In addition to filling balloons at birthday parties, helium can be found scattered throughout the cosmos. To date, however, scientists have struggled to detect the ubiquitous element on distant worlds, even though the gas is certain to be there. But that's now changed, thanks to the discovery of helium on a Jupiter-sized world located 200 light-years from Earth — but that's only part of the story.
Artist's conception of exoplanet WASP-107b as it passes in front of its host star. The Jupiter-sized world features a distinctly comet-like tail. Image: ESA/Hubble, NASA, and M. Kornmesser
"Helium is the second-most common element in the universe after hydrogen. It is also one of the main constituents of the planets Jupiter and Saturn in our Solar System," Jessica Spake, the astronomer who made the discovery, said in a statement. "However, up until now helium had not been detected on exoplanets — despite searches for it."
But now, using Wide Field Camera 3 on NASA's Hubble Space Telescope, Spake's team managed to detect this strangely elusive substance, marking the first time that helium has been detected on a planet outside of our Solar System. The key to the finding was the use of infrared spectra to study the exoplanet's atmosphere, whereas previous attempts used ultraviolet and optical wavelengths.
This study now shows that the composition of exoplanetary atmospheres can indeed be studied at longer wavelengths. The details of this discovery were published yesterday in Nature.
The planet is called WASP-107b, and it's in orbit around a small, orange star. It's roughly the same size as Jupiter, but it's exceptionally light, featuring a mere 12 per cent of Jupiter's mass. So in addition to hosting detectable amounts of helium, it's now the least dense planet known to astronomers.
What's more, WASP-107b — a so-called Hot Jupiter — takes just six days to make a complete orbit around its host star. This exoplanet is barely able to hold on to its atmosphere, which extends out into space for thousands of miles, giving the planet a distinctly comet-like tail.
Spake, a PhD student at the University of Exeter in the UK, estimates that WASP-107b's atmosphere is bleeding out into space at a rate between 0.4 and 4 per cent of its atmosphere's total mass every billion years. The extended atmosphere is caused by the planet's extremely low gravity and the intense ultraviolet radiation pouring out from its host star.
To detect the WASP-107b's helium, the researchers used the tried-and-true transit method, in which a planet can be studied as it passes in front of its host star. Spake's team used Hubble's Wide Field Camera 3 to measure the amount of near-infrared stellar light being transmitted through its decaying atmosphere. The researchers detected a specific wavelength associated with helium.
They're confident what they saw was really helium, they say, as the amount measured was five times stronger than any false signal could possibly produce.
"The strong signal from helium we measured demonstrates a new technique to study upper layers of exoplanet atmospheres in a wider range of planets," said 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."