“This all changed with the launch of NASA’s Kepler Mission, which continually monitored a patch of sky for brightness variations in 150,000 stars. Any dip in brightness can be caused by a planet passing in front of its star, blocking a small fraction of its light. In its four-year run, Kepler detected and confirmed nearly 2,000 planetary systems, ranging from “Hot Jupiters” to frozen, rocky worlds. Intriguingly, a select few lie within the Goldilocks zone where liquid water could exist because the planet isn’t too hot or too cold.