See Me Clumpin', I'm Accretin'

Scientists observing the fairly nearby star HD 100546 have noticed that within a cloud of dust and gas circling it, something's happening.

Specifically, they think they're seeing a planet form; a gas giant larger than Jupiter. The most common current theory for planetary formation is that they form from the clouds of dust and gas that remain when a star finally acquires enough mass to begin nuclear fusion. The cloud may be evenly distributed or it may already have thick spots, or the radiation from the new star may cause ripples that make even more clumps.

These clumps have greater gravity than just individual dust particles, so as they orbit their star, they will gather up the dust particles that they pass by. Some clumps will attract other clumps. making even bigger ones, and by this process -- over a looooong period of time -- we get planets of various sizes and compositions. Some form as balls of rock, like Mars or our own Earth. Others are gas, like Jupiter, or a mixture of ice and gas, like Neptune. The distance from the star plays a large role in what kind of planet forms.

When astronomers studied one of the gas clouds orbiting HD 100546, they saw what seemed to be a more coherent mass within the disc of orbiting gas. A couple of other explanations are possible, but they will continue to observe the dust cloud and see what develops. By doing so, and testing what they see against current planetary formation theory, they can learn quite a bit about the formation of our own planet, which none of us were around to see.

The interesting thing is that since HD 100546 is 335 light years away, the planetary formation may already have advanced quite a bit. We're seeing the star and its dust cloud as it looked in the late 1600's; if we develop a way to travel that distance in any practical period of time, we may find out the answer there long before we would ever see it here.