Pebbles from Heaven: Tracking planets in the making

Recording radio waves from the region around a young star, astronomers have for the first time documented a key step in the rocky road to planethood: making pebbles. The standard recipe for planet formation starts with a disk of gas, dust, and ice swirling around a newborn star. Particles within the disk coalesce into nuggets and then ever-larger clumps, which over several million years grow into a planet.

RAW MATERIAL. Pebbles dominate this sketch of a planet-forming disk around star TW Hydrae. B. Saxton, NRAO, AUI, NSF

Although astronomers have found dusty disks around many young stars, they had never before seen direct evidence that dust grains actually gather into pebbles. To detect the clumps, David Wilner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his colleagues relied on the Very Large Array (VLA), a network of radio telescopes near Socorro, N.M. The array picks up centimeter-long radio wave emissions, the radiation that centimeter-size pebbles would tend to emit.

Besides choosing the best detector, the team had to find the right star. It had to be young enough to still have a planet-forming disk but old enough to be past the time when radio emissions from the star’s own early growth would confound signals from the disk. At 10 million years of age, TW Hydrae fit the bill. It’s part of a group of young stars that lies 180 light-years from Earth.

The team’s observations indicate that particles with a diameter of 1.0 to 1.4 cm lie within the disk that surrounds TW Hydrae, Wilner and his colleagues report in the June 20 Astrophysical Journal Letters. The disk extends to more than four times Pluto’s distance from the sun and is heavy enough to make several planets.

“Their detection of the disk with the VLA at radio wavelengths is trailblazing and definitely exploits the potential of our radio telescopes to understand planet formation,” comments Paul Kalas of the University of California, Berkeley.

Additional evidence hints that at least one planet has already formed around TW Hydrae, Wilner says. Using previous infrared observations of the disk, Nuria Calvet of Harvard-Smithsonian created computer simulations suggesting that there’s a gap in the inner part of the disk. The gap would extend to a distance similar to the diameter of the asteroid belt in our solar system. A planet sweeping away dust might have cleared such a gap, the team asserts.

The pebble-size debris is relatively cold, indicating that it doesn’t reside any closer to the star than approximately the distance between Jupiter and the sun, notes Kalas. “This could mean that the pebbles [closer to the star] have become boulders and the boulders have become planets,” he says. “This is one of the of the most promising systems to search for baby planets.”

Ben Zuckerman of the University of California, Los Angeles cautions that a strong wind from the star—rather than the gravity of a planet—could have created an inner gap in the disk. But regardless of whether or not TW Hydrae harbors full-grown planets, “it’s nice to have the observational confirmation [of dust coalescing into pebbles] that we all believed was going on,” he adds.

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