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Is this the lightest exoplanet imaged so far?

Jenny Winder, News Writer
Jun 5, 2013, 7:00 UTC

Sen—A team of astronomers has caught on camera what is probably the least massive planet to be directly observed outside the Solar System.

The object, which has been designated HD 95086 b, appears as a faint but clear dot close to the star HD 95086 in the constellation of Carina (The Keel). A later observation also showed that it was slowly moving along with the star across the sky. This suggests that it is in orbit around the star. Its brightness also indicates that it has a predicted mass of only four to five times that of Jupiter.

Astronomers have confirmed the existence of over 850 planets orbiting stars other than the Sun. Almost all were found using indirect methods that could detect the effects of the planets on their parent stars, the dips of brightness produced when planets crossed in front of them (the transit method), or the wobble caused by the gravitational pull of planets in their orbits (the radial velocity method). So far, only a dozen exoplanets have been directly observed.

"Direct imaging of planets is an extremely challenging technique that requires the most advanced instruments, whether ground-based or in space," says Julien Rameau (Institut de Planetologie et d'Astrophysique de Grenoble, France), first author of the paper announcing the discovery. "Only a few planets have been directly observed so far, making every single discovery an important milestone on the road to understanding giant planets and how they form."

The team used NACO, the adaptive optics instrument mounted on one of the 8.2-metre Unit Telescopes of the European Southern Observatory's Very Large Telescope (VLT). This allows astronomers to remove most of the blurring effects of the atmosphere and obtain very sharp images.The observations were made using infrared light and differential imaging, which improves the contrast between the planet and dazzling host star.

The star itself is a little more massive than the Sun and surrounded by a debris disc. Astronomers have identified it as an ideal candidate to harbour young massive planets. The youth of this star, just 10 to 17 million years, leads astronomers to believe that this new planet probably formed within the disc of debris.

This picture shows the sky around the young star HD 95086 in the southern constellation of Carina. Image credit: ESO/Digitized Sky Survey 2. Acknowledgement:  Davide De Martin.

The newly discovered planet orbits the young star at a distance of around 56 times the distance from the Earth to the Sun, twice the distance from Neptune to the Sun. The whole system lies some 300 light-years away from us.

"Its current location raises questions about its formation process. It either grew by assembling the rocks that form the solid core and then slowly accumulated gas from the environment to form the heavy atmosphere, or started forming from a gaseous clump that arose from gravitational instabilities in the disc" explains Anne-Marie Lagrange, another team member. "Interactions between the planet and the disc itself or with other planets may have also moved the planet from where it was born."

Another team member, Gaël Chauvin, concludes, "the brightness of the star gives HD 95086 b an estimated surface temperature of about 700 degrees Celsius. This is cool enough for water vapour and possibly methane to exist in its atmosphere. It will be a great object to study with the forthcoming SPHERE instrument on the VLT. Maybe it can also reveal inner planets in the system, if they exist."