How do scientists figure out whether a planet is rocky or gaseous?

Density is one of the most useful clues about planetary composition. If a planet is very dense, it is likely rich in rock and metal, like Earth or Mercury; if it has a low density, it probably contains large amounts of gas or ice, like Jupiter or Neptune. Scientists combine mass measurements with radius measurements, often from transits and radial velocity data, to estimate density. Spectroscopy can then reveal atmospheric gases or surface minerals, refining the picture. This method is central to studying both Solar System planets and exoplanets around other stars.