Thread: Exoplanets - can someone explain?

hi,

I have a few questions about the recent discoveries of exoplanets we have been discovering of late.

Generally, these planets have masses and size in excess of Jupiter, orbit their stars in 'days' with orbitals the diameter of our own Mercury - around stars that seem to be well on their way to red-giants, ie large.

What I have been wondering is if anyone has calculated what the orbital velocities must be, the rates of rotation of the planets and the stars and how it is physically possible for these systems to be stable according to the known laws of physics.

Why dont these planets, that must have huge orbital velocity fly apart or get flung off orbit.

It all seems a bit impossible to me... Or perhaps there is some funky relativity thing going on I dont quite understand - that means it all looks a bit magnified and sped up when we observe these systems.

As far as I can tell by a quick review of the Exo-planet list, the super Jupiters that orbit close in to a star tend to orbit small red dwarf stars. For example, HD 41004 B b, which has a mass 18.4 times that of Jupiter, orbits at a distance of 2.6 million km, in a period of 1.3 days. It orbits a star with a mass of only 0.4 that of the Sun.

and using the formula for orbital period:

T = 2*pi* sqrt(a³/(G*M))

a = average distance of orbit
G = gravitational constant
M = mass of the star.

we get 2 * pi* sqrt((2.6e9)³/(6.673e-11/(8e29)) = 114007.7 sec = 1.3 days

This is exactly the right orbital speed.

Part of why all the planets they find are like this is because of how they observe them. The planet has to actually cause its star to "wobble" due to its rotation.

You're right that a planet with enough mass, and orbiting near enough to be able to actually move its star around when it orbits should probably be a very rare occurrence.

What's that tell you?

If the only ones they can see are ones that are doing something extremely uncommon........... and they *are* seeing a few of these, just imagine how many ordinary planets must be out there that they aren't seeing.