planet formation is violent, slow and messy
Planet formation is violent, slow and messy
11:45 19 October 04
NewScientist.com news service
The collisions that spawn planets are bigger and take place over longer periods of time than previously thought, say astronomers who studied nearly 300 stars with NASA's Spitzer Space Telescope. The new cache of data gives astronomers fresh hope that Earth-like planets might be common in the universe.
Planets are thought to grow as dust collides and sometimes sticks together in the discs around young stars. Until now, most theorists suspected the process was relatively smooth, with dust clumps building up gradually through successive collisions.
Some collisions would not result in a bigger clump, but a new cloud of dust, which would spread into rings around the central star. In a few tens of millions of years, these so-called debris discs were expected to disappear, being pulled into or blown away from the star, or swept up by growing planets.
Now, the Spitzer Space Telescope has revised that scenario by observing 266 nearby stars, 71 of which revealed the infrared glow of a debris disc. The discs display a surprising diversity, with some giant collisions occurring around stars considered much too old to sustain discs.
Grinding events
Massive discs were found circling some stars as old as 330 million years at extremely close range. At such close distances, any dust there would survive for less than a million years before being sucked into the star. That suggests the bright discs were replenished relatively recently by crashes between objects hundreds of kilometres across.
"These are really grinding, catastrophic events," says team member George Rieke, an astronomer at the University of Arizona in Tucson. He and colleagues described the results, which will be published in the Astrophysical Journal, in a NASA briefing on Monday.
"We know violent collisions were frequent in the early Solar System," says Scott Kenyon, an astronomer at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, commenting on the results.
He notes the Moon itself is thought to be the product of a smash between the infant Earth and another body the size of Mars, and that the Moon's many craters testify to a violent past.
Previously, Kenyon says, only a small number of debris discs had been imaged, providing astronomers limited insight into how similar our Solar System is to others in the universe. "If you only have six frames of any movie, you don't have a good idea of what the plot is," he says.
The new observations bolster the widely accepted theory that it takes between 10 million to 100 million years to form planets, Kenyon says. "But the end stages, where the debris left over is being cleared out, may take longer than originally thought."
The discs are likely to harbour planets, say the researchers. And because Spitzer probes warmer gas than previous satellites, the candidate planets orbit at distances similar to Earth's from the Sun - as opposed to Neptune's.
"These observations give us some hope that terrestrial planet formation is common," says Kenyon. They may also lead to more accurate estimates of the number of Earth-like planets that exist and to an understanding of how such violent collisions affect the stirrings of life, the team says.