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New Space Telescope Observations Upset Theories of Planet Formation

The leading theory of how planets form is being revised because of new observations from the U.S. Spitzer Space Telescope. The process is not as smooth and quiet as once thought, but rather the result of violent collisions between massive space rocks.

If you look at the dust under your bed long enough, you will see it clump together into bigger dust balls. This is a rough analogy to the process astronomers thought caused planets to form.

The theory suggests that the disk of dust swirling around a star, a remnant of that star's formation, merges through gravity. U.S. space agency astronomer Jonathan Gardner says the grains gather to form small clumps that attract other clumps to make bigger ones, and so on relatively smoothly over a few million years, or so it was thought.

"Particles in the disk clump together, forming planetessimals, which then collide with each other, building up to form the planets. The planets then sweep up or dissipate the disk," he said. "Now, we will have to rewrite the text books because these observations reveal that view to be wrong."

But the Spitzer Space Telescope has observed a more varied, longer, and more violent planet forming environment in nearly 300 stars at distances between 20 and 500 light years from Earth.

A team led by University of Arizona astronomer George Rieke found that the earlier view of planet formation was just the beginning of the story, not the entire tale.

"There are lots of steps that go from a huge number of motes [particles] of dust to a terrestrial planet like the Earth," he said. "Previously we've known where this started and we've known where it ended, but we've had only a few clues about what happened in between."

Now, Mr. Rieke's team has observed that intermediate period with the Spitzer observatory. He found that planet formation is not only the gradual gathering of material through gravitational attraction between dust grains and eventually the collisions of the small bodies they become. There seems to be a long subsequent period of massive collisions between rocky bodies as big as mountain ranges that break them down and cause them to reform into bigger ones.

Mr. Rieke says previous views did not account for this stage. "Some earlier theories had planets build a little bit like a vacuum cleaner would make the dust build. That is, you have a large planet, say the dust in the bag of the vacuum cleaner, and you keep sucking more dust grains in and joining them rather than having planets that are a couple hundred miles (kilometers) in diameter run into each other or even bigger objects running into each other, creating the huge destruction that then has to reassemble itself into a full-sized planet."

The Spitzer telescope, which was launched 14 months ago, saw the dusty aftermaths of these collisions with its powerful infrared vision. The dust was warmed by the central star and the telescope measured this heat radiation, which is emitted in wavelengths below visible red light.

By measuring the infrared radiation around the stars, the astronomers concluded that some older stars had more massive dust disks than would have been expected under the old notion of planet formation. Only collisions between massive objects would account for this much dust so late in the age of the stars.

Theorist Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts says the new findings help us understand how our own solar system formed.

"These really large collisions between 10 and 100 mile-across objects are really important and occur at much later times than we thought previously," he said. "We have a history in our solar system, but we don't know how typical that is. The exciting thing theoretically is to show that these collisions are very common, they are very frequent in the early stages, and that probably means that planets are forming around all these stars."

Before the Spitzer telescope, older infrared observatories had discovered only a few dozen planet-forming disks around stars older than a few million years. Spitzer's far more sensitive instruments allowed it to detect the dim heat of many more discs, leading to this new view that will cause old theories to be revised.