How the laws of physics constrain the size of alien raindrops

Whether they’re made of methane on Saturn’s moon Titan or iron on the exoplanet WASP 76b, alien raindrops behave similarly across the Milky Way. They are always close to the same size, regardless of the liquid they’re made of or the atmosphere they fall in, according to the first generalized physical model of alien rain.

“You can get raindrops out of lots of things,” says planetary scientist Kaitlyn Loftus of Harvard University, who published new equations for what happens to a falling raindrop after it has left a cloud in the April Journal of Geophysical Research: Planets. Previous studies have looked at rain in specific cases, like the water cycle on Earth or methane rain on Saturn’s moon Titan (SN: 3/12/15). But this is the first study to consider rain made from any liquid.

“They are proposing something that can be applied to any planet,” says astronomer Tristan Guillot of the Observatory of the Côte d’Azur in Nice, France. “That’s really cool, because this is something that’s needed, really, to understand what’s going on” in the atmospheres of other worlds.

Comprehending how clouds and precipitation form are important for grasping another world’s climate. Cloud cover can either heat or cool a planet’s surface, and raindrops help transport chemical elements and energy around the atmosphere.

Clouds are complicated (SN: 3/5/21). Despite lots of data on earthly clouds, scientists don’t really understand how they grow and evolve.

Raindrops, though, are governed by a few simple physical laws. Falling droplets of liquid tend to default to similar shapes, regardless of the properties of the liquid. The rate at which that droplet evaporates is set by its surface area.

“This is basically fluid mechanics and thermodynamics, which we understand very well,” Loftus says.

She and Harvard planetary scientist Robin Wordsworth considered rain in a variety of different forms, including water on early Earth, ancient Mars and a gaseous exoplanet called K2 18b that may host clouds of water vapor (SN: 9/11/19). The pair also considered Titan’s methane rain, ammonia “mushballs” on Jupiter and iron rain on the ultrahot gas giant exoplanet WASP 76b (SN: 3/11/20). “All these different condensables behave similarly, [because] they’re governed by similar equations,” she says.

The team found that worlds with higher gravity tend to produce smaller raindrops. Still, all the raindrops studied fall within a fairly narrow size range, from about a tenth of a millimeter to a few millimeters in radius. Much bigger than that, and raindrops break apart as they fall, Loftus and Wordsworth found. Much smaller, and they’ll evaporate before hitting the ground (for planets that have a solid surface), keeping their moisture in the atmosphere.

Eventually the researchers would like to extend the study to solid precipitation like snowflakes and hail, although the math there will be more complicated. “That adage that every snowflake is unique is true,” Loftus says.

The work is a first step toward understanding precipitation in general, says astronomer Björn Benneke of the University of Montreal, who discovered water vapor in the atmosphere of K2 18b but was not involved in the new study. “That’s what we are all striving for,” he says. “To develop a kind of global understanding of how atmospheres and planets work, and not just be completely Earth-centric.”