Have scientists seen planets in the making?

12:23pm, May 2, 2001
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Astronomers may finally have glimpsed a key step in the construction of a planet.

Both theory and observations suggest that the doughnut-shape disks of gas and dust that surround many newborn stars are the spawning grounds for planets. The Hubble Space Telescope has spied many such protoplanetary disks. Since 1995, astronomers have found evidence of more than 60 fully formed planets as massive as Jupiter orbiting nearby stars.

Yet astronomers had yet to spy a crucial intermediate step: To make a planet, the fine dust grains within a disk must coalesce into larger particles. Now, researchers say they've seen just that.

Studying the disk of a young star in the Orion nebula, a star-birthing region 1,500 light-years from Earth, scientists say they have evidence that dust particles, roughly the diameter of particles in cigarette smoke, have begun to clump. Henry B. Throop of the Southwest Research Institute in Boulder, Colo., and his colleagues, including Mark J. McCaughrean of the Astrophysical Institute in Potsdam, Germany, describe their study in the April 26 Science Express, the online version of Science.

Using Hubble, the collaborators examined the Orion disk along one of its outer edges, through which bright background light can pass. The team's key finding is that just as much visible light as longer, near-infrared wavelengths passes through the grains.

The only explanation for that finding, Throop says, is that the grains in this part of the disk are considerably larger than the near-infrared wavelength his team recorded. The team says the grains must be at least 5 micrometers in diameter, fully 10 to 20 times the size of particles inferred to reside in other observed disks. The inner part of the Orion disk may have assembled even larger, boulder-size lumps, Throop speculates.

"This seems to be the best proof so far that interstellar dust grains have begun the process of coagulating together in suspected protoplanetary disks," says Alan P. Boss of the Carnegie Institution of Washington (D.C.).

McCaughrean told Science News, however, that he's skeptical of his colleagues' interpretation of the data. Other Hubble images, although not as sharp, hint that more near-infrared light than visible light makes it through the edge of the disk, he notes. That suggests the grains are still small, 0.5 micrometer or so.

Moreover, previously published observations by McCaughrean and his collaborators indicate that the inner part of the same disk contains small dust grains that show no evidence of coagulation. Throop maintains that those particular grains, because they lie at the poles of the newborn star, may be irrelevant to planet making. However, says McCaughrean, the turbulent disk could have fine dust particles in many other regions.

Planet formation in Orion had better be quick, Throop notes. Ultraviolet light from the hot, massive star that illuminates the nebula will soon erode 90 percent of Orion's disks, boiling away most of their gas. As a result, a gas giant like Jupiter would have to form more rapidly than the standard model allows.

Calculations by Throop's team nevertheless show that grains at the core of the disks could rapidly grow to the size of gravel, at which point they could withstand bombardment by ultraviolet light and leisurely evolve into rocky planets or asteroids.

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