A new study has found that the size of raindrops can help scientists identify potentially habitable exoplanets.

Scientists have designed several ways to deduce whether a particular planet is capable of supporting life as we know it. These include methods from measuring their distances from the parent stars to analyzing their atmospheres.

Now, researchers have found a new method to identify potentially habitable worlds.

In a recent paper, Harvard researchers found that understanding the behavior of raindrops on other exoplanets would not just reveal the ancient climate on those planets, but also help us identify potentially habitable worlds that lie outside our solar system.

The study suggests that raindrops are very similar across different planetary environments. Even on planets that differ as drastically as Earth and Jupiter.

These raindrops also form a vital component of the precipitation cycle for all planets. And understanding how individual raindrops behave could provide us with a better representation of rainfall in complex climate models.

Scientists say clouds and precipitation are really complicated and too complex to model completely. However, they are looking for simpler ways to understand how clouds evolve, and the first step is whether cloud droplets evaporate in the atmosphere or make it to the surface as rain.

To that end, size matters. A bigger drop will break apart due to insufficient surface tension, regardless of whether it’s water, methane, or superhot liquid iron as on an exoplanet called WASP-76b. A smaller drop, on the other hand, will evaporate before hitting the surface.

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For the study, the team identified three properties of raindrops: the drop shape, falling speed, and evaporation speed.

Drop shapes are the same across different rain materials. Also, contrary to popular belief, these raindrops aren’t actually tear-shaped. They are spherical when small and hamburger bun-shaped when large. The shape primarily depends on how heavy the drop is.

Further, the falling speed depends on its shape as well as gravity and the thickness of the surrounding air.

And finally, the evaporation speed is more complicated. This one is influenced by atmospheric composition, pressure, temperature, relative humidity, and more.

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Taking these raindrop properties into account, the research team found that across a wide range of planetary conditions, only a very small fraction of the possible raindrop sizes in a cloud can reach the surface of the planets.

This finding on how raindrops behave has implications for precipitation efficiency, convective storm dynamics, and rainfall rates–all of which are properties that will help us understand planetary radiative balance and (in the case of terrestrial planets) rainfall-driven surface erosion.

Moreover, these results could also guide us as we model cloud cycles on exoplanets.


The most interesting exoplanet with rain, as we mentioned before, is WASP-76b, a giant exoplanet where instead of water, the rainfall consists of extremely hot vapor droplets of iron.

This world lies about 650 light-years away in the constellation Pisces.

Since it receives thousands of times more radiation from the Sun than Earth does, the planet is extremely hot, with temperatures reaching 4,350 degrees Fahrenheit (2,400 degrees Celsius) on its dayside.

WASP-76b is also tidally locked. It keeps one face to its star, much as our moon always has one side facing Earth.

The temperatures on the dayside of this exoplanet are scorching, high enough for metals to vaporize. But the nightside is cool, and winds carry an iron rain from the dayside to the night side.

Scientists published the findings of this study in the Journal of Geophysical Research: Planets.

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