Question: I've heard that scientists can use the amount of light absorbed by a planet to determine what the planet is like if it is really far awaye.g the atmosphere or something like that. How does this work?

Sara Imari Walker answered on 17 Jun 2011:

Hi Rachel! Another great question from you!! You are absolutely correct. There is a whole new science emerging in the last decade centered on characterizing exoplanets (those are planets around other stars) based on the light we receive from them. The technical term for what scientists are studying is the emission spectrum of the atmosphere (ooo! new buzz word!). Every atom and every molecule has a unique emission spectrum. Here’s some quantum physics for you: when light interacts with atoms it can excite electrons. When those electrons de-excite they re-emit that light at specific energies corresponding to set wavelengths of light (i.e. the colors of the rainbow are different wavelengths of the visible spectrum of light). The important thing is that each element and each kind of molecule has a unique set of wavelengths. It is like a fingerprint. So we can identify atoms and molecules based on their spectra. In fact, this is precisely how we identified that the Sun is made of Hydrogen! The spectrum of the Sun matches exactly that of hydrogen studied here on Earth. We have used spectra to identify what giant interstellar clouds are made of and even entire galaxies. The same goes for exoplanets, as long as we can get light from them we can identify what their atmospheres are made of. The challenge with exoplanets though is that we only get a teeny tiny amount of light from them so it is hard to resolve their spectra. And, what makes it even tougher is that the atmosphere can have a lot of different stuff in them so it is hard to identify what all the components are. We have a lot to learn, but their are a lot of great scientists working in this area. Many are young since the field is so new, so its an exciting thing!!