How Do Planets Form?
For centuries, astronomers and philosophers wondered how our solar system and its planets came to be. As telescopes advanced and space probes were sent out to explore, we learned more and more about our solar system, which gave us clues to how it might have taken shape.
But were our ideas right?
We could only see the end result of planet formation, not the process itself. And we had no other examples to study. Even with the knowledge gained about our solar system, we were left to wonder, are there other planetary systems out there, and did they form like ours? Discoveries made by the Hubble Space Telescope are helping us fill in key pieces to the puzzle of how planets form.
A cloud collapses to form a star and disk. Planets form from this disk.
According to our current understanding, a star and its planets form out of a collapsing cloud of dust and gas within a larger cloud called a nebula. As gravity pulls material in the collapsing cloud closer together, the center of the cloud gets more and more compressed and, in turn, gets hotter. This dense, hot core becomes the kernel of a new star.
Meanwhile, inherent motions within the collapsing cloud cause it to churn. As the cloud gets exceedingly compressed, much of the cloud begins rotating in the same direction. The rotating cloud eventually flattens into a disk that gets thinner as it spins, kind of like a spinning clump of dough flattening into the shape of a pizza. These "circumstellar" or "protoplanetary" disks, as astronomers call them, are the birthplaces of planets.
As a disk spins, the material within it travels around the star in the same direction. Eventually, the material in the disk will begin to stick together, somewhat like household dust sticking together to form dust bunnies. As these small clumps orbit within the disk, they sweep up surrounding material, growing bigger and bigger. The modest gravity of boulder-sized and larger chunks starts to pull in dust and other clumps. The bigger these conglomerates become, the more material they attract, and the bigger they get. Soon, the beginnings of planets — "planetesimals," as they are called — are taking shape.
When the Sun was young, it was encircled by a rotating disk of gas and dust. From this disk formed the planets.
The planets inherited their motion from the disk and today act like cars on a racetrack, all orbiting the Sun in the same direction and in roughly the same flat plane.
In the inner part of the disk, most of the material at this point is rocky, as much of the original gas has likely been gobbled up and cleared out by the developing star. This leads to the formation of smaller, rocky planetesimals close to the star. In the outer part of the disk, though, more gas remains, as well as ices that haven't yet been vaporized by the growing star. This additional material allows planetesimals farther from the star to gather more material and evolve into giants of ice and gas.
As each planetesimal grows bigger, it starts clearing out the material in its path, snatching up nearby, slow-moving rubble and gas while gravitationally tossing other material out of its way. Eventually, the debris in its path thins out and the planetesimal has a relatively clear lane of traffic around its star.
Hundreds of these planetesimals are forming at the same time, and inevitably they meet up. If their paths cross at just the right time and they're moving fast enough relative to each other, SMASH! — they collide, sending debris everywhere. But if they slowly meander toward one other, gravity can gently draw them together. They form a union, merging into a larger object. If the participants are farther apart, they might not physically interact but their gravitational encounter can pull each body off course. These wayward objects start to cross other lanes of traffic, setting the stage for additional collisions and other meetings of the rocky kind.
After millions of years, countless encounters between these planetesimals have cleared out much of the disk's debris and have built up much larger — and many fewer — objects that now dominate their regions. A planetary system is reaching maturity.
Piecing Together the Evidence
The Hubble Space Telescope has the best resolution of any telescope that observes visible light (the kind of light we see with our eyes). However, it still can't resolve planets outside our solar system very well because they are so far away. For example, you can take a much better and more detailed picture of a person if you stand five feet away than you can if you stand 50 feet away. Similarly, in Hubble images, planets in our solar system look big and have great detail because these planets are relatively close.
But planets orbiting other stars are light-years away, and so they only appear as small, faint dots — if we can see them at all. In order for Hubble to see extrasolar planets as well as it sees, say, Mars or Jupiter, Hubble would have to have a mirror about 100 miles wide! (Hubble's main mirror is actually less than 100 inches, or 2.5 meters, wide.)
How do we know all this? In part, because Hubble's exceptional vision has uncovered evidence in the disks around stars. This evidence helps to piece together the story of how planets form.
Until recently, we had only one planetary system — our own — to study in our attempt to understand how planets form. But in less than two decades, the Hubble Space Telescope has worked with other telescopes to open a window onto the mystery of planet formation. Hubble's ability to peer into nearby nebulae and to probe the regions around neighboring stars has shown us planetary systems under construction, the conditions planets form in, and even a planet orbiting another star. Hubble's discovery of a planet circling Fomalhaut replaced speculation with direct evidence that some of the strange features it has seen in disks could be caused by developing planets. Hubble's revelations have sometimes confirmed our ideas and sometimes showed us things we never imagined, all the while helping us better understand how planets form.