How can we know the size, composition, and atmospheric makeup of distant exoplanets? NASA explains the details in this Alien Atmospheres video.
By observing periodic variations in the parent star’s brightness and color, astronomers can indirectly determine an exoplanet’s distance from its star, its size, and its mass. But to truly understand an exoplanet astronomers must study its atmosphere, and they do so by splitting apart the parent star’s light during a planetary transit.
The basic idea is, when a liquid comes in contact with something really hot—about twice as hot as the liquid’s boiling point, although it changes on certain factors like the size of the drop—the liquid never comes in direct contact with the surface; vapor acts as a barrier that keeps the two separated. When you flick drops of water on to a pan to check the heat, that skittering you see and hear is because of this effect.