Raindrops on Distant Worlds May Hold the Key to Finding Habitable Planets

A raindrop, the Harvard researchers say, is a good place to start when looking for another world like ours. A new study from the university argues that any habitable planet is bound to have rain, and that whether those drops actually reach the ground could help narrow the hunt for life beyond the solar system.

The paper was published in the Journal of Geophysical Research. Kaitlyn Loftus is its lead author and Robin Wordsworth its senior author. Wordsworth describes the raindrop as a vital component of the precipitation cycle on every planet. The team's reasoning is practical. Modeling a planet's entire water cycle is a complicated undertaking, while studying individual drops is far simpler. The first question is whether a drop evaporates on the way down from the clouds or survives the trip to the surface.

The answer depends on the drop's size, what it is made of, how fast it falls, and what happens to it along the way. Size sets the limits. A drop that is too large falls apart, whether it is water, methane, or liquid iron, the last of which has been spotted on the exoplanet WASP-76b. A drop that is too small evaporates before it can land. The researchers sought the size that is just right, a Goldilocks zone for raindrops, as the habitable zone is for planets.

To find it, they narrowed their focus to three properties: the shape of the drops, how they fall, and what happens as they evaporate. All raindrops begin the same way regardless of their material. Small ones are spherical, not teardrop-shaped as many people picture them. As they grow, they flatten, something like pizza dough. Shape matters because it determines fall speed, which also varies with a planet's gravity and the thickness of the air around the drop.

Evaporation is the hardest piece, because many variables feed into it: atmospheric conditions, pressure, temperature, and relative humidity, among others. Combining the three properties, the study concludes that only a very small fraction of the raindrop sizes that can form in a cloud will actually reach the surface.

The source article notes that rain falls on very different worlds, though not always as water. It lists sulphuric acid on Venus, diamonds on Neptune, methane on Titan, glass on HD 189733b, and iron on OGLE TR 56b. The researchers found that raindrops are a common denominator across such different environments, and that rain on Earth and on Jupiter is remarkably similar in its behavior.

The practical payoff is in the models. Wordsworth says that understanding how individual drops behave lets scientists represent rain better in complex climate models. Loftus says the behavior can guide how cloud cycles are modeled on exoplanets. Finding a planet that could support human life is described as a needle-in-a-haystack problem, and rain is meant to work like a magnet for pulling out the likeliest candidates. What to watch next is whether these drop-level rules get built into the climate models used to study distant worlds.