The transit method used by the Kepler space telescope has made it possible to determine whether an exoplanet is rocky or gaseous. How?

rocky planets usually have mountains, and during the transit you can see the mountains sticking up out of the edge of the planet in silhouette

actually, there is no way to determine the density: both the transit and Doppler methods yield the MASS of the planet, but unfortunately not the RADIUS

in the transit method, the density is obtainable directly from the period of rotation, and the big heavy giant gas planets have a higher density than the small rocky Earth$-$like planets

the transit method yields the RADIUS of the planet, and when combined with the MASS of the planet $($obtained from the Doppler method$),$ this yields the DENSITY of the planet.... $...$and rocky planets have a higher density than gas planets