Astronomers using the Very Large Telescope (VLT) have caught an unprecedented glimpse of an exoplanet moving in its orbit around a distant star. Called Beta Pictoris b, the exoplanet has been directly imaged in two separate points covering nearly half of its orbit. The achievement could prove a significant stepping stone in our understanding of how planetary systems, including our own solar system, formed.

Since the discovery of the first extra-solar planet – or exoplanet – in 1995, over 450 more have subsequently been unveiled. Most of these other worlds have been indirectly inferred from their effect on the motion or light of their host stars. This latest research adds Beta Pictoris b to the handful of exoplanets actually captured on camera. The planet, with a mass of about nine Jupiters and orbiting as far from the star Beta Pictoris as Saturn does from the Sun, had been predicted to exist ever since its host star became the first to be found with a dusty disc of debris encircling it in 1984.

Subsequent investigation found that this disk was significantly warped and the presence of a planet was hypothesized to account for it. However, direct evidence of its existence had eluded researchers until now. "There were all these clues that there should be a planet orbiting the star but these were indications and not proof. Before these images we couldn't be sure," lead investigator Anne-Marie Lagrange, of the Laboratoire d'Astrophysique de l'Observatoire de Grenoble (LAOG), tells

Planet or rogue?

Not only have we confirmed it is a planet but we have seen it move almost half an orbit Anne-Marie Lagrange, LAOG

In fact, Beta Pictoris b was first directly imaged in 2003 using the European Southern Observatory's VLT, but the team couldn't be sure that it was a planet and not some other rogue source. It was spotted again in 2009 and improvements in observing techniques allowed astronomers to conclude that it is indeed an exoplanet – and that it had moved from its original position. "The source we detected in 2003 was in the north-east part of the disc and now it is in the south-west of the disc. Not only have we confirmed it is a planet but we have seen it move almost half an orbit," Lagrange says.

Beta Pictoris completed a little under half an orbit in six years, allowing Lagrange and team to estimate that the giant planet orbits at a distance 8–15 times further away than the Earth orbits the Sun. Pinning down its motion opens the door to calculating many of the planet's other characteristics, a chance Lagrange is keen to exploit.

"It is important because on a very short time scale, in terms of an astronomer's life, we have the opportunity to follow the complete orbit of an extrasolar planet. This will allow us to far more accurately model the history of the planet's formation," she says.

Modelling planet formation

This unprecedented chance to watch a planet sweep through the dust around its star is one that fellow planet hunter Carole Haswell, of the Open University, finds exciting. "This research is going to have a big impact. There is going to be a lot of theoretical work that will use this discovery as a touchstone to make sure that they are doing the right thing when it comes to modelling planet formation," she says.

Coel Hellier of Keele University who, like Haswell, studies exoplanets but was also not involved in the research, sees echoes our own planetary system in Lagrange's findings. "What we are seeing in Beta Pictoris is probably a very young example of what previously happened in our own solar system; Jupiter and Saturn probably formed in the same way as the planet seen in Beta Pictoris," he explains.

The observations are described in Science Express.