Genevan Researchers Observe Exoplanet Inflate Like a Balloon

Helium is a rare element on Earth but abundant in the Universe. After hydrogen, it is the main component of stars and gaseous giant planets. Notwithstanding its ubiquity, helium was only discovered recently in the atmosphere of a gaseous giant by an international team with astronomers from the University of Geneva (UNIGE), Switzerland. Genevan researchers led the team, who observed for the first time and in great detail how this gas escapes from the overheated atmosphere of an exoplanet, literally inflated with helium.

Helium is the second most copious element in the Universe. Since the year 2000, it’s been predicted to be one of the best possible tracers of the atmospheres of exoplanets, which are planets that orbit around other stars than the Sun. It took astronomers nearly two decades to detect it. It was hard to see as a result of the bizarre observational signature of helium, situated in the infrared, out of range for most of the instruments that were previously used.

The discovery took place earlier in 2018, due to the Hubble Space Telescope observations, which proved tricky to interpret. Team members from UNIGE, members of the National Centre for Competence in Research PlanetS, had the idea of pointing another telescope outfitted with a brand-new instrument — a spectrograph called Carmenes.

Like a rainbow, a spectrograph decomposes the light of a star into its component colours. The “resolution” of a spectrograph is a measure showing the number of colours that can be revealed. While the human eye is unable to distinguish any colour beyond red without an adapted camera, the infrared eye of Hubble can detect hundreds of colours there. This proved sufficient to identify the coloured signature of helium. The instrument Carmenes, equipped on the 4-metre telescope at the Observatory of Calar Alto in Andalusia, Spain, can distinguish over 100’000 colours in the infrared!

This high spectral resolution enabled the team to observe the position and speed of helium atoms in the upper atmosphere of a gaseous Neptune-size exoplanet, four times bigger than the Earth. Situated in the Cygnus (the Swan) constellation, 124 light-years from home, HAT-P-11b is a “warm Neptune” (a decent 550°C!), twenty times closer to its star than our planet is from the Sun. Romain Allart, Ph.D. student at UNIGE and first author of the study said that the new observations are so exact that the exoplanet atmosphere is undoubtedly inflated by the stellar radiation and escapes to space.

Vincent Bourrier, co-author of the study and member of the European project FOUR ACES* backed these observations by numerical simulation. Due to the simulation, it is possible to track the trajectory of helium atoms. Bourrier explained that helium is blown away from the day side of the planet to its night side at over 10’000 km/h because it is such a light gas, it escapes easily from the attraction of the planet and forms an extended cloud all around it. This gives HAT-P-11b the shape of a helium-inflated balloon.

This result paves the way to observe the extreme atmospheric conditions reigning in the hottest exoplanets. The Carmenes observations portray that these studies, which were believed to only be feasible from space, can be accomplished with better precision by ground-based telescopes outfitted with the proper instruments.

What’s more, UNIGE astronomers are very much involved in the design and use of two new high-resolution infrared spectrographs, similar to Carmenes. One of them, known as SPIRou, has just begun an observational campaign from Hawaii, while the UNIGE Department of astronomy is home to the first tests of the Near Infrared Planet Searcher (NIRPS), which will be installed in Chile at the end of 2019.

The results of their work are published in Science.