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Future of upper atmospheric characterisation of exoplanets with spectroscopy

Periodic Reporting for period 2 - FOUR ACES (Future of upper atmospheric characterisation of exoplanets with spectroscopy)

Reporting period: 2018-12-01 to 2020-05-31

Planets beyond our Solar System (exoplanets) exhibit a huge variety of sizes, levels of insolation and structures. Understanding this diversity is the key to reconstruct the history of all planetary systems, including our own. One key science driver is the possibility of life on another planet. To find out, we analyse the atmospheres of exoplanets, hoping to find chemical disequilibrium in their composition. Finding life on another planet will have a strong impact on how we understand the origin of life on Earth. The main challenges are to find exoplanets similar to Earth, determine whether they are able to host life and probe their atmospheres with sufficient precision to detect their chemical composition. We are able to do that for exoplanets extremely close to their stars, which are too hot to be habitable. Studying these hot exoplanets, we realised that they are losing their atmosphere, which is literally evaporating. Atmospheric evaporation produces huge clouds of gas around the planet, which yield strong observational signatures. In contrast, the signature of atoms or molecules low in the atmosphere of a planet are much more difficult to detect. Therefore, the main idea behind this ERC project is to use atmospheric evaporation as a magnifying glass to probe the atmospheric properties of exoplanets, from gaseous giants to Earth-like planets.
The FOUR ACES core team is composed by the PI, two PhD students and two postdoctoral researchers. Previous team members include two postdoctoral researchers, now off to permanent positions in academia and the private sector.

As of 31 May 2020, the team has published 68 articles in international refereed journals, receiving a total of 1163 citations (source: NASA ADS). Among these 68 articles, 6 were published in high-impact interdisciplinary journals (5 in Nature and 1 in Science). FOUR ACES team members are lead authors of 17 of articles out of these 68 (5 are led by the project PhD students), including 3 articles in Nature. Green open access has been enforced for all publications through the arXiv server.

Milestone results are
- The detection of iron condensation across the nightside of an ultra hot gas giant exoplanet (Ehrenreich et al. 2020, Nature 580, 597; PR:
- The discovery of a misaligned planetary system, where the planet is evaporating (Bourrier et al. 2018a, Nature 553, 477; PR:
- The detection of metallic vapour in a giant exoplanet hotter than a star (Hoeijmakers et al. 2018, Nature 560, 453; PR:
- The first detection of helium escaping from a bloated exoplanet (Spake et al. 2018, Nature 557, 68) and the first detailed measurement of the shape and dynamics of an escaping cloud of helium around an exoplanet (Allart et al. 2018, Science 362, 1384; PR:
- The detection of a second case of evaporating Neptune-mass exoplanet (Bourrier et al. 2018b, A&A 620, A147; PR:
We have demonstrated that atmospheric evaporation is common for Neptune-mass exoplanets and can be strong enough to explain the lack of such planets very close to their stars. This is the first strong clue of the impact that stellar irradiation has on the evolution of exoplanets. We have also opened the way for a new way to probe escaping atmospheres by uncovering the signature of helium in several exoplanets and use 3D models to interpret the signature. This signature is readily observable using high-resolution spectroscopy in the near-infrared, meaning that we can now observe atmospheric escape from the ground (previously, only the Hubble Space Telescope could detect the hydrogen escaping from exoplanets in the ultraviolet). In contrast with the hydrogen signature, the helium signature is unaffected by the interstellar medium and we can thus access most known exoplanets, creating the first big survey for atmospheric evaporation. These ground-breaking results were not expected until the end of the project. We will thus use it to substantially orient the scientific programmes of several new instruments the team is involved with, including ESPRESSO at the VLT and NIRPS at the ESO 3.6m telescope. With ESPRESSO, we could scan the surface of a giant exoplanet; we measured a difference in the chemical composition at two location on the planet. We concluded from this that iron atoms present as a gas on the evening side of the planet must condense (rain or snow) during the night. This provides a new way to trace extreme climates on highly irradiated exoplanets.
Artist's impression showing the condensation of iron at the twilight zone of an ultrahot gas giant
Artist's impression showing the orbital misalignment of an evaporating Neptune-mass exoplanet
Artist's impression showing the detection of escaping helium from a Neptune-mass exoplanet
Artist's impression showing the discovery of metallic vapour on an extremely hot gas giant