An artist's concept shows what the planets of a system known as Trappist-1 might look like. The system contains seven known planets, all of which are roughly as big as Earth. Three of the planets are in the star's habitable zone, which is the distance from the star where liquid water could exist on the surface. Much of what astronomers know about the planets was obtained by Spitzer Space Telescope, which observes the sky at infrared wavelengths. The telescope has been in space for 15 years. [NASA/JPL/Caltech]