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nspande
2009-May-04, 02:59 AM
Hi there all,

Ok, so I read a bunch about astronomy, listen to Astronomy Cast, the Jodcast, and read Universe Today on occasion. I've recently been thinking about exoplanets, and have a question about how astronomers determine their sizes.

I understand that the mass of a planet will alter the motion of its star, and that the larger the planet, and the closer it is, the more the motion of the star will be altered. I also know that most planets that have been discovered recently are more than multiple Jupiters in mass.

Here is my question. How is it possible to determine whether it is a single planet causing the disruption in mass, versus multiple planets? For example, if I have a 1 Jupiter mass planet at 1au, and a 5 Jupiter mass planet at 3au (merely examples, I know that won't actually work out) how do I distinguish between that and an 8 Jupiter mass planet at, say, 4au?

Thanks,
Nathan Spande

George
2009-May-04, 03:17 AM
The 1AU planet will be orbiting faster than the 3AU planet, thus the variations that led to their discovery will be notably different over time.

WayneFrancis
2009-May-06, 03:37 AM
The reason you can tell it is 1 planet and not multiple planets is the odds are that 2 or more planets will not have identical orbits so that their gravitational attraction will not be in sync. It is the same way we detect multiple planet systems. The wave caused by the wobble of the star has other waves on top that are not synchronized with the main wave.

chornedsnorkack
2009-May-06, 09:57 AM
The reason you can tell it is 1 planet and not multiple planets is the odds are that 2 or more planets will not have identical orbits so that their gravitational attraction will not be in sync. It is the same way we detect multiple planet systems. The wave caused by the wobble of the star has other waves on top that are not synchronized with the main wave.

If the planets are in 1:2 resonance then their gravitational attraction is in sync. And looks much like one planet on an elliptical orbit.