An exoplanet is a planet that orbits a star other than our Sun. The prefix "exo" means outside, and these planets are far away from us. Exoplanets are discovered using indirect methods such as measuring the dimming of a star that happens to have a planet pass in front of it, called the transit method, or monitoring the spectrum of a star for the tell-tale signs of a planet pulling on its star and causing its light to subtly Doppler shift. Direct imaging is rare and only a handful of exoplanets have been found this way. Exoplanets come in a wide variety of sizes, from gas giants larger than Jupiter to small, rocky planets about as big around as Earth or Mars. Some exoplanets are hot enough to boil metal or locked in deep freeze. They can orbit their stars so tightly that a "year" lasts only a few days, or they can orbit two suns at once. Exoplanets are extremely far away, several light-years away at their closest, and most are too far to even dream of sending space probes to. As of October 1, 2020, we have discovered 4,354 exoplanets in 3,218 planetary systems, and about 22% of those systems have more than one planet.