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Raindrops on Exoplanets Remain Similar to Those on Earth, Harvard Researchers Discover

Hoffman Laboratory houses the Department of Earth and Planetary Sciences.
Hoffman Laboratory houses the Department of Earth and Planetary Sciences. By Ryan N. Gajarawala
By Natalie L. Kahn and Simon J. Levien, Crimson Staff Writers

A pair of Harvard researchers discovered that falling raindrops on other planets remain similar in size and behavior despite widely different atmospheric conditions, according to a study published in the Journal of Geophysical Research: Planets last month.

Kaitlyn “Kait” Loftus, a fourth-year Ph.D. candidate in Earth and Planetary Sciences, and environmental science professor Robin D. Wordsworth used computer models to study raindrop shape, falling speed, and evaporation speed, ultimately determining that raindrop size is relatively constant across disparate planetary environments.

Loftus — the study’s lead author — said she began thinking about this research project in fall 2019 as a “starting point” to model how the water cycle works on other planets.

“It kind of started off like a little pet project,” Loftus said. “And then I went down a rabbit hole and managed to convince my advisor that this is also an interesting problem.”

Wordsworth, who supervised the project, said the paper was unique due to its approach of starting with the “knowledge of physics” and then extrapolating its applications.

“How can we understand this process as completely as possible, and how can we generalize that to lots of different possible conditions?” he said of the study’s guiding principles.

Loftus said she and Wordsworth studied raindrops as opposed to other aspects of the water cycle like clouds and snow due to their “close to spherical” shape and ease to model in simulations.

“There’s something elegant about them,” Loftus said. “They are very simple in a way because of the fact that they are liquid, so every raindrop behaves very similarly, in contrast to snow or snowflakes [which] are very complicated.”

Wordsworth said one big challenge was “deciding on how many processes to include.”

“Cloud microphysics is a very complicated problem, and lots of different things that go into it and trying to do everything at once for totally generalized conditions — you would probably need to write a paper that was 100 pages long or longer,” he said.

Loftus said she found the results of her 28-page paper — the discovery that raindrops remain consistent across planets — to be “surprising.”

“There’s all kinds of sensitivities to like how strong gravity is, how thick the air is, what the air is made out of,” Loftus said, referring to the atmospheric conditions of exoplanets. “A lot of these dependencies end up cancelling out, and the basic behaviors of how raindrops work is actually quite similar across broad planetary conditions.”

Loftus added, however, that her paper is a “laying-the-groundwork study,” and that more components of how rainfall precipitates can be included in future computer models.

She explained that she and Wordsworth did not model the “birth of a raindrop,” which she described as “fundamental.” In addition, she said the pair did not study snow or other solid particles.

Loftus said understanding raindrop patterns across planets using computer models can help scientists piece together large-scale climate models.

To assemble a computerized climate model that spans the globe, “we have to basically abstract what’s happening on the level of clouds, which includes kind of precipitation and rain,” Loftus said. “It’s more like an art than necessarily a science of how you abstract these very small-scale processes to put them into big models.”

Wordsworth said he admires the way Loftus approached building her model to address a difficult topic.

“The real challenge is to build a model that’s just complicated enough to include the key things you care about, but otherwise, as simple as it can be, and I think that’s what [Kaitlyn] has achieved here,” he said.

—Staff writer Natalie L. Kahn can be reached at Follow her on Twitter @natalielkahn.

—Staff writer Simon J. Levien can be reached at Follow him on Twitter @simonjlevien.

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