Scientists have been saying for some time now that helium should be readily detectable in the atmospheres of gas giant planets — after all, this is the second-most common element in the universe, and we know it is plentiful at Jupiter and Saturn. The problem has been how to detect it, an issue which this morning’s story brings into sharp relief. At the University of Exeter (UK), Jessica Spake has put data from the Hubble telescope’s Wide Field Camera 3 to good use, finding an abundance of helium in the upper atmosphere. The planet in question is the puffy WASP-107b, and this marks the first helium detection of the inert gas on an exoplanet.

Some 200 light years from Earth in the constellation Virgo, WASP-107b shows little similarity to anything in our own Solar System. It was discovered in 2017 and is one of the lowest density planets yet found, a world that, although roughly the same size as Jupiter, has only 12 percent of its mass. In a tight six-day orbit around its K-class primary, the planet has an atmosphere that apparently — judging from the amount of helium that Spake and team have found — extends tens of thousands of kilometers into space.

Image: The exoplanet WASP-107b is a gas giant, orbiting a highly active K-type main sequence star. The star is about 200 light-years from Earth. Using spectroscopy, scientists were able to find helium in the escaping atmosphere of the planet — the first detection of this element in the atmosphere of an exoplanet. Credit: ESA/Hubble, NASA, M. Kornmesser.

Finding helium here is no surprise, but the story is interesting because of the detection methods used. We analyze exoplanet atmospheres when a transiting world passes in front of its star as seen from Earth, leaving spectral evidence of the constituent elements. Such transmission spectroscopy has allowed the investigation of other extended atmospheres, working at ultraviolet and visible wavelengths. Spake’s team did its work in the infrared.

“The strong signal from helium we measured demonstrates a new technique to study upper layers of exoplanet atmospheres in a wider range of planets,” says Spake. “Current methods, which use ultraviolet light, are limited to the closest exoplanets. We know there is helium in the Earth’s upper atmosphere and this new technique may help us to detect atmospheres around Earth-sized exoplanets – which is very difficult with current technology.”

WASP-107b’s low mass relative to its size makes retaining its atmosphere a problem. The planet’s extended atmosphere is gradually diminishing, with about ~0.1-4% of its total mass disappearing every billion years. Exacerbating the effect is stellar activity on the host star, radiation which is absorbed by the atmosphere that causes it to heat still further. As Drake Deming notes in an article that appears in the same issue of Nature as the Spake paper, the primary star is riven by magnetic fields, producing a strong ultraviolet flux.

The result is a gaseous tail in which the authors detected helium atoms. The detection problems are complex, for atoms in an extended tail like this tend to relax into their ground state, absorbing ultraviolet light, and that absorption as a function of time or wavelength, says Deming, can be complex. Spake’s work leverages the fact that helium atoms are also found in a metastable state, one that can absorb near-infrared stellar light, and measurements at these wavelengths are easier to interpret. This is also useful because while Earth’s atmosphere is opaque to ultraviolet, ground-based telescopes can see helium’s near-infrared signal.

Spake was working with Hubble data, but this kind of analysis may become available on the ground. Our next generation of extremely large telescopes should be able to home in on escaping atmospheres around other exoplanets. We would expect helium to persist in Neptune-class planets where lighter hydrogen has already escaped, but escape rates may also be helpful when it comes to other aspects of exoplanet composition. Again I turn to Deming:

Heavier elements such as carbon and oxygen would be slow to escape, and could in principle be present in exoplanetary atmospheres in concentrated amounts. These heavier elements are key to understanding both how planets form and how they acquire their atmospheres. For planetary astronomers, an escaping atmosphere that is rich in heavy elements is something of a cosmic treasure, providing ample scientific opportunities to study planetary formation and evolution. Spake and colleagues’ detection of helium in WASP-107b will enable astronomers to look for atmospheres that are rich in helium, and perhaps in heavier elements, thereby opening a new subfield of exoplanetary science.

The paper is Spake et al., “Helium in the eroding atmosphere of an exoplanet,” Nature 557 (02 May 2018), 68-70 (abstract).