REVEALing Signatures of Habitable Worlds Hidden by Stellar Activity

Informations projet

REVEAL

N° de convention de subvention: 101118581

DOI

10.3030/101118581

Date de signature de la CE 12 Janvier 2024

Date de début 1 Avril 2024

Date de fin 31 Mars 2030

Financé au titre de

• European Research Council (ERC)

Coût total

€ 6 831 455,50

Contribution de l’UE

€ 6 831 455,00

Coordonné par

UNIVERSITAET GRAZ
Austria

Periodic Reporting for period 1 - REVEAL (REVEALing Signatures of Habitable Worlds Hidden by Stellar Activity)

Période du rapport: 2024-04-01 au 2025-09-30

Résumé du contexte et des objectifs généraux du projet

For millennia, humans have asked whether Earth-like planets exist beyond the Solar System and whether they might show signs of life. Today, this question has become experimentally accessible. Extreme-precision radial-velocity spectrographs are approaching the sensitivity required to detect Earth-mass planets in habitable zones, while facilities such as the James Webb Space Telescope aim to probe the atmospheres of rocky exoplanets.

This progress is now limited by a fundamental astrophysical barrier. Stellar surface inhomogeneities caused by magnetic activity and convection introduce variability signals that contaminate the subtle signatures of small exoplanets, affecting both planet mass measurements and the interpretation of atmospheric spectra. Overcoming this “stellar variability problem” is therefore essential for further progress toward detecting Earth analogues and potential biosignatures.

The ERC Synergy project REVEAL addresses this challenge by combining expertise in stellar magnetohydrodynamics, spectral synthesis, observations, and data analysis. Its central objective is to build a physically grounded framework that links stellar surface physics directly to the observables used in exoplanet detection and characterization.

A cornerstone of the project is the construction of realistic three-dimensional magnetohydrodynamic models of stellar atmospheres across a broad range of spectral types. Using the MURaM code, REVEAL has carried out simulations from Sun-like stars down to ultra-cool M dwarfs, providing a physically consistent “ground truth” for stellar surface variability. Building on this foundation, the project advanced spectral synthesis, enabling the first starspot spectra based directly on 3D MHD simulations, and introduced a new approach to quantify the sensitivity of spectral lines to granulation using the ergodic nature of stellar convection. These developments underpin forward-modeling tools that connect spatially resolved simulations to disk-integrated observables.

REVEAL further addresses the impact of stellar variability on exoplanet atmospheric studies. The project showed that stellar activity can interfere with measurements of morning–evening temperature contrasts on transiting planets and identified diagnostics to distinguish stellar contamination from genuine planetary signals. Observationally, the Sun is used as a benchmark and this approach is extended to planet-hosting stars, including the use of JWST observations of flares on TRAPPIST-1 to infer the spectra of active surface features.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

During the reporting period, REVEAL carried out a coordinated program of numerical modeling, spectral synthesis, data analysis, and interpretation of high-precision observations to address the impact of stellar variability on exoplanet detection and characterization.

A major effort focused on first-principles modeling of stellar surface variability. Using the 3D radiative magnetohydrodynamics code MURaM, the consortium simulated stellar photospheres across a wide range of spectral types, from Sun-like stars to late M dwarfs. These simulations enabled the calculation of emergent spectra from realistic 3D atmospheres and delivered the first starspot spectra based directly on magnetoconvective simulations (Smitha et al. 2025), providing essential inputs for modeling stellar contamination in transmission spectroscopy and radial-velocity measurements.

Within REVEAL, a novel approach was developed to quantify the sensitivity of spectral lines to granulation based on the ergodic nature of stellar convection, enabling efficient mitigation of granulation-induced radial-velocity variability (Sowmya et al., submitted).

On the observational side, the project analyzed ultra-precise data from facilities such as JWST. REVEAL demonstrated how stellar magnetic activity can interfere with measurements of exoplanet atmospheric properties, including morning–evening temperature contrasts inferred from transit light curves (Kostogryz et al. 2025), and identified diagnostics to separate stellar and planetary signals. In parallel, the consortium addressed methodological systematics in transit spectroscopy, quantifying biases from commonly used limb-darkening prescriptions and providing a robust recipe for reliable transmission-spectrum extraction (Keers et al. 2024). Additional work reassessed assumptions in stellar atmosphere modeling, showing that pressure broadening has been systematically overestimated in cool stars (Glidden et al., submitted).

REVEAL also contributed to a broader assessment of model-dependent biases in interpreting stellar and planetary spectra. This includes work extending stellar contamination concepts to emission spectroscopy (Fauchez et al. 2025) and studies showing how gravity and metallicity bias spectral diagnostics in ultracool dwarfs (Davoudi et al. 2025). Supporting results further strengthened the project’s foundations, including models of solar/stellar UV variability (Sowmya et al. 2025) and empirical constraints on magnetic surface features in ultracool dwarfs derived from JWST flare observations (Vasilyev et al. 2025).

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

REVEAL has delivered results that go beyond the current state of the art in both stellar and exoplanet research.

First, the project replaced simplified, empirical descriptions of stellar activity with a physics-based framework. By coupling 3D MHD simulations with realistic spectral synthesis and forward modeling, REVEAL enables stellar variability to be treated as a predictable astrophysical signal rather than an irreducible noise source (Smitha et al. 2025; Seager & Shapiro 2025). A key methodological advance is a new, computationally efficient framework to quantify granulation-induced radial-velocity variability based on the ergodic nature of stellar convection (Sowmya et al., submitted).

Second, REVEAL demonstrated that widely used assumptions in exoplanet analysis are not universally valid. This includes the breakdown of 1D spot models for cool stars (Smitha et al. 2025), the finding that faculae can be dark on cool and metal-poor stars (Shapiro & Seager et al., submitted), and the identification of limb-darkening prescriptions that introduce spurious spectral features at JWST precision (Keers et al. 2024).

Third, the project identified previously unrecognized stellar contamination channels, such as activity-induced ingress–egress asymmetries that can mimic planetary day–night temperature contrasts (Kostogryz et al. 2025). Their distinct wavelength dependence provides practical diagnostics to disentangle stellar and planetary signals.

REVEAL further established a Synergy feedback loop between stellar physics and exoplanet observations: JWST data were used not only to study planets, but also to diagnose deficiencies in stellar atmosphere models, such as pressure broadening in cool dwarfs (Glidden et al., submitted). The project also showed that stellar contamination extends beyond transmission spectroscopy to emission and secondary-eclipse observations (Fauchez et al. 2025), and that retrieval outcomes depend sensitively on molecular inventories (Niraula et al. 2025). Together with evidence for gravity- and activity-related biases in stellar diagnostics (Davoudi et al. 2025), these results reinforce the central REVEAL message: robust exoplanet characterization requires simultaneous progress in stellar modeling, spectral synthesis, and interpretation strategies.