The size of raindrops can help identify potentially habitable planets outside of our solar system

One day, humanity could walk on another habitable planet. This planet may look very different from Earth, but one thing will sound familiar to you: rain.

In a recent article, Harvard researchers found that raindrops are remarkably similar in different planetary environments, even planets as radically different as Earth and Jupiter. Understanding the behavior of raindrops on other planets is essential not only for revealing the ancient climate on planets like Mars, but also for identifying potentially habitable planets outside of our solar system.

“The cloud life cycle is really important when we think about the habitability of the planet,” said Kaitlyn Loftus, graduate student in the Department of Earth and Planetary Sciences and lead author of the article. “But clouds and precipitation are really complicated and too complex to be modeled completely. We are looking for simpler ways to understand how clouds change, and a first step is to find out if the cloud droplets are ‘evaporate into the atmosphere or reach the surface as rain. “

“The humble raindrop is a vital component of the precipitation cycle for all planets,” said Robin Wordsworth, associate professor of environmental science and engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and main author of the article. “If we understand how individual raindrops behave, we can better represent precipitation in complex climate models.”

A critical aspect of the behavior of raindrops, at least for climate modelers, is whether or not the raindrop makes it to the surface of the planet, since water in the atmosphere plays an important role in climate. planetary. To this end, size matters. Too big and the drop will shatter due to insufficient surface tension, whether it is water, methane, or superheated liquid iron like on an exoplanet called WASP-76b. Too small and the drop will evaporate before it hits the surface.

Loftus and Wordsworth identified a Goldilocks zone for the size of raindrops using just three properties: the shape of the drop, the rate of fall, and the rate of evaporation.

The shapes of the drops are the same for different rain materials and depend mainly on the severity of the drop. While many of us can imagine a traditional teardrop-shaped droplet, raindrops are actually spherical when they are small, becoming squashed as they grow until they take on a shape like the top of a hamburger bun. The speed of fall depends on this shape as well as the gravity and thickness of the surrounding air.

The rate of evaporation is more complicated, influenced by atmospheric composition, pressure, temperature, relative humidity and more.

Taking all of these properties into account, Loftus and Wordsworth found that under a wide range of planetary conditions, calculating the fall of raindrops means that only a very small fraction of the possible drop sizes in a cloud can reach the surface. .

“We can use this behavior to guide us as we model the cycles of clouds on exoplanets,” Loftus said.

“The knowledge we gain from thinking about raindrops and clouds in various environments is essential to understanding the habitability of exoplanets,” Wordsworth said. “In the long run, they can also help us better understand the climate of the Earth itself.”