Water common but scarce in exoplanets

Exoplanet atmospheres appear to be unexpectedly low in water vapour scientists say, suggesting these planets may not have formed in the same manner as worlds in our own Solar System.

The discovery came from the first-ever tabulation of exoplanet atmospheres — a type of research that has only now become possible, says Nikku Madhusudhan, an astrophysicist at the University of Cambridge, UK. 

It was only five years ago, he says, that his team was able to obtain good enough spectroscopic data from the Hubble Space Telescope to be able make the first detection of water in an exoplanet atmosphere. 

Now, he says, 19 exoplanets have been found to show signs of water in their atmospheres, with 14 having sufficiently good data to reveal the amount.

That’s enough, he says, to show a clear trend: by Solar System standards, exoplanets simply do not have enough water. 

“It’s not a one-off effect. All are depleted compared to what we see in the Solar System.”

Not that any of these planets is earthlike. So far, only giant planets like Jupiter, Saturn, Uranus and Neptune can be studied well enough across interstellar distances for astronomers to find signatures of water vapour in their atmospheres. 

But the low levels of water in those that have been measured holds for a wide range of planets, Madhusudhan says, from “mini-Neptunes” 10 times the size of the Earth, to “super-Jupiters” more than 600 times more massive than our home world.

Not that the finding is entirely about water, important as it is on our own planet. 

Water is important as a detectable carrier of oxygen — the third most abundant element in the Universe. And it is the oxygen that is important from an astrophysical point of view. 

In the Solar System, oxygen (in the form of water) appears to be substantially more plentiful in the atmospheres of the giant planets than in the Sun. But it’s not similarly plentiful in exoplanets, even when compared to the oxygen content of their own stars. 

“The water abundances are significantly lower,” Madhusudhan says.

What this means, he adds, is “that the picture we have of planets forming with a substantial accretion of water ice and dust is incomplete. These planets are finding a way to form without accreting ‘enough’ water ice.” 

One possibility, he says, is that the planets in his study formed far out from their stars, where water ice — and oxygen — were scarce, then migrated inward. 

Or, it’s possible that estimates of the amount of water vapour in our own giant planets are overstated. 

One of the ironies of the field, Madhusudhan says, is that it’s currently easier to measure water vapour in exoplanet atmospheres than in our own giant planets. 

“Ours are too cold,” he says. (The planets in his study all lie close enough to their stars to have temperatures at least as warm as the Earth, and in some cases, substantially hotter.) 

One of the main goals of NASA’s Jupiter-orbiting Juno mission, Madhusudhan adds, has been to measure Jupiter’s water content but, so far, that’s not succeeded. 

Meanwhile, he says, the focus is on continuing to study exoplanet atmospheres. “We look forward to increasing the size of our sample.”

The research was reported in Astrophysical Journal Letters.