Hello I am studying astro independently and have a question on keplers 3rd law and my math.(adsbygoogle = window.adsbygoogle || []).push({});

I set myself with a few givens with Kepler's laws

My first question is, I remember reading M had to be in solar masses, is this true?

For p^2=4π^2/G(M1+M2)/ a^3

Given: a= 1 AU

m1= .31 M☉(Used Gliese 581)

m2= .00095 M☉ (Used Jupiter)

We all know the gravitional constant is 6.67*10^-11

I plug in everything and get

39.47/6.67*10^-11(.31+.00095) *1^3

39.47/6.67*10^-11(.31095) *1^3

39.47/11.44031908×10^12

This is where I always screw up. In my notes I got 10.31065 I don't know how but after this I did:

39.47/10.31065

3.828

√p^2=√3.828

p=1.95 Years

Otherwise I would have got 12000132.94

Also when do I use p^2=a^3/(M1+M2) ?

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# Kepler's law questions

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