# Just a quick question;

1. Jan 27, 2009

### bassist_13

How do we know the mass and density of the planets in our solar system?
My physics teacher at college said that they were calculated using their orbits and the orbits of any moons they may have, but did not go into any more detail than that.

2. Jan 27, 2009

### mgb_phys

You can easily measure the mass of an object if anything else is orbiting around it.
All you need to know is the distance between the planet and it's moon and the time it takes the moon to go around.
You don't need to know the mass of the moon (assuming it's much less than the planet), this means you can't use the orbit to work out the mass of the orbiting object.
so we can use the Earth's orbit around the sun to work out the mass of the Sun but not the mass of the Earth, we can then use the Moon's orbit around the Earth to work out the mass of the Earth but not the moon. To measure the mass of the moon we have to put something else in orbit around it - like a spaceship

If your interested the formula is
time =$$2\pi \sqrt{radius^3 / GM }$$

Where M is the mass of the planet

Last edited: Jan 27, 2009
3. Jan 27, 2009

### bassist_13

Thank you, that has cleared that up for me