From an Earth-centric point of view, rain of course means falling water. We can have storms with falling dust — I experienced a few of those while a reporter in India — but rain is pretty much exclusively H2O falling from the clouds. But as the study of exoplanets moves aggressively into the realm of characterizing these distant planets after they are detected, the concepts of rain and clouds are changing rapidly.
We already know that it rains methane on the moon Titan, sulfuric acid on Venus and ammonia, helium and, yes, water, on Jupiter and Saturn. Some have even posited that carbon — in the form of graphite and then diamonds — falls from the “clouds” of Saturn and Jupiter, but the eye-catching view is widely disputed.
Now the clouds of exoplanets large and small are being rigorously scrutinized not only because they can potentially tell researchers a great deal about the planets below, but also because especially thick clouds have become a major impediment to learning what many exoplanet atmospheres and even surfaces are made of. Current telescopes and spectrometers just can’t see much through many of the thick ones.
Here’s why: The chemical compositions of many exo-planetary clouds are so profoundly different from what is found in our solar system. Hot gas exoplanets, for instance, tend to have clouds of irons and silicates — compounds that are in a gas form on the surface (such as it is), then rise into the atmospheres and form into grain-like solids when they get higher and colder. For some smaller exoplanets, the composition tends to be salts such as zinc sulfide and potassium chloride.
The process of identifying the make-up of different clouds is very much a work in progress, as is an understanding of how thick or how patchy the clouds may be.… Read more