For the first time, astronomers have been able to directly follow the motion of an exoplanet as it moves from one side of its host star to the other. The planet has the smallest orbit so far of all directly imaged exoplanets, lying almost as close to its parent star as Saturn is to the Sun. The star, beta Pictoris is only 12 million years old, and so this exoplanet confirms that gas giant planets can form very rapidly—in only a few million years—within such circumstellar disks, and experts say that this discovery validates the theory that these unique, gaseous disk structures can be used as a sort of “fingerprint” to characterize their embedded planets.
Beta Pictoris is 75% more massive than our Sun, and is located about 60 light-years away towards the constellation of Pictor (the Painter). This is one of the best-known examples of a star surrounded by a dusty debris disc. 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,” said 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.”
This exoplanet, dubbed Beta Pictoris b, was thought to have been spotted first in 2003, and then was first imaged back in 2008. But the astronomer couldn’t rule out definitively that the possible planet wasn’t just a foreground or background object. These new observations confirm that, indeed, the object is a gas giant planet orbiting the star.
Other 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.
Only about ten exoplanets have been imaged, Beta Pictoris b, has the smallest orbit known so far. It is located at a distance between 8 and 15 times the Earth-Sun separation — or 8-15 Astronomical Units — which is about the distance of Saturn from the Sun.
“The short period of the planet will allow us 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,” said student researcher Mickael Bonnefoy.
The planet has a mass about nine times that of Jupiter, 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.
The team used the NAOS-CONICA instrument mounted on one of the 8.2-metre Unit Telescopes of ESO’s Very Large Telescope (VLT).
These most recent observations, taken during autumn 2009, revealed the object on the other side of the disc from where it was seen in 2008, and after a period of hiding either behind or in front of the star (in which case it is hidden in the glare of the star). This confirmed that the source indeed was an exoplanet and that it was orbiting its host star. It also provided insights into the size of its orbit around the star.
“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,” said team member Gael Chauvin.
“The recent direct images of exoplanets — many made by the VLT— illustrate the diversity of planetary systems,” said 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.”