View Full Version : What does current theories say about planets?

2008-Sep-25, 10:55 AM
First: I admit the following questions may appear a little confused (not only because of possible erratic english). My excuse is the news we get from research of extrasolar planets -or "planetlike/planetsized objects" seems confusing. Is there any good or favored theories at the moment?
Is there any reason these kind of not easily detected objects should typically orbit a central star? Or are "freefloating" - rogues - probably more common? - Or entirely "dark systems"?
Another related question: May the "typical solar(or dark) system" be rather stable - a lifelong celestial "family", or may stars and planets frequently change partners(like a lot of people do - each eyar at least one divorce and marriage - and a lot of time a life as single)?
Now and then a collision (as in recent article).
We should of corse expect theorists to avoid biases and cirkular arguments. - Like basing theoretical models on observations,because some objects will be hard or impossible to detect before instruments improve.
So, this reader is a bit confused. DCan the professional community offer a more clear picture?

Jeff Root
2008-Sep-25, 05:26 PM
Stars and planets form together, when a giant molecular cloud collapses
either because it cools off and condenses or because it is compressed
by nearby supernovae, or both. When the density of particles becomes
great enough, collisions become more frequent. Even if the particles do
not stick together, at least one of the particles will likely have a reduced
speed relative to the cloud's center of mass. Gravity pulls them toward
the center of the cloud. When the friction between colliding particles
becomes great enough, the whole cloud collapses into a disk. The high
density of particles in the disk makes collisions frequent, and the low
relative speeds make particles likely to stick together by static charge
when they collide. Planetesimals grow from these small clumps. The
larger they become the better the planetesimals are able to hold on to
particles and other planetesimals when they collide. Gravity keeps them
together. If they become large enough, they are planets, otherwise
they are asteroids and comets.

Asteroids and comets are easily thrown out of a forming planetary
system when they pass close to a very massive planet. Planets, though,
are almost never thrown out of the system they form in. Only a close
encounter with a star from a different planetary system would be able
to throw a large planet out of its own system.

Stars are so small in relation to the distances between them, that
collisions are *extremely* rare.

-- Jeff, in Minneapolis