Exocomets Are As Common As Exoplanets
January 8, 2013

Exoplanets Are Not Alone, Exocomets Are Just As Common

April Flowers for redOrbit.com - Your Universe Online

A beautiful byproduct of our solar system's formation, comets trailing wispy tails across the night sky are icy leftovers from 4.6 billion years ago when the planets coalesced from rocky rubble.

Six likely comets around distant stars were recently discovered by astronomers at the University of California, Berkeley and Clarion University suggesting that comets — dubbed "exocomets" — are just as common in other stellar systems with planets.

Out of the 10 stars now thought to harbor comets, only one is known to also harbor planets. However, the fact that all 10 stars have massive surrounding disks of gas and dust — a signature of exoplanets — makes it likely that all 10 harbor planets as well.

“This is sort of the missing link in current planetary formation studies,” said Barry Welsh, a research astronomer at UC Berkeley´s Space Sciences Laboratory. “We see dust disks — presumably the primordial planet-forming material — around a whole load of stars, and we see planets, but we don´t see much of the stuff in between: the asteroid-like planetesimals and the comets. Now, I think we have nailed it. These exocomets are more common and easier to detect than people previously thought.”

Welsh presented the findings of this study at the meeting of the American Astronomical Society. Welsh, along with Sharon L. Montgomery of the Department of Physics at Clarion University, also reported three of the new exocomets in the October 2012 issue of Publications of the Astronomical Society of the Pacific.

The current theory of planet formation, according to Welsh, is “interstellar dust under the influence of gravity becomes blobs, and the blobs grow into rocks, the rocks coalesce and become bigger things — planetesimals and comets — and finally, you get planets.”

Astronomers know that disks of gas surround many stars and dust, one of the closest of these, beta-Pictoris (B-Pic), was reported to have comets in 1987. A large planet was found orbiting B-Pic in 2009, about 10 times larger than Jupiter. Subsequently, three other stars, including one discovered by Welsh in 1998, were found to have comets.

“But then, people just lost interest. They decided that exocomets were a done deal, and everybody switched to the more exciting thing, exoplanets,” Welsh said. “But I came back to it last year and thought, ℠Four exocomets is not all that many compared to the couple of thousand exoplanets known — perhaps I can improve on that.´”

The "snowball" comets are small, only 3-13 miles in diameter, making it sound difficult to detect them. According to Welsh, however, once comets are knocked out of their parking orbit in the outer reaches of a stellar system and fall toward a star, they heat up and evaporate, creating a tell-tale absorption line in the spectrum of a star such as what we see with Halley and Comet ISON.

Using the 2.1-meter telescope of the McDonald Observatory in Texas, the six new exocomet systems were discovered during three five-night-long observing runs between May 2010 and November 2012. The scientists uncovered weak absorption features that were found to vary from night to night using the telescope's high-resolution spectrograph. This varying outcome is attributed to large clouds of gas emanating from the nuclei of comets as they neared their central stars.

Welsh's detection technique works best with very young type A stars. These stars are about 5 million years old, and all of the newly discovered exocomets - 49 Ceti (HD 9672), 5 Vulpeculae (HD 182919), 2 Andromedae, HD 21620, HD 42111 and HD 110411 — are found with these types of stars. Given a higher resolution spectrograph, Welsh believes he would be able to detect comets around the older and yellower G and F stars where most exoplanets have been found.

Planets are the only thing that could knock a comet out of its orbit and make it fall toward its star, and all the evidence suggests that these dusty type A stars should have planets.

“If it quacks, waddles and has feathers, then it´s probably a duck,” he said.