 Image stolen from [Exoplanets](http://books.google.com/books?id=VlSVmxgPgGYC&pg=PA441&lpg=PA441&dq=%22terrestrial+planet+atmospheres+and+biosignatures%22&source=bl&ots=-qh9W-ZVsx&sig=7XdxzQFF-kdKhxOJAnrqEnCpUHM&hl=en&sa=X&ei=nNhrVKm0M9CAygShgYLYBA&ved=0CDEQ6AEwAw#v=onepage&q&f=true)
