Raindrops on other planets could be key in search for life beyond the solar system

• A new study by Harvard University finds that any habitable planet is bound to have rain.
• Whether or not drops manage to reach the surface can help further narrow down the search for life on planets outside the solar system.
• This depends on the size of the drops, what they’re made up of, how fast they fall and what happens if they manage to survive the fall.

Finding another planet that can support human life is like looking for a needle in a haystack. But rain should serve as a magnet to detect the most likely candidates.

A new study by researchers at Harvard finds that raindrops are a common denominator across different planetary environments. Even when you compare the Earth to Jupiter — the rain is remarkably similar.


Trying to study the entire water cycle of a planet is a complicated endeavor. But studying raindrops is much simpler. The first step merely involves figuring out whether or not raindrops evaporate before reaching the surface or survive the journey down from the clouds.

“If we can understand how individual raindrops behave, we can better represent rain in complex climate models,” said Robit Wordsworth, senior author of the paper published in the Journal of Geophysical Research.

Size matters
In trying to determine whether or not a raindrop reaches the surface of a planet, the size of drops matter. If it’s too big, the drop will fall apart regardless of whether it's made up of water, methane, or even liquid iron — as spotted on the exoplanet dubbed WASP-76b.



If the drop is too small, it will evaporate before it's able to make a splash landing.

Rain on different planets:

Planet	Type of rain
Venus	Sulphuric acid
Neptune	Diamonds
Titan	Methane
HD 189733b	Glass
OGLE TR 56b	Iron

The Goldilocks zone of raindrop size
The researchers at Harvard University narrowed down three aspects of a raindrop to determine what size would be ‘just right’ — the Goldilocks zone (habitable zone) — to find out whether or not the rain reaches the surface.

This includes the shape of the drops, how they fall and what happens when they evaporate.

All raindrops start out the same way, regardless of the material they are made up of. While many of us picture teardrop shaped rain, raindrops are actually spherical when small. As they grow larger, they become more squashed, like pizza dough.

The shape is important since it determines the speed at which the drops fall. This speed can vary depending on gravity as well as the thickness of the surrounding air.

Evaporation is where things get a little more complicated because there are more variables involved — like atmospheric conditions, pressure, temperature, relative humidity and other factors.

By using the three properties of drops, the study determines that there is only a very small fraction of possible raindrop sizes within a cloud that can reach the surface.

“We can use this behavior to guide us as we model cloud cycles on exoplanets,” said Kaitlyn Loftus, the lead author of the paper.