Exosolar planets are planets outside our solar system. They are planets that are hundreds of light-years away. These are planets that are usually discovered by the way they distort the light of the stars around them--no clear photos here.
So, how do scientists determine what a planet is made of?
The answer involves a little math. To get the density of a planet, one must know the mass and volume, since the formula for density is mass divided by volume.
Determining an exosolar planet's mass involves observing the star it orbits. Because of their gravitational pull on one another, the star will "wobble" slightly. By measuring how much the planet makes the star "wobble", it's then possible to determine an accurate mass for the planet.
Volume calculations involve making observations while one celestial body eclipses another. The degree to how much the light of one is dimmed by another makes for a good estimate of volume.
Knowing both of these figures allows for the calculation of density.
Furthermore, the general atmospheric makeup can be determined, if one has a spectrograph and light filtering past the observed planet. If certain frequencies are missing, then that's a sign they have been absorbed by the planet's atmosphere, and thus are present.
Citations:
Holladay, April. (2006). How scientists determine the composition of each planet. Retrieved February 6 2009, from: http://www.usatoday.com/tech/columnist/aprilholladay/2006-09-25-measuring-planets_x.htm
Strobel, Nick. (2007) Planetary science. Retrieved February 6 2009, from: http://www.astronomynotes.com/solarsys/s2.htm
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