Alien raindrops falling on your head could be very similar to Earth's

With a nod to B.J. Thomas' 1969 pop hit, "Raindrops Keep Falling On My Head," it appears that no matter where you are in the universe being pelted with liquid droplets originating from alien skies, the size and structure of those individual precipitation units basically remains the same as Earth's.

As described in a new research paper published in the Journal of Geophysical Research: Planets, Harvard University researchers have provided a more unified theory on the physics of cloud formation and precipitation cycles on exoplanets whose weather conditions differ from our Big Blue Marble.

In their study, scientists discovered that raindrops are actually similar when examined across various planetary environments, even heavenly bodies as dissimilar as Earth and Jupiter. By absorbing the characteristics of raindrops on other planets, climatologists, astronomers, and planetary geologists can learn more about ancient climates on neighboring bodies such as Mars, enabling them to target habitable planets beyond our solar system. 

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“The lifecycle of clouds is really important when we think about planet habitability,” said lead author Kaitlyn Loftus, a graduate student in the Department of Earth and Planetary Sciences. “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.”

According to the Harvard press release, a vital aspect of raindrop behavior is determining if raindrops do drift to the surface of a planet, due to atmospheric water being such an important part of a planet's climate.

If the drop is too large it will break up for lack of surface tension, no matter if that particle is composed of water, methane, or even superheated liquid iron as found on the exotic exoplanet WASP-76b. Droplets that are too small succumb to quick evaporation before falling to a surface. 

“The humble raindrop is a vital component of the precipitation cycle for all planets,” said senior author Robin Wordsworth, Associate Professor of Environmental Science and Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). “If we understand how individual raindrops behave, we can better represent rainfall in complex climate models.”

Loftus and Wordsworth pinpointed a "just-right" Goldilocks zone for sizing up raindrops by deciding upon a trio of identifying properties: drop shape, falling speed, and evaporation speed. 

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As traditional artistic depictions of a single raindrop might be teardrop-shaped, precipitation particles appear spherical when small enough but become more flattened as they form into larger units, eventually resembling a hamburger bun lid in shape.

In trying to determine falling speeds, the researchers calculated in gravity and how thick the atmosphere was during rainfall periods. Evaporation rates include many more variables like planetary atmosphere composition, temperature, humidity, and pressure affecting each drop.

Adding up all these requirements and possibilities, Loftus and Wordsworth found that while considering a broad range of exoplanet conditions, a tiny fraction of drops had the right stuff to make it all the way to the ground.

“The insights we gain from thinking about raindrops and clouds in diverse environments are key to understanding exoplanet habitability,” explained Wordsworth. “In the long term, they can also help us gain a deeper understanding of the climate of Earth itself.”