Astronomers Discover ‘Cosmic Treasure’ Around Exoplanet WASP-107b
Helium is abundant throughout the universe, but detection of the element in exoplanet atmospheres has eluded astronomers — until now.
Since helium is the second most abundant element in the universe, exoplanet hunters had thought that the element should be easily found in the atmospheres of planets orbiting other stars. It turns out, however, that finding helium has been a lot harder than expected. But by looking in a different wavelength of light, astronomers have found for the first time helium in an exoplanet atmosphere.
Using the Hubble Space Telescope and looking in infrared, rather than the usual ultraviolet, spectrum, an international team of scientists has studied exoplanet WASP-107b, about 200 light-years away. The amount of helium detected was more than five times greater than what could be caused by a false signal from normal activity from the star it orbits. This led the team to determine that this world has a bizarre atmosphere that is somewhat like a comet’s tail, stretching tens of thousands of kilometers into space in a tenuous cloud.
“We detected an escaping atmosphere with infrared light, for the first time,” Jessica Spake of the University of Exeter said in an email to Seeker. “We look forward to studying many more escaping atmospheres with this new technique.”
Spake is the lead author of a new study, published in the journal Nature, on the discovery.
The team wrote that even though early theoretical models predicted helium to be among the most readily detectable elements in the atmospheres of exoplanets — especially for those planets that have large atmospheric envelopes — until now, only hydrogen and a few other elements with low abundances have been identified in eroding, exoplanet atmospheres. Astronomers say it’s not that the helium isn’t there, it has just been difficult to detect.
Studying the atmospheres of distant exoplanets is challenging, even for Hubble. When an exoplanet passes between its star and Earth, a tiny fraction of the star’s light passes through the planet’s atmosphere, providing a brief glimpse into the composition, thickness, and temperature of the gases that surround the planet. Most of these gases are visible in ultraviolet light.
“Until now, most escaping exoplanet atmospheres had been detected at ultraviolet wavelengths, using the Lyman-alpha line,” Spake explained. “The only telescope which can do this is Hubble, and it is technically very challenging.”
However, Lyman-alpha radiation is absorbed by the interstellar medium, the clouds of material between the stars, so that technique only works for a handful of the closest planetary systems. Only three exoplanets so far have been found to have these large, escaping or eroding atmospheres.
But since infrared light is not absorbed by the interstellar medium, Spake and her colleagues used seven hours of observing time with Hubble’s Wide Field Camera 3 to look at the atmosphere of WASP-107b in near-infrared. The added benefit is that stable helium atoms are more visible in infrared light.
“We hope this detection will open up a whole new avenue in the study of exoplanet atmospheres,” Spake said. “We hope to answer questions like: How long could an Earth-sized planet orbiting close to an M-dwarf (a relatively small and cool red dwarf star) retain its atmosphere? Or what are the most common planets in our galaxy like — are they rocky cores with hydrogen/helium envelopes, or are they mostly water? Perhaps we can answer these questions by measuring helium absorption on more exoplanets with escaping atmospheres.”
WASP-107b is a strange, Jupiter-sized world that has a very low density, only about 12 percent Jupiter’s mass. It was discovered in 2017 with the help of the Wide Angle Search For Planets (WASP), which uses two robotic telescopes in the Canary Islands and South Africa. The team that made the discovery called it “fluffy,” and Spake said she would classify WASP-107b as a fluffy, lightweight world as well.
“This is one of the lowest density planets known to science,” Spake said. “Because it is so low in density it must have a large hydrogen/helium envelope, and its water abundance is consistent with solar abundances. We are very curious about exactly why the density of this planet is so low.”
The host star is smaller and cooler than our sun, but is very magnetically active. As the giant exoplanet orbits the star every six days, ultraviolet radiation from the star energizes the planet’s atmosphere, causing it to form a gaseous tail.
“From a previous work we know there are clouds (and water) in its lower atmosphere,” Spake said, “and our data also supports this. Our work showed that the upper, escaping part of WASP-107b’s atmosphere probably extends tens of thousands of kilometres above the clouds in its lower atmosphere, so everything about this planet is very fluffy indeed!”
In a commentary on the paper, also published in Nature, astronomer Drake Deming from the University of Maryland said the discovery opens a new chapter in the study of exoplanetary atmospheres, a 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,” Demming said, adding that even though looking in infrared is more difficult, astronomers are up to the challenge.
Spake told Seeker that she believes the team’s findings came at the perfect moment.
“There is a suite of new ground-based, high-resolution telescopes with the infrared capabilities necessary to detect helium coming online soon built to find small planets around M-dwarfs,” Spake said. “Not only that, but the James Webb Space Telescope will be able to detect helium on exoplanets with a higher precision than Hubble or ground-based instruments, and with it we may be able to detect the atmosphere of an Earth-sized planet around a small star.”
WASP-107b is one of the prime exoplanet targets already chosen for early observations with the new telescope, which NASA is set to launch in 2020.