High definition and time-resolved studies of exoplanet atmospheres: a new window on the extreme diversity of the exoplanet zoo

Project Information

exoZoo

Grant agreement ID: 805445

DOI

10.3030/805445

Project closed

EC signature date 3 October 2018

Start date 1 September 2019

End date 31 August 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 500 000,00

EU contribution

€ 1 500 000,00

Coordinated by

THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF OXFORD
United Kingdom

Periodic Reporting for period 5 - exoZoo (High definition and time-resolved studies of exoplanet atmospheres: a new window on the extreme diversity of the exoplanet zoo)

Reporting period: 2025-01-01 to 2025-08-31

Summary of the context and overall objectives of the project

After 31 years and 6000+ discoveries of planets orbiting other stars, we have barely scratched the surface of the seemingly infinite forms that other worlds take. Each study of a new exoplanet atmosphere reveals a rich, eclectic zoo of conditions. From the blistering heat of hot Jupiters and the hazy mini-Neptunes, to rocky planets abound in their diversity, as reflected in our own Solar system, from sulphuric acid clouds to arid deserts; they are vastly different to our cosy home on Earth. How a universal star-planet formation process could create and evolve such differing worlds is unclear, and how many of these result in conditions that could support life, is unknown. Is life common, rare, or singular, and what is its resilience in such vastly different environments from Earth? We live at a special time where we will have the technological power to measure this for the first time in human history with the Extremely Large Telescope (ELT) due online at the end of the 2020s. It is the driving force behind this ERC project (exoZoo) where we will ensure we are ready to use it to search for life beyond Earth; the next great scientific leap.

To do this, the overall objectives of this ERC project are to open three new observational avenues that will leverage the ELT to study exoplanet atmospheres in exquisite detail. It connects atmospheric properties to formation and appearance, and enables the search for signs of life, aka biosignatures, such as oxygen and other gases in their atmospheres. These three new avenues are i) Multi-resolution: combining ground-based high resolution spectroscopy with space-based observations to place the best constraints on the composition and structure of exoplanet atmospheres and connect this to their formation processes; ii) Reflected light: to access wavelengths containing key biosignature gases, and iii) Variability: using cutting-edge optical components to isolate and monitor light directly from exoplanets, mapping out their appearance as they rotate and searching for their exomoons.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

By exploring exoplanet atmospheres across multiple spectral resolutions we achieved multiple successes:

- first detection of molecular features using high resolution cross-correlation spectroscopy (HRCCS) in the M-band (4-5 micron);
- first detection of SiO in an sub-stellar atmosphere with HRCCS;
- first detection of CO lines in emission in a hot Jupiter;
- first demonstration that 3D atmospheric dynamics, such as offset hotspots, can be revealed in high resolution spectroscopic phase curves through Bayesian retrievals;
- measured chemical and cloud constraints for a sub-Neptune atmosphere with HRCCS as well as achieved with space-based telescopes (JWST).

Our success with the M-band means that METIS/ELT (3-5 microns) is a viable and powerful machine for biosignature hunting. We further contributed to the first discovery of hydroxyl radical (OH) emission and the use of its individual spectroscopic bands as an exoplanet thermometer. We also demonstrated the importance of including disequilibrium modelling in HRCCS.

We made major breakthroughs in the development of HRCCS for reflected light studies with the ELT, using the world’s largest optical telescope, the combined 16-m VLT, to observe the reflective atmosphere of a hot Neptune. The data were so sensitive that they ruled out the presence of titanium oxide. This sensitivity is extremely promising for using the technique to study biosignatures with the ELT. We simulated the capability of HARMONI/ELT to do this for the nearest rocky exoplanet, Proxima b, resulting in urgent interventions on the design of its coronagraph to enable it. We further made recommendations for future space-based reflected light instruments, responding to calls from ESA’s Voyage 2050, and setting coronagraphic requirements for the Habitable Worlds Observatory.

Using state-of-the-art optics and advanced data processing, we monitored light directly from a planetary mass companion to a repeatable 4% precision level using ground-based telescopes, the most precise to date. Our studies are not yet limited by a noise floor, indicating that sub-1% precision may be possible, which would allow mapping of exoplanet weather systems and the search for their transiting exomoons. This innovative and novel technology combination will become a primary exoplanet characterisation tool for the ELT's METIS, ANDES, and PCS.

We have disseminated our results via numerous talks at international academic conferences, including 11 keynote talks; as well as through public engagement, reaching ~2 million listeners on BBC Radio 4’s In Our Time (https://www.bbc.co.uk/programmes/m0025vvd).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We made four notable advances in the state of the art.

First was overcoming the thermal background noise that dominates HRCCS the M-band, opening up this new wavelength region for exoplanet studies. We did this through novel processing algorithms to remove the contamination from Earth’s atmosphere.

Next, we developed the HRCCS Bayesian likelihood framework to encode vital 3D information about the planet’s atmospheric dynamics through an orbital phase-dependent scaling factor. Rather than measure the brightness of the planet as it rotates, we instead measured how the spectral line depth changes, revealing changing thermal structure caused by offset hotspot regions. Crucially, this 3D information came without needing to run a full 3D retrieval (currently prohibitively slow to run). We achieved this at a resolution of just R~15,000 (much lower than the typical R~100,000 for HRCCS). This was largely due to the fast orbital speed of the hot Jupiter we observed, which showed that e.g. very close in lava worlds could be studied with large ground-based telescopes, trading resolution for more photon collecting power.

We advanced reflected light HRCSS in several ways. We showed that for systems where the planet orbits faster than the star rotates, that its reflected light spectrum can be significantly broadened. This reduces the number of lines that can be detected with HRCCS, and must be accounted for when determining observing time. Our 16-m VLT study showed that cloud deck altitudes can be measured with HRCCS in reflected light, but only if sufficient absorption lines from the planet can be observed. While cloud decks can be reflective, this does not always mean that HRCCS will be successful in detecting the planet’s gases, a key consideration when exploring the diversity of rocky worlds with the ELT.

Finally, key to our success in exoplanet variability monitoring was the use of novel vector apodizing phase plates to null the light from the host star and provide a simultaneous reference, then projecting it onto an integral field spectrograph. This reduced contamination by spreading out the light and enabling removal of problem wavelength regions, before recombination into a white light curve. This approach draws on space-based techniques to extract precision light curves (differential spectrophotometry), but its novel twist is that it is applied directly to the planet rather than the host star.

The exoZoo logo showing the goals of fingerprinting exoplanet atmospheres, mapping their features, a