NASA's Spitzer Space Telescope has revealed the first known system of seven Earth-size planets around a single star - a tiny, nearby, ultra-cool dwarf star called TRAPPIST-1. Three of these planets are located in the habitable zone, the area around the parent star where a rocky planet is most likely to have liquid water. The discovery sets a new record for greatest number of habitable-zone planets found around a single star outside our solar system. All of these seven planets could have liquid water under the right atmospheric conditions, but the chances are highest with the three in the habitable zone. Over 21 days, NASA's Spitzer Space Telescope measured the drop in light as each planet passed in front of the star.