[lssndrdn]

To find out what it's made of, they are thinking of using spectroscopy.

To find out the density, it's mass over volume. They infer the mass by the planet's distance from its star and by its orbiting speed, and they get the volume by the amount of light it takes away from the star when it pases in front of it.

[bioweek]

Couldn't any mass planet be going any speed at any distance around a star? I'm not sure why these factors would depend on mass.

[hugh_]

You're correct -- the orbital speed and path tell you nothing about the mass. A grain of dust will follow exactly the same orbital path as, say, Jupiter.

It's the redshift/blueshift of the actual star that you need to measure.

[bioweek]

Oh so when the planet is between us and its star, it pulls the star towards us which blueshifts the light?

That sounds hard to detect.

[fh]

> That sounds hard to detect.

Well, obviously you need really precise spectrometers, but just because the effect is tiny, doesn't mean that you can't detect it.