CEA

It is now almost ten years since astrophysicists first detected planets outside our own solar system. Today, more that 200 exoplanets have been identified. New planets form in the disc of gas and dust surrounding young stars. Paradoxically, we know relatively little about these discs, especially those around stars with a greater mass than the Sun.

Astrophysicists from CEA/Dapnia and the University of Paris 7, in collaboration with the Grenoble Astrophysics Laboratory, the Orsay Space Astrophysics Institute and the University of Groeningen in the Netherlands, have succeeded in mapping the disc surrounding the star HD 97048. This particular young star (just a few million years old compared with the 4500 million years of the Sun) has 2.5 times the mass of the Sun and shines 40 times brighter. HD97048 is 600 light-years from Earth in the constellation of Chamaeleon (southern hemisphere). It was observed using the Visir infrared camera which provides high-acuity images of the infrared emissions from celestial bodies. This instrument, designed and built by CEA/Dapnia and the Astron Institute***** in the Netherlands for the European Southern Observatory, has been permanently installed on the Melipal telescope, one of the four 8-metre telescopes built by the ESO on Mount Paranal in Chile.

The images from Visir show a disc in its early stages of development extending out to a radius of at least 370 astronomical units******. The geometry of this disc is unusual in that it is not flat, but widens out as the distance from the star increases, reaching an eventual thickness of 360 astronomical units. Although predicted by some models, this is the first time that such a disc has been directly observed around a massive star. With such a geometry, every point on the surface of the disc receives light from the star. This light is absorbed by the dust at the surface of the disc which keeps the disc relatively ‘hot’ even at a distance from the star.

This morphology is only possible if the disc still contains a large quantity of gas. In the case of HD97048, the mass of gas is estimated to be at least ten times the mass of Jupiter. A large quantity of dust (more than fifty times the mass of the Earth) has also been observed. This is an excellent example of a protoplanetary disc as it contains sufficient material for planets to form. In the coming months, this disc will be the target of a number of observation programmes, with the particular aim of zooming in on internal denser regions of the disc where embryonic planets may already have formed.

*European Southern Observatory.
**Department of Astrophysics, Particle Physics, Nuclear Physics and the Related Instrumentation.
***Exoplanets are planets that orbit stars other than the Sun.
****Science Express is the online version of the journal Science. Information published on Science Express will also be published in the next issue of Science.
*****ASTRON: The Netherlands Foundation for Research in Astronomy.
******One astronomical unit (AU) is equal to the distance between the Earth and the Sun.