Kepler's 3rd Law, circular orbit, 2 planets

[zwingtip]

Homework Statement

http://www.its.caltech.edu/~tmu/ph1a/FPs/fp12.htm
this is the question.

Homework Equations

$$T^2=\frac{4\pi^2a^3}{GM}$$

$$E=-\frac{GMm}{2a}$$

The Attempt at a Solution

I have none. My question is, does the fact that there are two planets affect the equations for period and energy? Without knowing this, I can't solve any of it. Help? Slightly urgent. This problem is due tomorrow at 4pm PST.

 

[Schr0d1ng3r]

Well, since m << M, I would say that you can ignore any effects which the planets have on each other, so no, you don't need to change the period and energy equations.

 

[zwingtip]

Thanks.