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



Raindrops also keep falling on exoplanets

One day, humanity may step down on another habitable planet. This planet may look very different from Earth, but one thing will feel familiar – the rain.

In a recent article published in JGR planets, Harvard researchers found that raindrops are remarkably similar across different planetary environments, even planets as drastically different as Earth and Jupiter. Understanding the behavior of raindrops on other planets is the key to not only revealing the ancient climate on planets such as March but identify 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 at the Department of Earth and Planetary Sciences and lead author of the paper. “But clouds and precipitation are really complicated and too complex to model completely. We are looking for simpler ways to understand how clouds evolve, and a first step is whether cloud droplets evaporate into the atmosphere or make it to the surface like rain. ”

“The humble raindrop is a vital component of the rainfall 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 senior author of the paper. . “If we understand how individual raindrops behave, we can better represent precipitation in complex climate models.”

An essential aspect of raindrop behavior, at least for climate models, is whether the raindrop reaches the surface of the planet because water in the atmosphere plays a major role in the planet’s climate. For that purpose, size matters. Too large and the fall will break apart due to insufficient surface tension, whether it is water, methane or overheated, liquid iron as on a exoplanet called WASP-76b. Too small and the droplet will evaporate before hitting the surface.

Loftus and Wordsworth identified a Goldilocks raindrop size zone using only three properties: drop shape, drop rate, and evaporation rate.

“The insight we gain by thinking about raindrops and clouds in different environments is the key to understanding the habitability of the exoplanet.”
Robin Wordsworth, Associate Professor of Environmental Science and Engineering

Droplet shapes are the same across different rain materials and depend primarily on how heavy the droplet is. While many of us might imagine a traditional teardrop-shaped drop, raindrops are actually spherical when small and get squeezed when they get bigger until they transition to a shape like the top of a hamburger bun. Falling speed depends on this shape as well as gravity and the thickness of the ambient air.

Evaporation rate is more complicated, affected by atmospheric composition, pressure, temperature, relative humidity and more.

Taking all these properties into account, Loftus and Wordsworth found that the calculation falling over a wide range of planetary conditions only means that a very small fraction of the possible droplet sizes in a cloud can reach the surface.

“We can use this behavior to guide us when modeling sky cycles on exoplanets,” Loftus said.

“The insight we gain by thinking about raindrops and clouds in different environments is key to understanding the habitability of the exoplanet,” Wordsworth said. “In the long run, they can also help us gain a deeper understanding of the Earth’s climate itself.”

Reference: “Physics of Falling Raindrops in Different 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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