It's a mess. A new study reveals that young planetary systems elsewhere are surprisingly violent, with huge collisions (inset) that produce bright but short-lived bands of dust.

Planet Building Isn't Pretty

Newborn planets in other solar systems endure "catastrophic" collisions for hundreds of millions of years, according to a new astronomical survey of nearby stars. An infrared telescope has revealed fresh clouds of dust from gigantic smashups between rocky bodies, signs that planet-building and destruction take longer and are more violent than astronomers had assumed.

Planet hunters have found more than 130 extrasolar planets, but nearly all are bloated gas giants like Jupiter. Evidence for worlds like Earth remains indirect. Astronomers infer the presence of small rocky objects that give rise to such planets by detecting warm disks of dusty particles girdling young stars. These "debris disks" are constantly fed by collisions among rocky bodies--the larger of which can survive and grow by continued accretion--because the tiny dust grains quickly fall onto the central star or get blown out of the planetary system. Astronomers didn't know how a debris disk changes as a parent star gets older because there weren't enough detailed studies of stars at various ages.

That has changed thanks to NASA's Spitzer Space Telescope, an infrared satellite launched last year (ScienceNOW, 18 December 2003). Spitzer is conducting a systematic study of the dust around more than 300 nearby stars, each about 2.5 times as massive as the sun. Initial results--described 18 October at a NASA teleconference--reveal numerous disks chock-full of warm dust at about the same distances from their stars as Venus, Earth, and Mars are from ours. The thickest dust disks are most prominent for the youngest stars, less than 100 million years old. But even for stars with ages of several hundred million years, Spitzer found a startling number of bright dusty clouds. The dust is so fleeting that it must have arisen from collisions within the last million years, says astronomer George Rieke of the University of Arizona, Tucson. "These are really grinding, catastrophic events," he says, noting that the doomed objects must be hundreds of kilometers across to produce as much dust as Spitzer sees.

Spitzer's observations represent "a fantastic series of snapshots" of how planets assemble even in chaotic settings, says theorist Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "Clearing out the debris [from planet formation] may take longer than we thought." The abundance of violent collisions provides reassuring evidence that there are enough rocky shards to accumulate Earth-like planets, Kenyon adds.

Related sites
Spitzer Space Telescope
MIPS instrument on Spitzer, used in study
Primer on planet formation
Scott Kenyon's home page

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