Common, Yet Rare

ESA/Hubble, M. Kornmesser

In science fiction, when an advanced alien civilization visits Earth, they always want something. Often it’s our precious water. Earth is a water-rich world, and (as far as we know) life depends upon water. But water isn’t that rare in the cosmos. Water is made of hydrogen and oxygen. Hydrogen is by far the most abundant element, and Oxygen is the third most abundant. Number two is helium, which doesn’t react chemically, so water is really common.

But just because water is cosmically plentiful, that doesn’t mean that planets are brimming with oceans. Of the terrestrial planets in our solar system, Earth is the only one that is water-rich, and even Earth has less water than some icy moons of the outer planets, like Europa. So what about planets orbiting other stars?

It’s only in the past few years that astronomers have been able to detect water on exoplanets. They do it by studying the light that passes through the atmosphere of the exoplanet when it passes in front of its star. By studying the absorption spectra of the starlight, they can see whether the planet’s atmosphere contains water, and how much water there is.

Recently a team studied the atmospheres of 19 exoplanets.¹ These were large planets like Neptune or Jupiter, rather than smaller, Earth-like worlds. But it is the first survey to see how common water is on exoplanets. The team detected water on 14 of the worlds, which suggests that most exoplanets have water. This is what you would expect, given the molecule’s abundance.

But the team also found something rather surprising. While water was often present, it wasn’t typically abundant. This was true for a wide range of exoplanets, from hot Jupiters to much cooler worlds. So while water is common in the universe, it may tend to evaporate off planets. That would mean Earth is rather rare.

Interestingly, while we know the water abundance of these exoplanets, we don’t know the same about the large planets in our solar system. So we don’t know whether the planets of this survey are typical or unusual. But if exoplanets are typically dry worlds, then perhaps aliens would be interested in our water after all.