Scientists sleuthing in nearby nebulae with the Hubble Space Telescope have seen flat disks of gas and dust around half of the young stars they looked at, adding evidence to support the tantalizing prospect that planets may be awaiting discovery outside our solar system.
C. Robert O'Dell of Rice University said Wednesday that photos taken by the space telescope of part of the Orion Nebulae showed at least 15 stars surrounded by what he characterized as "protoplanetary disks," or bands of dust of the type thought to form planets.
Astronomers have long known about what is thought to be a protoplanetary disk girdling Beta Pictoris, a relatively mature star. Earlier this year, astronomers revealed infrared images of similar disks around eight other, younger stars in the enormous gas cloud embracing the Taurus and Auriga constellations.
However, Stephen E. Strom, a University of Massachusetts astronomer who specializes in the study of star formation, said at a NASA press conference Wednesday that the material detected in that work "was very thin and nebulous when compared to the thick disks that we see here."
No one has yet made a confirmed sighting of planets outside the solar system, though they are generally believed by scientists to exist. The visual sighting by O'Dell and his colleagues of so many disks around relatively new stars--those roughly 1 million years old--supports a "nebular hypothesis" of planet formation.
"Hubble certainly has not found planets . . . but it has found all these disks," said Ray Villard of the Space Telescope Science Institute at Johns Hopkins University in Baltimore. "Whatever (eventually) happens to these things, the fact you can see so many of them confirms many theories of planet formation."
The nebular hypothesis, which was advanced by Immanuel Kant and Pierre Simon Laplace two centuries ago, suggests that the solar system condensed from a cloud of dust and gas. The sun swirled into being first, followed by a disk around its equator that eventually condensed into planets.
This hypothesis is supported by the fact that the planets--except Mercury, which is closest to the sun, and Pluto, which is farthest away--lie in a flat plane within three degrees of one another and within six degrees of the sun's equator.
The planets--again, slightly excepting Mercury and Pluto--also all revolve in nearly circular orbits in the same direction that the sun itself is rotating.
Key to the theory is the idea of disks around young stars. That is what was observed by the space telescope.
"This provides the first real visual demonstration that these inferences are correct," said Strom. "I regard this as dramatic proof."
O'Dell said the disks found around those new stars in the Orion Nebulae could resemble what the Earth and its solar system neighbors may have looked like when they were forming around the sun 4.6 billion to 5 billion years ago.
The part of the Orion Nebulae where these new stars were observed is 1,500 light-years from Earth. A light-year is about 6 trillion miles.
"The disks are a missing link in our understanding of how planets like those in our solar system form," he said.
He added that the abundance of disks seen by the telescope suggests that planetary building blocks may not be rare.
"We now have direct evidence that material that can make planets exists around about half of the stars like our sun," O'Dell said. "It's much more common than we thought before."
If what appears to be common in Orion is extrapolated throughout the universe, there may indeed be billions of stars that had the opportunity to form planets.
Still to be confirmed is how often that opportunity is realized. Theorists suspect that material in some disks could simply fail to condense because it is first blown away by the solar winds of nearby stars or for some other reason.
Finding actual planets around other stars is the next step for astronomers.
Direct observation of distant planets would be difficult because such objects are much smaller and dimmer than stars. But circumstantial evidence--such as a slight wobble in solar rotations, presumably caused by gravity exerted by revolving masses--hints that other planets are out there.
Villard, of the Space Telescope Institute, said discovery of such new planets may be possible with the new wide-field optical camera that is scheduled to be installed late next year in the Hubble.
The new instrument, a more powerful version of the camera used to locate O'Dell's disks, is designed to compensate for the satellite's flawed main mirror.