Although helium is a rare element on Earth, it is ubiquitous in the Universe. It is, after hydrogen, the main component of gaseous stars and planets. Despite its abundance, helium was only recently detected in the atmosphere of a gaseous giant by an international team, including astronomers at the University of Geneva (UNIGE) in Switzerland. The team, this time led by Genevan researchers, noted in detail and for the first time how this gas escapes the superheated atmosphere of an exoplanet, literally inflated with helium. The results are published in Science.
Helium is the second most abundant element in the Universe. Foreseen since 2000 as one of the best possible plotters of the atmospheres of exoplanets, these planets orbiting around stars other than the Sun, it took 18 years for astronomers to actually detect it. It was difficult to detect because of the very peculiar observational signature of helium, located in the infrared, out of reach of most of the instruments previously used. The discovery came earlier this year, thanks to observations from the Hubble Space Telescope, which proved difficult to interpret. Members of the UNIGE team, members of the National Center for Research Competence PlanetS, had the idea of pointing to another telescope equipped with a new instrument – a spectrograph called Carmenes.
Detecting color of planets with Carmenes
A spectrograph breaks down the light of a star into its component colors, like a rainbow. The "resolution" of a spectrograph is a measure that indicates the number of colors that can be revealed. While the human eye can not distinguish any color beyond red without an adapted camera, Hubble's infrared eye is able to identify hundreds of colors there. This proved to be sufficient to identify the colored signature of helium. The Carmenes instrument, installed in the 4-meter telescope at the Calar Alto observatory in Andalusia, Spain, is able to identify more than 100,000 colors in the infrared!
This high spectral resolution allowed the team to observe the position and velocity of the helium atoms in the upper atmosphere of a gaseous exoplanet, the size of Neptune, four times larger than Earth. Located in the constellation Cygnus (Swan), 124 light years away from home, HAT-P-11b is a "hot Neptune" (a decent 550 ° C!), Twenty times closer to its star than Earth "We suspect that this proximity to the star could impact the atmosphere of this exoplanet," says Romain Allart, a doctoral student at UNIGE and the first author of the study. "The new observations are so precise that the exoplanet atmosphere is undoubtedly inflated by stellar radiation and escapes into space," he adds.
A planet inflated with helium
These observations are supported by numerical simulation, led by Vincent Bourrier, co-author of the study and member of the European project FOUR ACES *. Thanks to the simulation, it is possible to trace the trajectory of helium atoms: "Helium is blown from the day side of the planet to its night side at more than 10 thousand km / h", explains Vincent Bourrier. "Because it is such a light gas, it easily escapes the attraction of the planet and forms a cloud extended around it." This gives the HAT-P-11b the shape of a balloon inflated with helium.
This result opens a new window to observe the extreme atmospheric conditions that prevail in the hottest exoplanets. Carmenes's observations show that such studies, long thought only of space, can be more accurately achieved by terrestrial telescopes equipped with the right kind of instruments. "These are exciting times for the search for atmospheric signatures in exoplanets," says Christophe Lovis, senior lecturer at UNIGE and co-author of the study. In fact, UNIGE astronomers are also heavily involved in the design and exploitation of two new high resolution infrared spectrographs, similar to Carmenes. One of them, called SPIRou, has just started a Hawaiian observation campaign, while the UNIGE Department of Astronomy hosts the first tests of the Near Infrared Planet Researcher (NIRPS), which will be installed in Chile in late 2019. The result will increase the interest of the scientific community by these instruments. Their number and geographical distribution will allow us to cover the whole sky, in search of the evaporation of exoplanets, "concludes Lovis.
* Four ACES, Future of the Upper Atmospheric Characterization of Exoplanets with Spectroscopy, is a project funded by a Consolidator donor of the European Research Council (ERC) under the European Union Research and Innovation 2020 Program (subsidy agreement n ° 724427) .
Université de Genève. .