How do astronomers measure the mass of a planet they cannot weigh?

Planets are too distant to place on a scale, so astronomers infer mass through gravity. In the radial velocity method, a planet causes its star to wobble, and the size of that wobble depends on the planet’s mass. In transit timing variations, small changes in the timing of transits can reveal gravitational interactions between planets in the same system. When a planet’s size is known from transits and its mass from wobble measurements, scientists can estimate density and infer whether it is rocky, icy, or gaseous. This combination is key to studying exoplanet composition.