| EXOPLANETS - Capturing distant worlds on film
Very close to the young brown dwarf is a cold object, clearly visible. This sensational picture is causing a frenzy of excitement among astronomers who interpret it(1) as the first picture ever taken of a planet orbiting a star outside the Solar System (an exoplanet). It was taken by a team from the astrophysics laboratory in Grenoble (FR), using the NACO optical system on the European Southern Observatory’s (ESO) Very Large Telescope in Chile.
The presence of a planet, comparable to those in our own Solar System, orbiting the 51 Pegase b star – a star similar to our own Sun – was first detected in 1995. This exoplanet had never been seen before, although its existence had been deduced from the tiny changes to the light emitted by its star when it passed in front of the observation telescope.
|In the centre, in blue, the brown dwarf 2M1207. On its left, in red, the celestial body identified as its exoplanet.|
Since then the world of astronomy has been in a frenzy of excitement due to a series of discoveries of this kind, telling us a great deal about the very diverse mechanisms of planetary system formation. It is knowledge of this kind that may one day lead us to indications of extraterrestrial life. About 2 000 stars similar to the Sun are today being observed and the presence of 123 exoplanets have already been inferred by the star hunters.
The picture taken from the ESO’s Mount Paranal Observatory in Chile, in June 2004, marks a dramatic step forward in the search for an actual – rather than deduced – sighting. “This picture is a first step towards opening up a whole new field in astrophysics: the study of planetary systems using imaging and spectroscopy,” explains Anne-Marie Lagrange of the Grenoble Observatory (FR) which participates in the work carried out at the ESO. “Direct observation will enable astronomers to characterise much more precisely the physical structure and chemical composition of exoplanets.”
The exoplanet was caught orbiting an astral body known as 2M1207. Located just (in space terms) 230 light years from Earth, this young brown dwarf – that is, a cool and dim star, akin to a ‘failed sun’, whose mass and temperature are too low to sustain nuclear fusion – was itself only identified very recently, in April 2004. It lies in the TW Hydrae stellar association, a group of nearby stars, just a few million years old. “Whatever their nature, substellar objects are hotter and more brilliant when they are young (10 million years old) and are, therefore, more easily detectable than older objects of similar mass,” stresses the ESO astronomer Gaël Chauvin, coordinator of the joint European-US team that has been studying this phenomenon since 1998.
As for this young exoplanet itself, it is still ‘warm’ (1 000°C), which explains its visibility. It is five times the mass of Jupiter (or almost 1 600 times that of the Earth) and its orbital trajectory is about 55 times the distance between the Earth and the Sun (8 billion km). Its light spectrum shows the presence of water molecules. For Christophe Dumas, another astronomer engaged in this adventure, “it is incredible to actually be able to see this weak source of light in real time. It is a strange feeling when you think that this may be the first picture ever seen of a planetary system other than our own.”
(1) Although this interpretation is considered to be very probable (over 99%), it will be another one or two years before it will be possible to give 100% confirmation that the object identified is a planet. That is why the exoplanet is still referred to as a Giant Planet Candidate Companion (GPCC).