Astronomers have for a decade been studying very young stars for evidence of circumstellar disks of matter which theory suggests are the starting points for planets in a solar system. By 1990, they had spotted several young stars only a few million years old which seemed to have such disks based on observations made at infrared and radio wavelengths. These barely discernible disks are flattened collections of gas and dust which orbit the central star, and a 100 year old theory ( Kant-Laplace Dust Cloud Hypothesis) predicts that planets grow from such a primordial 'proto-planetary disk'. Theoretical models of how stars are formed also suggest that such disks ought to be very common because this is how a rotating, collapsing body 'solves' its so-called angular momentum problem.
The above picture was obtained by C.R. O'Dell (Rice U.), NASA, in 1994. A close-up of the Orion Nebulae, reveals what seem to be disks of dust and gas surrounding newly formed stars. These fuzzy blobs, called "proplyds", appear to be infant solar systems in the process of formation. Of the five stars in this field which spans about 0.14 light years, four appear to have associated proplyds - three bright ones and one dark one seen in silhouette against the bright nebula. A more complete survey of 110 stars in the region found 56 with proplyds.
Two very bright, naked eye stars, also seem to have circumstellar disks including Vega and a star called Beta Pictoris. The one around Beta Pictoris is, in fact, visible, if you first occult the light from the bright star itself. You can see a flattened disk of light about 50 times the size of our solar system. Careful studies of this disk now indicate that its central regions, about the size of our solar system, have been 'mysteriously' swept clean of small particles. But of course, if these disks can provide the conditions needed for forming planets, the process is not so mysterious at all. Many astronomers now feel that the central regions of the Beta Pictoris system have been swept clean because there are as-yet undetectable planets that have formed there and 'eaten up' all of the asteroidal material that once was there, and which now is only present in the outer regions of this disk.
Once the Hubble Space Telescope began to look at the sky with increased clarity, it made the spectacular discovery of an enormous number of proto-planetary disks ( Proplyds) associated with new-born stars in the Orion Nebula. What you will see in the Hubble Space Telescope images are pictures that show the multi-colored hues of the Orion Nebula, but you will also see many small, elongated clumps that are the Proplyds around the young stars. The environment of the Orion nebula is a harsh one. The luminous, massive stars that have formed there produce lots of UV radiation, and this together with the motion of the Proplyds through the gas in the nebula will cause some of these Proplyds to evaporate in time. Still, there are thousands of other interstellar clouds where Proplyds can continue to evolve, and may eventually form solar systems.
Like theory says, the observations of the Orion Nebula Proplyds show that they are very common. Nearly half of the young stars that have about the mass of the sun, sport these Proplyds. This could mean that half of all the stars you see in the sky could have had a proto-planetary disk phase, and if planets are not too difficult to form, there may be a LOT of solar systems out there!
Recently, planets have been detected around other stars. More on this can be found in my answers to the other questions in this Ask the Astronomer area.
Copyright 1997 Dr. Sten Odenwald
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