Every newborn star is encircled by a disk of gas and dust, like a broad, thick pancake. Most of the material in such a disk will be funneled into the star itself. But in many star systems, some of the material in the disk will give birth to planets. Just how that process works, though, is still a subject of intense debate.
The leading idea says that the microscopic grains of dust stick together to form clumps the size of pebbles. Their gravity then pulls the pebbles together to form chunks the size of a large city. Those chunks, in turn, stick together to form planets. This idea has some problems, but it seems to work better than the other leading contender.
That other idea says that dense clumps of material in the disk collapse under their own gravitational pull. This idea would work only at a pretty good distance away from the star. It could build giant planets like Jupiter, but not little ones like Earth.
It's possible that both processes are at work in different regions of the disk, giving birth to small planets close in, and giant planets farther out.
Scientists do agree that planets are born in a hurry. Observations show that about half of all stars have lost most of their surrounding gas and dust within about two million years of their birth. Since most stars live for billions of years, the era of planet formation ends in a hurry -- no matter how it happens.
We'll have more about the birth of planets tomorrow.
Script by Damond Benningfield, Copyright 2010
Today's program was made possible in part by the NASA Science Mission Directorate.