The Orion Nebula seems an unfriendly neighborhood to build a planet. Several massive stars batter this large cloud of gas and dust with intense radiation and powerful winds, and there's always the risk that one of them will violently explode. But new observations confirm that some of the first steps of planet formation are going on here.
The primary ingredients for planet-making can be found in the silicate dust grains that swirl in thick disks around newborn stars. But just how the microscopic grains clump together to form meter-sized rocks--and eventually planets--remains a mystery. Even more mysterious is that the process seems to be taking place in the highly treacherous Orion Nebula, 1500 light years away. Home to thousands of stars, some of which are 10,000 times brighter than the sun, the nebula's ultra violet (UV) radiation boils gas and small particles out of disks. Yet astronomers recently reported in Science (1 June 2001, p. 1686) that the first steps of planet formation were occurring here.
To confirm the findings, astronomer Marc Kassis of the Keck Observatory in Hawaii and his colleagues took infrared spectra of 10 disks in the nebula. Compared to the earlier observation, which relied on indirect calculations using visible light, the infrared analysis allowed the team to directly detect signals from the grains themselves. The data show that the grains in these disks are at least a few microns in size, an order of magnitude larger than they started. "It resembles cometary dust more than primitive interstellar dust," says co-observer John Bally from the University of Colorado in Boulder. The team presented its work 9 January at a meeting of the American Astronomical Society in Washington, D.C.
But what about all of the UV radiation? Astronomer Henry Throop of Southwest Research Institute in Boulder thinks it could actually promote the growth of grains. His research indicates that the radiation removes gas, whose turbulent motion would otherwise prevent clumping. But that same UV light could place an upper limit on size, says astronomer Thomas Henning of the Max-Planck-Institut für Astronomie in Heidelberg, Germany, boiling away gas necessary for giants such as Jupiter and Saturn to form. This, he believes, suggests that our own solar system didn't start out in an Orionlike environment.