Giant planets made in a jiffy
June 10, 2010 at 1:59 PM ET
ESO Click for video: This diagram traces the dusty disk around Beta Pictoris, with the positions of a giant planet marked at two points in its orbit. Click on the image to watch an "ESOcast" video, or click here for a larger version of the diagram..
Astronomers say their years-long observations of a gas giant orbiting a young star demonstrate that such planets can form in just a few million years, which is a surprisingly short span of time in cosmic terms. The findings, published online today by the journal Science
, mark another advance for the booming field of exoplanetary science
. Just five years ago, the ability to make direct observations
of planets beyond our own solar system seemed like a distant dream
. Today's report about Beta Pictoris and its planetary companion demonstrate that such views are becoming part of the routine for the planet search. Beta Pictoris, a 12 million-year-old star located 60 light-years from Earth in the constellation Pictor, is surrounded by a warped, dusty disk that has been seen with increasing clarity by a succession of telescopes. Astronomers have long suspected that planets were being formed there, because the disk's warp hinted at some internal gravitational perturbation. Sure enough, two years ago, a team of astronomers
using the European Southern Observatory's Very Large Telescope spotted a big blip
that fit the pattern for a planet nine times as massive as Jupiter, orbiting at about Saturn's distance from our own sun. Many of the same astronomers were in on the findings announced today, which also relied on VLT's views. They tracked the blip around the star between 2003 and 2009, and found that the blip continued to follow its predicted planetary orbit. That was important, because the additional observations ruled out alternate explanations for the blip - for example, that it was actually just a background star.
ESO An artist's conception shows Beta Pictoris b in orbit around its parent star.
The astronomers wanted to be sure they had it right because of Beta Pictoris' age. Twelve million years is a long time in human terms, but it's a very short time for the computerized models that scientists have developed to explain how giant planets are built up from the gas and dust surrounding a star. Most of the other star systems where planets have been detected are much older, "preventing the validation of models," the astronomers said. In contrast, Beta Pictoris demonstrates that gas giants can be created in a jiffy. "Because the star is so young, our results prove that giant planets can form in disks with time spans as short as a few million years," the research team's leader, Anne-Marie Lagrange of France's Laboratoire d'Astrophysique de l'Observatoire de Grenoble, said in an ESO news release. The researchers also said the position of Beta Pic's giant planet is consistent with a particular model for planet formation known as "hot start" core accretion, which allows for planets to be formed by the contraction of the hot cloud of gas surrounding a young star. The latest findings about Beta Pic and other star systems suggest that "super-Jupiters could be frequent byproducts of planet formation around more massive stars," said Gael Chauvin, a colleague of Lagrange's at LAOG. Alan Boss, a planetary scientist at the Carnegie Institution of Washington who was not involved in the Science study, has been focusing on a different model for planet formation known as disk instability. He said either model could explain the Beta Pic observations. "I would call it a toss-up as to which mechanism formed this guy," he said. Boss, who has written about the planet search in a book titled "The Crowded Universe,"
discussed the "gorgeous" images and the researchers' analysis in an e-mail message:"It looks like they have nailed down the case for a very low mass companion to Beta Pic, with a planetary-range mass a good bet, given the uncertainty in the gas giant planet interior models. ... The key thing here is that we seem to be well along our way to being able to do direct imaging of exoplanets, at least when they are (1) massive, (2) young, (3) nearby, and (4) not too close to their stars! That is a lot of preconditions, but it is a great start nonetheless. Someday theory will catch up, but for the moment, theorists are trailing far behind the observers."
The next step is to look even more closely with future telescopes - many of which are being equipped with spectroscopic instruments that could "sniff" the atmospheric chemistry of distant planets, in search of a whiff of biological activity. Although Beta Pictoris b isn't in its parent star's habitable zone, it could still serve as one of the targets for observations to come. "The short period of the planet will allow us to record the full orbit within maybe 15 to 20 years, and further studies of Beta Pictoris b will provide invaluable insights into the physics and chemistry of a young giant planet's atmosphere," student researcher Mickael Bonnefoy, a member of the Science study's team, said in the ESO news release. More on the planet search:In addition to Lagrange, Bonnefoy and Chauvin, the authors of "A Giant Planet Imaged in the Disk of the Young Star Beta Pictoris" include D. Apai, D. Ehrenreich, A. Boccaletti, D. Gratadour, D. Rouan, D. Mouillet, S. Lacour and M. Kasper. The study was published today on Science's Web site and will appear in a future issue of the journal.The Beta Pictoris star system is also thought to contain Pluto-sized planetary embryos farther out in its dusty disk. For more about Pluto and the planet search, check out my book, "The Case for Pluto." You can also join the Cosmic Log corps by friending me on Facebook or following b0yle on Twitter.