Orion Nebula Gives Clues To Formation of Planets

THE spectacular Orion Nebula has yielded "the best evidence to date that planetary systems are quite common" in our galaxy.

That's the way Ray Villard at the Space Telescope Science Institute in Baltimore sums up the Institute's latest release of Hubble Space Telescope discoveries.

Astronomer C. Robert O'Dell of Rice University in Houston and his colleagues have been using the orbiting observatory to look for what he calls "fine scale structure" in the famous nebula. What they found, he says, "are stars which have around them disks of dust and gas."

In fact, about half of a sample of 50 young stars still in the process of formation appear to have such disks.

Dr. O'Dell notes that, according to the standard theory in astronomy, stars form when a primordial cloud of dust and gas condenses. He explains that the kind of circumstellar disk his group has found "is the material that is not going to make it into the star ... [and] is available to build planets."

Up to now, astronomers have confirmed the presence of such so-called protoplanetary disks around only four stars - Beta Pictoris, Alpha Lyrae, Alpha Piscis Austrini, and Epsilon Eridani. O'Dell explains that these four stars are mature. He calls their disks "failed" planetary systems.

O'Dell also says that the stars his group is studying in the Orion Nebula "are very young stars, still in a state of collapse." It is quite possible that planets could form in their surrounding disks, which O'Dell calls "proplyds" for "protoplanetary disks."

The Hubble pictures show each of these proplyds as a thick disk with a hole in the middle where the star is located, according to the Space Telescope Science Institute announcement. They also show streams of material boiled off the disks and trailing away from the young stars in comet-like tails.

One picture shows a shock wave caused by material moving at the hypersonic speed of 148,000 miles an hour.

Dr. O'Dell explains further that such fast-expanding shells of gas may be the result of jets that came out of stars while they were in the process of formation. As they came out, the jets interacted with interstellar gas to form shock fronts.

The photos show details as small as 50 astronomical units. An astronomical unit is the average distance from Earth to the sun - 149.6 million kilometers or 92.95 million miles.

In this research, O'Dell is working with astronomer Jeff Hester of Arizona State University at Tempe and Rice graduate students Zheng Wen and Xi-Hai Hu. He emphasizes that no one has seen a planet in another star system yet.

However, he says that "finally, we are seeing some" of the details of star and protoplanetary disk formation that theory has predicted.

He adds, "I've worked on the Space Telescope [program] for 20 years and this has to be the funnest stuff."

The Orion Nebula is a vast cloud of gas and dust about 1,500 light-years from Earth. Seen with a small telescope, it appears as a bright greenish mist in the middle of the star pattern that forms the "Sword" of the constellation Orion. It shines by flourescent light that is energized by ultraviolet radiation from nearby stars. It can be seen with binoculars, or even by the naked eye on clear dark nights.

The Orion Nebula has intrigued astronomers and poets for centuries. The 19th-century English poet Alfred Tennyson wrote of its "regions of lucid matter taking form" in his poem "Merlin and Vivien." Now O'Dell says that this latest discovery "ties into the whole idea of Orion being a stellar nursery."

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