The size of the raindrops helps identify potentially habitable planets outside our solar system



Raindrops also keep falling on exoplanets

One day, humanity may set foot on another habitable planet. That planet may look very different from Earth, but one thing will be familiar to you: rain.

In a recent article published in JGR planets, Harvard researchers found that raindrops are remarkably similar in different planetary environments, even planets as drastically different as Earth and Jupiter. Understanding the behavior of raindrops on other planets is key not only to reveal ancient weather on planets like Mars but identifying potentially habitable planets outside our solar system.

“The life cycle of clouds is really important when we think about the habitability of the planet,” said Kaitlyn Loftus, a graduate student in the Department of Earth and Planetary Sciences and lead author of the paper. “But clouds and precipitation are really complicated and too complex to fully model. We are looking for simpler ways to understand how clouds evolve, and a first step is whether the cloud droplets evaporate in the atmosphere or reach the surface in the form of 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 Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and author. main article. . “If we understand how individual raindrops behave, we can better represent rain in complex climate models.”

An essential aspect of the behavior of raindrops, at least for climate modelers, is whether or not the drop reaches the surface of the planet because water in the atmosphere plays an important role in the planetary climate. To that end, size matters. Too large and the drop will break due to insufficient surface tension, regardless of whether it is water, methane, or superheated liquid iron as in a exoplanet called WASP-76b. Too small and the droplet will evaporate before hitting the surface.

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

“The insights we get from thinking about raindrops and clouds in various environments are key to understanding the habitability of exoplanets.”
Robin Wordsworth, Associate Professor of Environmental Science and Engineering

The shapes of the drops are the same in different rain materials and depend mainly on the weight of the drop. While many of us can imagine a traditional teardrop drop, raindrops are actually spherical when small and squash as they grow until they are shaped like the top of a hamburger bun. The rate 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.

By taking all these properties into account, Loftus and Wordsworth found that under a wide range of planetary conditions, the mathematics of falling 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 cloud cycles on exoplanets,” Loftus said.

“The insights we get from thinking about raindrops and clouds in various environments are key to understanding the habitability of exoplanets,” Wordsworth said. “In the long term, they can also help us gain a deeper understanding of Earth’s climate.”

Reference: “The Physics of Raindrops Falling in Various Planetary Atmospheres” by Kaitlyn Loftus and Robin D. Wordsworth, March 15, 2021, JGR planets.
DOI: 10.1029 / 2020JE006653

This research was supported by the National Science Foundation through grant AST-1847120.

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