I am an observational astronomer. I have spent many nights using telescopes to study stars and to measure the masses of planets that orbit them. These days I work with space telescopes such as Kepler, K2, and TESS to measure the sizes of transiting planets. Together the masses and radii tell us a lot about the bulk properties of the planets, such as the density and whether they might have rocky surfaces where the temperature is right for water to be liquid. Our goal is to find and characterize planets enough like the Earth that we can imagine that life as we know it might be comfortable there.