Posted: 24 November, 2008
Inside the debris disc of Beta Pictoris lies a newly discovered object. If confirmed as a gas giant, it will be the first image of a planet that is as close to its host star as Saturn is to the Sun.
The hot star Beta Pictoris is one of the best known examples of stars surrounded by a debris disc – the dusty remains of past collisions among planetary building blocks and asteroids. Previous observations of the system revealed a peculiar shape to the disc, a secondary inclined disc, and comets tumbling towards the central star. With a projected distance from the star of only eight times the Earth-Sun distance, the new object discovered by a team of French astronomers using ESO's Very Large Telescope is most likely a giant planet that astronomers suspected might be lurking there all along.
"These are indirect, but tell-tale signs that strongly suggest the presence of a massive planet lying between five and ten times the mean Earth-Sun distance from its host star," says team leader Anne-Marie Lagrange. "However, probing the very inner region of the disc, so close to the glowing star, is a most challenging task."
This composite image represents the close environment of Beta Pictoris as seen in near infrared light. The newly detected source is more than 1000 times fainter than Beta Pictoris, aligned with the disc, at a projected distance of eight times the Earth-Sun distance. Image: ESO/A.-M. Lagrange et al.
The team imaged the Beta Pictoris system around five years ago using the NAOS-CONICA instrument (NACO) mounted on one of the 8.2 metre units of the Very Large Telescope (VLT). Recently, a member of the team re-analysed the data in infrared to seek the trace of a companion to the star. "For this, the real challenge is to identify and subtract as accurately as possible the bright stellar halo," explains Lagrange. "We were able to achieve this after a precise and drastic selection of the best images recorded during our observations."
A feeble, point-like glow well inside the star's halo was discerned, with independent analysis throwing up the same result: the companion is real. "Our observations point to the presence of a giant planet, about eight times as massive as Jupiter and with a projected distance from its star of about eight times the Earth-Sun distance, which is about the distance of Saturn in our Solar System," says Lagrange.
However, the team cannot yet rule out the possibility that the candidate companion could be a foreground or background object, and so the team will have to make some more observations to confirm the nature of the discovery, although the fact that the object lies in the plane of the star’s disc strongly implies that it is bound to the star and its proto-planetary disc. "Moreover, the candidate companion has exactly the mass and distance from its host star needed to explain all the disc's properties. This is clearly another nail in the coffin of the false alarm hypothesis," adds Lagrange.
Once confirmed, this candidate companion will be the closest planet from its star ever imaged directly, although planets have been inferred orbiting closer to their host stars in other studies using different methods. By studying different planetary systems, scientists can learn about the different formation processes of planets at varying distances from their host stars.
"Direct imaging of extrasolar planets is necessary to test the various models of formation and evolution of planetary systems. But such observations are only beginning,” says team member Daniel Rouan. “Limited today to giant planets around young stars, they will in the future extend to the detection of cooler and older planets, with the forthcoming instruments on the VLT and on the next generation of optical telescopes."
Beta Pictoris is 12 million years old and located about 70 light years away from Earth towards the constellation Pictor.