Hubble finds first exoplanet helium
Scientists looking for methane found something else entirely, and celebrated. Lauren Fuge reports.
In a happy accident, astronomers have made the first-ever discovery of helium in the atmosphere of a planet outside the solar system.
“Helium is the second-most common element in the universe after hydrogen,” explains Jessica Spake from the University of Exeter in the UK, lead author of the team which made the discovery.
“It is also one of the main constituents of the planets Jupiter and Saturn in our solar system. However, up until now helium had not been detected on exoplanets — despite searches for it.”
Cue WASP-107b, a gas giant discovered last year orbiting an active star 200 light-years away. Spake and her team used the Hubble Space Telescope to capture the spectrum of light passing through the colossal planet’s upper atmosphere, revealing its weird architecture.
Though the planet is the size of Jupiter, it has only 12% of its mass, making it one of the lowest density exoplanets known. Its gas is puffed out in a massive but diffuse atmosphere that is constantly bombarded by harsh radiation from its host star — meaning the planet may resemble a super-sized comet, with a tail of gas streaming off for tens of thousands of kilometres into space.
It was within this wispy wake that astronomers spotted tell-tale signs of helium. They were actually looking for methane, but instead stumbled across a giant spike in the spectrum that turned out to be helium, boosted to an excited “metastable” state by the star’s relentless radiation. Their results are published in the journal Nature.
“This is a really cool result — but not one I’d say is unexpected!” says Jonti Horner, an astrophysicist from the University of Southern Queensland who was not involved in the discovery.
He points out that astronomers have long predicted the abundance of helium in the atmospheres of distant worlds.
“But that said, just because it is there doesn’t mean it is easy to detect — after all, the science of studying the atmospheres of planets around other stars is really still in its infancy,” he explains.
“It isn’t through lack of trying, but simply that these kinds of observations are amazingly hard to do.”
Previously, astronomers have mainly studied exoplanet atmospheres at shorter wavelengths such as optical and ultraviolet. The first detection of oxygen and carbon, for example, was made in the ultraviolet.
But this limits astronomers to studying nearby exoplanets. Spake and her team, on the other hand, made their observations in the infrared, using a new technique to detect metastable helium that will allow them to probe exoplanets further out into space.
Since helium is present in Earth’s upper atmosphere, Spake says “this new technique may help us to detect atmospheres around Earth-sized exoplanets — which is very difficult with current technology.”
Horner notes that we are still “a long, long way from that now — but each discovery like this is another step along the way.”