The question of whether it rains on other planets has a definite answer: yes, it does. However, the substance falling from the sky is rarely water. Precipitation on worlds across the galaxy adheres to the same fundamental laws of physics that govern Earth’s weather, but the materials involved are shaped by wildly different temperatures, pressures, and chemical compositions. This leads to a spectacular diversity in planetary meteorology, where rain can be made of substances ranging from simple hydrocarbons to molten metal. The existence of an atmosphere and a temperature gradient is enough to create clouds and precipitation, even if the resulting showers are entirely alien.
The Universal Requirements for Planetary Rain
For any form of precipitation to occur, a world must possess a substantial atmosphere and a temperature-pressure profile that allows a chemical substance to transition between a gas and a liquid state. This process is essentially a condensation cycle, regardless of whether the substance is water or something more exotic. The atmospheric layer where this condensation happens acts like Earth’s troposphere, where temperatures decrease with altitude.
Gravity plays a significant role by pulling the condensed droplets downward, creating the rainfall effect. The same physical principles of fluid dynamics and thermodynamics determine how large a droplet can grow before it breaks apart or evaporates. Liquid precipitation, whether it is water on Earth or molten iron on an exoplanet, tends to fall within a narrow size range, averaging only a few millimeters in radius. Drops much larger than this break up, while smaller ones often evaporate before reaching the ground or a lower atmospheric layer.
The Unique Case of Liquid Water Precipitation
Liquid water rain, the kind familiar on Earth, is comparatively rare because it requires a very specific set of conditions. The planet’s surface temperature and atmospheric pressure must align with the triple point of water, a precise balance where water can exist simultaneously as a solid, liquid, and gas. Earth is currently the only body in the Solar System where liquid water consistently falls from the clouds and pools stably on the surface.
Mars once hosted vast oceans and rivers, but no longer experiences liquid water rain due to its extremely thin atmosphere and low average temperature. Any trace of water vapor quickly turns into ice, frost, or sublimates directly from solid to gas. Venus holds a significant amount of water in its atmosphere, but the runaway greenhouse effect makes the surface temperature far too high. Water vapor is trapped high above the planet, unable to condense into liquid droplets that could survive the scorching descent to the ground.
Exotic Rains in Our Solar System
Beyond Earth, precipitation cycles within our Solar System involve much stranger compounds. Saturn’s largest moon, Titan, is the only other world known to have stable liquid bodies on its surface, which are lakes and seas of liquid methane and ethane. The moon experiences a complete hydrological cycle, where gaseous methane condenses in the cold, dense atmosphere and falls as methane rain, replenishing these hydrocarbon reservoirs. Intense, monsoon-like methane downpours have been observed on Titan, sometimes dumping a foot of liquid in a single event.
Venus features a dense cloud layer composed primarily of sulfuric acid droplets. This potent acid condenses and falls as rain in the upper atmosphere, but the intense heat near the surface causes the liquid to evaporate completely around 15 to 25 kilometers above the ground. This phenomenon, where precipitation vaporizes before impact, is called virga.
In the deep atmospheres of the gas giants, Jupiter and Saturn, powerful storms drive more complex precipitation, including ammonia and water ice. These may form semisolid “mushballs” of precipitation. The immense pressures and temperatures within the ice giants Uranus and Neptune are theorized to compress methane into carbon. This carbon then solidifies into diamonds that fall through the interior layers.
Extreme Weather on Distant Exoplanets
The most bizarre forms of weather occur on exoplanets orbiting distant stars, particularly the ultra-hot Jupiters. These massive planets orbit incredibly close to their parent stars, resulting in atmospheric temperatures that can vaporize rock and metal. On WASP-76b, the intense heat on the day side vaporizes metals, including iron, which are then swept by extreme winds to the cooler night side.
As the iron vapor moves to the dark side, it cools rapidly and condenses into liquid iron droplets. This results in a continuous shower of molten iron rain on the night side of the planet.
Another example is WASP-121b, where temperatures reach over 2,500 degrees Celsius, vaporizing aluminum and other heavy elements. On the cooler side of this tidally locked world, these metal vapors condense into solid-state precipitation, forming rains of corundum, the mineral that makes up rubies and sapphires.
Finally, on planets like HD 189733b, evidence of silicate particles suggests the possible presence of clouds that would rain molten glass or silicates driven by high-speed winds.