# Homework Help: Determining the orbital radius of extrasolar planets

1. Sep 7, 2010

### aggieg

1. The problem statement, all variables and given/known data
Basically I don't understand how you do this - the course I'm doing keeps mentioning the orbital radius of extrasolar planets but doesn't tell you HOW to find it?

2. Relevant equations

3. The attempt at a solution

2. Sep 7, 2010

### Delphi51

We have a formula relating the period of the orbit to its radius. It is easy to observe the period, so the radius can then be calculated. The longer answer is that the formula has the mass of the star in it and that mass is not easy to observe. I think some guessing is involved but I don't really know. Stars are very interesting and a lot is known about how they work, so perhaps there is a way to guess the mass fairly well from observations of the star's characteristics.

3. Sep 7, 2010

### aggieg

Hi Delphi
Yes, it's actually the formula I'm trying to find
Best wishes
AggieG

4. Sep 7, 2010

### Delphi51

There is a lovely way to find the satellite formula for circular motion. You begin by saying Fc = Fg
meaning that the centripetal force required to hold the planet in circular motion is provided by the gravitational force between planet and star. Then fill in the detailed formulas for Fg and Fc (choosing the one with the period T in it rather than the one with velocity).