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.
Small clumps of material within a
disk stick together to form larger clumps. Eventually these clumps grow to
become 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.
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.
