June 11, 2010
Astronomers have been able to directly follow the motion of an exoplanet as it moves to the other side of its host star. The planet has the smallest orbit so far of all directly imaged exoplanets, lying as close to its host star as Saturn is to the Sun.
Only 12 million years old, or less than three-thousandths of the age of the Sun, Beta Pictoris is 75% more massive than our parent star.
It is located about 60 light-years away towards the constellation of Pictor (the Painter) and is one of the best-known examples of a star surrounded by a dusty debris disc, which are composed of dust resulting from collisions among larger bodies such as planetary embryos or asteroids. They are larger versions of the zodiacal dust band in our Solar System. The disc around Beta Pictoris was the first to be imaged and is now known to extend up to about 1000 times the distance between the Earth and the Sun.
Earlier observations showed a warp of the disc, a secondary inclined disc and comets falling onto the star. "Those were indirect, but tell-tale signs that strongly suggested the presence of a massive planet, and our new observations now definitively prove this," says team leader Anne-Marie Lagrange. "Because the star is so young, our results prove that giant planets can form in discs in time-spans as short as a few million years."
Recent observations have shown that discs around young stars disperse within a few million years, and that giant planet formation must occur faster than previously thought. Beta Pictoris is now clear proof that this is indeed possible..
The short period of the planet will allow the team to record the full orbit within maybe 15-20 years, and further studies of Beta Pictoris b will provide invaluable insights into the physics and chemistry of a young giant planet's atmosphere.
The planet has a mass of about nine Jupiter masses and the right mass and location to explain the observed warp in the inner parts of the disc. This discovery therefore bears some similarity to the prediction of the existence of Neptune by astronomers Adams and Le Verrier in the 19th century, based on observations of the orbit of Uranus.
"Together with the planets found around the young, massive stars Fomalhaut and HR8799, the existence of Beta Pictoris b suggests that super-Jupiters could be frequent byproducts of planet formation around more massive stars," explains Gael Chauvin, a member of the team.
Such planets disturb the discs around their stars, creating structures that should be readily observable with the Atacama Large Millimeter/submillimeter Array (ALMA), the revolutionary telescope being built by ESO together with international partners.
A few other planetary candidates have been imaged, but they are all located further from their host star than Beta Pictoris b. If located in the Solar System, they all would lie close to or beyond the orbit of the furthest planet, Neptune. The formation processes of these distant planets are likely to be quite different from those in our Solar System and in Beta Pictoris.
"The recent direct images of exoplanets — many made by the VLT— illustrate the diversity of planetary systems," says Lagrange. "Among those, Beta Pictoris b is the most promising case of a planet that could have formed in the same way as the giant planets in our Solar System."
Casey Kazan via ESO