Is water, water everywhere? NASA/JPL-Caltech

A FARAWAY world is steaming. Astronomers have found water vapour in the atmosphere of an exoplanet called 51 Pegasi b — and achieved the feat using a brand new technique.

Detected over 20 years ago, 51 Pegasi b was the first known “hot Jupiter” – a Jupiter-like world orbiting close to its star. It isn’t the first such planet to have water spotted in its atmosphere, but it is the first non-transiting one.

All previous water detections relied on planets transiting – slipping in front of – their host star as seen from Earth. That’s a slight problem, in that most exoplanets never make transits.


In 2015, astronomers estimated that there could be as many as 200 small stars with planetary systems that are closer to us than the nearest star with a transiting Earth-size planet. “If we really want to understand our local neighborhood of stars and their potentially habitable planets, then we need a technique that works on non-transiting systems,” says Jayne Birkby at the Harvard-Smithsonian Center for Astrophysics.

That’s the beauty of what Birkby and her colleagues have done using the Very Large Telescope in Chile. They observed 51 Pegasi b and its host star side by side, which doesn’t typically allow light from the two bodies to be disentangled.

But the team didn’t just take a single snapshot. Instead, they watched the system for 4 hours, capturing part of the planet’s orbit. As the planet shifted away from and then towards Earth, its light shifted towards redder and then bluer wavelengths, thanks to the Doppler effect (arxiv.org/abs/1701.07257). Analysing its spectrum allowed the team to pick out its atmosphere from that of the star as well as Earth’s, and spot a watery signature.

“If we want to understand our local neighbourhood of stars and their planets, we need this technique”

“The detection is pretty rock solid,” says team member Matteo Brogi, at the University of Colorado, Boulder. It’s also an important step towards detecting water molecules in smaller, more habitable worlds.

Birkby is particularly keen to use this method with the Extremely Large Telescope, now under construction in Chile. “We think that’s probably going to be our best chance for looking at the atmosphere of Proxima b,” she says – referring to the Earth-like planet spotted a mere 4.2 light years away last year.

This article appeared in print under the headline “Water seen on planet 50 light years away”