TESS is a space telescope, launched April 18, 2018, designed to follow up on the discoveries of planets orbiting stars other than the Sun. In particular, it is studying planets around particularly bright stars, which generally are among the closest to Earth. Its observations will allow astronomers to determine the sizes and masses of the planets it studies, which reveals their density. That can tell us whether the planets are dense and rocky, like Earth; big and fluffy, like Jupiter and Saturn; or somwhere in between, like Neptune. Additional observations can reveal whether a planet has an atmosphere, and perhaps tell us something about the atmosphere's composition. The TESS observations also will reveal supernova explosions, variable stars, and other astronomical phenomena.