Planet copyThe Solar System gas giants Jupiter and Saturn orbit our Sun at large distances, where solar light carries only little heat. The situation is quite different in extrasolar planetary systems like HD 189733 and WASP-69. Here two gas giants orbit their stars at such small distances that a complete revolution, corresponding to one "year" on the planet, takes only a few days. In such proximity, stellar irradiation strongly heats the gas giants, which are therefore called hot Jupiters. One of the exciting questions in exoplanet research is how the atmospheres of hot Jupiters react in their extreme environments and if the planets even constantly endure evaporation.

Planetary transits are invaluable opportunities to observe such phenomena. When a planet moves in front of its host star, stellar light traverses the planetary atmosphere, which imprints its spectral signature on the traversing light. Indeed, transit observations with the Hubble Space Telescope have shown that hot Jupiters lose large amounts of material. This evaporated material can form a tail behind the planet, similar to the tails produced by comets in the Solar System. Until now evaporating planetary atmospheres could only be studied in ultraviolet light with spaceborne instrumentation.

Now a detection of helium in the atmospheres of hot Jupiters with ground-based instrumentation has been achieved for the first time. On Thursday, a team of researchers from Germany and Spain presented their discovery in two publications in Science and Astronomy & Astrophysics. Observations obtained with the high-resolution spectrograph CARMENES mounted at the 3.5m-telescope of the Calar Alto observatory revealed the signal at even higher spectral resolution than possible with the Hubble Space Telescope.

Astronomers from the Hamburg Observatory took the lead in the analysis of HD 189733 b, one of the best studied hot Jupiters to date. They found that helium in the atmosphere of HD 189733 b removes a small but well-detectable fraction of the infrared stellar light during the transit. The resulting absorption line contains abundant information about the planetary atmosphere. Particularly, the atmospheres appear to be rather compact and does not form a long comet-like tail. Because of the high data quality, the team could even measure atmospheric wind velocities, which indicate that the atmosphere moves differently on the two sides of the planet. "Maybe the atmosphere rotates even faster than the planet itself, a curiosity known from Venus", says Michael Salz, who is the first author of the publication. This so-called superrotation has never been observed in such elevated atmospheric layers and it certainly affects the distribution of the irradiated heat over the planetary surface.

A somewhat different picture emerged for the planet WASP-69 b, which was studied by a team from Spain. The absorption signal is even stronger in this system than in HD 189733 b and it still persists after the planet has passed over its host star. "Clearly, this planet is trailed in a comet-like tail by helium and likely other gases that have been evaporated from the planet", reports Lisa Nortmann, who is the lead author of the Science publication.

So far, we expected that the strength of the helium signature should depend directly on the stellar irradiation in the ultraviolet and X-ray range. And indeed, the analysis of three planetary atmospheres less intensely irradiated than those of WASP-69 b and HD 189733 b did not result in a measurable helium absorption signal. The presented work clearly proves the scientific potential offered by these novel observations to study the atmospheres of a large number of planets with ground-based instrumentation.


- Contact person at the observatory: Michael Salz
- Astronomy & Astrophysics publication
- Science publication
- Press release of the MIN faculty of the University of Hamburg (german)
- Press release of the Instituto de Astrofisica de Canarias - IAC

Image Credit:
Gabriel Perez Diaz (IAC)