Planets are formed when a disk of dust orbiting a newly minted star condenses into blobs. The beginning and end of this process has been observed, but the intermediate steps have only been modeled. However, a team of astronomers now reports observations of this process in the intermediate stages. The conditions under which this observation is made are unusual, which is apparently the reason that the observation was possible.
Dust-sized material can be detected around distant stars because it gets warmed up by the star’s light and emits infrared light,…Large planets … have enough gravitational pull to produce wobbles in the motions of the stars they orbit, and these wobbles can be detected…But there’s no such indicator for the in-between matter. “When it comes to sand or boulders, they don’t emit light directly, and they don’t produce wobbles,” explains Winn, assistant professor of physics in MIT’s Kavli Institute for Space Science and Astrophysics. “So these in-between stages are still just theoretical.”The team’s new findings may have changed all that. The discovery of clear signs of sand-sized grains came as a complete surprise to the researchers, Winn says, as they studied a star called KH 15D, whose behavior had seemed just downright weird.Longer, darker dimmingLike an ordinary variable star of a type called eclipsing binaries, it seemed to go through regular, periodic dimming, but these dark spells lasted for much longer and were much darker than could be explained by one star blocking another. The star’s odd behavior was first documented in 1995 by William Herbst, an astronomer at Wesleyan University, and his graduate student Kristin Kearns. Later, observers noticed that the dark periods were getting longer and longer, and soon astronomers figured out that the dimming seemed to be caused by an unusually oriented disk of material surrounding a pair of stars.Like a windowshade drawn to block the light, the disk was blocking the view of the stars. But as they circled each other on long elliptical orbits, one of the stars would periodically get far enough to the side to peek out from behind the shade, allowing its light to be seen. And the angle was slowly changing, obscuring the star for longer and longer periods.Another surprise, seen from ChileThat solved the mystery of this oddball star’s on-and-off appearance, but the star had another surprise in store. Using the powerful Magellan telescope in Chile, a joint venture in which MIT is a partner, they noticed that when the star went dark it didn’t disappear completely: There was still a bit of light there.Analysis of the spectrum showed a revealing clue: Like most stars, light, as seen from Earth, was shifted toward the red end of the spectrum, showing that it was moving away from us. During the dark spells, the opposite happened–the light shifted toward the blue, indicating it was coming toward us, at exactly the same speed it had been retreating.The team soon figured out what this enigmatic shift meant. While the starlight was blocked by one part of the disk, the other side of the disk–the part farther away from us–was catching the star’s rays and reflecting them back toward us, like a huge mirror. As the star sped away from us, its mirror reflection appeared to be coming toward us.The light was not reddened, as it would be by dust particles, so the team calculated that in order to reflect the star’s light in this way, without changing its color, the disk had to be made of particles that are about 1 millimeter across–the size of a typical grain of sand.”We don’t know for sure” if the grains will indeed go on to form planets in this particular oddball stellar system, Winn says. “But it seems reasonable that the material is representative of the material that could have existed around our sun” as the planets began to form. And eventually, Winn says, “we ought to be able to tell what it’s made of.”
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