way back -- and look at our own solar system.
"This seems to be direct evidence that at least something resembling the disk that makes up our solar system of planets and comets exists around other stars," Bjorkman says.
We have documentary proof that people used to have some pretty strange ideas about comets.
