A SpectroPhotometric Inquiry of Close-in Exoplanets around the Desert to Understand their Nature and Evolution

Project Information

SPICE DUNE

Grant agreement ID: 947634

DOI

10.3030/947634

EC signature date 12 October 2020

Start date 1 March 2021

End date 28 February 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 499 956,00

EU contribution

€ 1 499 956,00

Coordinated by

UNIVERSITE DE GENEVE
Switzerland

Periodic Reporting for period 3 - SPICE DUNE (A SpectroPhotometric Inquiry of Close-in Exoplanets around the Desert to Understand their Nature and Evolution)

Reporting period: 2024-03-01 to 2025-08-31

Summary of the context and overall objectives of the project

The goal of SPICE DUNE is to determine the origin of the hot Neptune desert, a deficit of Neptune-size planets on very short orbits. This mysterious feature contains the imprint of processes that shaped the population of exoplanets orbiting close to their star - and further. The relative role and the coupling of these processes, in particular atmospheric evaporation and orbital migration on secular timescales, remain unclear. This is because we lack optimal observational tracers to validate and constrain numerical models of exoplanet evolution. SPICE DUNE thus aims at gathering observations of escaping atmospheres from gas giants to super-hot rocky planets all around the desert. These data will drive the development of self-consistent models of upper atmosphere then used to derive the planets' erosion rate. We will combine for the first time these erosion rates with measurement of the planets' orbital architecture to constrain population syntheses coupling long-term orbital and atmospheric evolution of close-in planets. This ambitious approach, exploiting advanced modelling informed by the most relevant tracers, will unveil the evolutionary tracks of exoplanets and bring insights into their nature.

The proposed research addresses high-priority questions related to the origins of close-in planets, and has implications for the origin of both extrasolar systems and our own solar system's evolution.

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

SPICE DUNE articulates into three work packages (WPs): AGATE (Analyzing Giants Atmospheric Escape), JADE (Joining Atmosphere and Dynamics for Exoplanets), and JASPER (Judging the Amplitude of Small Planets ERosion). As of September 2024 the team has published 69 articles in international refereed journals, receiving more than 626 citations (source: NASA ADS). Out of these articles, 11 were led by SPICE DUNE team members (inc. 3 by the project PhD students). Green open access has been enforced for all publications through the arXiv server, and results have been disseminated through conferences, press releases, interviews and illustrations. The team obtained several open time observing programs on space-borne and ground-based instruments (including four programs led by the team PhD students, and the largest transit program ever granted on the VLT) and participates in guaranteed observation programs on cutting-edge instrumentation (JWST, HST, CHEOPS; ESPRESSO, NIRPS).

Main work and results :
- Pathfinder series of publications to study atmospheric escape and orbital architecture in a sample of planets around the Neptunian desert (DREAM I, Bourrier et al. 2023, 669, A63; DREAM II, Attia et al. 2023, 674, A120; DREAM III, Guilluy et al. 2023, 676, A130; PR: https://www.unige.ch/medias/en/2023/migration-tumultueuse-en-bordure-du-desert-des-neptunes-chaudes). This work led to ATREIDES (Bourrier et al. 2025, A&A, 701, A190), an international collaboration based on the first large program for transits with the VLT/ESPRESSO, which aims at measuring the orbital architectures of 60 close-in Neptunes (PR: https://www.unige.ch/medias/2025/le-programme-atreides-la-recherche-des-exo-neptunes-perdues).
- Discovery of the Neptunian Ridge, an overoccurrence of planets separating the Neptunian desert and savannah (led by a visiting PhD student to the team, Castro-González et al. 2024, A&A, 9, A250; PR: https://www.unige.ch/medias/2024-2/des-exoplanetes-nichees-entre-desert-et-savane-neptunienne).
- Design of ANTARESS, an advanced pipeline to process high-resolution spectra and extract planetary and stellar spectra (Bourrier et al. 2024, A&A, 45, A113). This pipeline allows the use of a technique developed by the team (the Rossiter-McLaughlin “Revolutions”) to measure the orbital architecture of exoplanets, used to discover a system with two planets on perpendicular orbits (Bourrier et al. 2021, A&A 654, A152; PR: https://www.unige.ch/medias/en/2021/les-orbites-renversantes-dun-systeme-multi-planetaire).
- Upgrade of the 3D numerical code EvE to simulate the escaping atmosphere of hot giant planets (Dethier & Bourrier 2023, A&A 674, A86) and account for the biases induced by the inhomogeneous stellar surface and the system orbital architecture (Carteret et al. 2024, A&A 683, A63). These works are part of a larger effort to promote a standard, holistic interpretation of high-resolution transit spectra (Allart et al. 2025, A&A, 25, A7).
- Development of the numerical code JADE to simulate the coupled atmospheric and dynamical evolution of a close-in planet system over secular timescales (Attia et al. 2021, A&A, 647, A40)
- Development of NIGHT, novel concept of spectrograph to survey helium escaping from exoplanets (Farret Jentink et al. 2024, MNRAS, 527, 3).

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)

The ANTARESS pipeline allows for a robust and homogeneous reduction and processing of high-resolution transmission spectra. Open-source and user-friendly, it aims at providing the community with a standard tool to produce high-quality master stellar spectra, extract stellar and planetary lines, and analyze them to measure, in particular, the orbital architectures of planetary systems. The new technique that we developed to measure orbital architectures gives access to planets that could not be studied previously, down to Earth-size and transiting faint, slow-rotating stars. As a result, we could build the ATREIDES collaboration which studies orbital architectures across the Neptunian landscape, a fascinating region displaying features like our Ridge detection which trace the evolutionary processes acting on close-in planets.

On the atmospheric side we upgraded beyond the state of the art our 3D code of evaporating exoplanets, EvE, to interpret transit spectroscopy datasets while accounting for contamination by the star. Next is the inclusion of an advanced model to describe extended upper atmospheres and interpret helium signatures. We are applying our model to measurements on specific systems acquired with ground-based (NIRPS) and space-borne (JWST) spectrographs, in preparation for the large survey that we will carry out with NIGHT, a novel concept of near-infrared spectrograph (narrow-band, high-resolution, compact, and portable) that we are developing. The final version of EvE will allow deriving the mass loss close-in exoplanets in a global framework accounting for the system orbital architecture and stellar surface properties.

Mass loss and orbital architecture measurements are used as constraints in JADE, an original model that we developed to couple the atmospheric and dynamical secular evolution of close-in exoplanets. We showed the critical feedback of the atmosphere on dynamical migration, suggesting a specific evolution for planets at the border of the Neptunian desert. We will use JADE to implement these evolutionary pathways in population synthesis and determine the origins of the various classes of close-in planets.

Artist representation of the RIDGE (credits: Elsa Bersier)

Artist logos of SPICE DUNE and its three work packages (credits: Elsa Bersier)

Artist representation of the SPICE DUNE project (credits: Elsa Bersier)

Artist representation of two planets found on perpendicular orbits around their star