The list of planets around distant stars continues to grow, but because many of these planets orbit stars much larger than our sun, they pose a bit of a conundrum. Theory and observation agree that smaller stars like our sun form in the midst of vast protoplanetary disks [like the false-color one pictured at right] that swirl for millions of years before planets accrete within them. But the much greater power of more massive stars might simply vaporize such disks, never allowing planets to form. Now astrophysicists in France have taken a step toward resolving this conundrum by observing a protoplanetary disk of dust and gas around a star 2.5 times as large as the sun, revealing that the even planets of such big stars probably form in much the same way as their smaller relatives.
Pierre-Olivier Lagage of the astrophysics branch of France's Atomic Energy Commission (CEA) and his colleagues trained the Very Large Telescope's imager and spectrometer atop Cerro Paranal in Chile on HD 97048--a young, bright star 180 parsecs (600 light-years) away in the Chameleon I dark cloud stellar nursery. Because such a star is 40 times more luminous than the sun and emits far more high-energy photons, it may simply blow away any surrounding gas. But looking at its infrared emissions with the new eight-meter telescope, the scientists noted some peculiarities best explained by the presence of a large disk of circumstellar material--at least 12 times larger in spread than the orbit of Neptune. "It is arranged in a very extended and flared disk, which contains enough gas and dust to spawn planets," Lagage says. "The disk appears as a precursor of debris disks such as the one around Beta Pictoris and provides the rare opportunity to witness the conditions prevailing prior to or during planet formation."
These observations, reported online September 28 in Science, rest on some simplifications: that any infrared re-emission comes solely from the surface of the disk and that the intensity of such emissions varies with the distance to the star following a mathematical power law. "The star light is mostly absorbed at the surface of the disk, so that the assumption that the emission is from the surface is also reasonable," Lagage notes. "Since the Science paper, we have used more sophisticated models which fully confirm our findings."
Planet formation models remain largely untested, and this disk may provide an opportunity to refine them; it has enough gas to form 10 Jupiters and enough dust to form 50 Earths. It also may provide the first glimpse of a planet in the making. It would be very difficult to see such protoplanets, if any yet exist, Lagage notes. "My first answer is that it would not be possible at the moment, but I have not envisaged all the various possibilities of observation."