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

One day, humankind may step foot on another habitable planet. That planet may look very different from Earth, but one thing will feel familiar — the rain.

In a recent paper, 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 key to not only revealing the ancient climate on planets like Mars but identifying potentially habitable planets outside our solar system.

“The lifecycle of clouds is really important when we think about planet habitability,” 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 model completely.

We’re looking for simpler ways to understand how clouds evolve, and a first step is whether cloud droplets evaporate in the atmosphere or make it to 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 senior author of the paper.

“If we understand how individual raindrops behave, we can better represent rainfall in complex climate models.”

An essential aspect of raindrop behavior, at least to climate modelers, is whether or not the raindrop makes it to the surface of the planet because water in the atmosphere plays a big role in planetary climate.

To that end, size matters.  Too big and the drop will break apart due to insufficient surface tension, regardless of whether it’s water, methane or superheated, liquid iron as on an exoplanet called WASP-76b. Too small and the drop will evaporate before hitting the surface.

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