C.4: Atmospheres and surfaces of small planets at high spectral resolution
The discovery of a large population of sub-Neptunes and super-Earths on close-in orbits remains one of the most significant achievements of exoplanet science to date. Unknown to our Solar System, these objects must be studied and understood in detail if we are to obtain a comprehensive picture of planet formation and evolution in our Galaxy. While precise mass and radius measurements will continue to provide basic characterisation of these objects through their bulk density, measurements of their atmospheric and surface composition are mandatory to make progress and lift the degeneracies that currently plague the modelling of their interior.
Fortunately, instrumental developments are about to make this a real possibility on a large scale. In particular, the transit spectroscopy technique has been developing at an extremely fast pace over the past few years and is now targeting exoplanets smaller than Neptune. Detection of molecular species and constraints on atmospheric scale height and temperature structure promise to bring much-needed insights into the nature of sub-Neptunes and super-Earths. The James Webb Space Telescope (JWST) will be able to play a critical role in the characterisation of this low-mass population through low-resolution transit spectroscopy. However, ground-based instrumentation is taking a more and more prominent role, especially through the use of high-resolution spectroscopy. In recent years, high-resolution spectrographs have detected various atomic and molecular species in many hot/warm Jupiters and Saturns and have been able to constrain their atmospheric dynamics through Doppler-shift measurements of spectral lines. This approach has been successfully developed by our team during the first two phases of the NCCR PlanetS. In particular, we were the first to detect helium in a Neptune-size exoplanet, HAT-P-11b (Allart et al. 2018), opening the way to detailed studies of atmospheric escape in low-mass planets. We intend to leverage and strengthen this expertise in Phase III.
Beyond sub-Neptunes and super-Earths, the ultimate goal of atmospheric studies is the characterisation of terrestrial planets in the habitable zone. We intend to become leaders in this field by developing and demonstrating a novel observing technique: high-contrast, high-resolution spectroscopy in reflected light. The basic idea is to combine a high-performance adaptive optics (AO) system to a high-resolution spectrograph.
The AO system spatially separates the planet from its star and increases the planet-to-star contrast at the location of the planet by up to 3 orders of magnitude. An integral-field unit then collects the planet light in the focal plane and sends it to the spectrograph through a bundle of single-mode optical fibres. Finally, the spectrograph spectrally separates the planet light from the remaining stellar light, taking advantage of the different spectral content and Doppler shift of the two signals. This final step allows the instrument to detect molecular features in exoplanet atmospheres at contrast levels of 10-7 to 10-8 relative to the host star. This enables reflected-light spectroscopy of exoplanets orbiting nearby stars, e.g., the temperate rocky planet Proxima b. Contrary to transit spectroscopy, reflected light can probe the deeper layers of exoplanet atmospheres, and even their surfaces. It thus would be able to probe surface features on habitable worlds, including ice caps or ocean glint. Developing the high-contrast, high-resolution approach is a major goal of this project and a critical step towards the future instruments from the European Southern Observatory (ESO): ELT-ANDES and ELT-PCS.
Work Package 1: Transit spectroscopy of sub-Neptunes and super-Earths with ESPRESSO and NIRPS.
In Phase III we intend to exploit the new spectrographs ESPRESSO and NIRPS to carry out an extensive atmospheric survey of sub-Neptunes and super-Earths. In particular, we will take advantage of a large observing time allocation for atmospheric studies within the NIRPS guaranteed-time (GTO) programme. We will aim at detecting molecules such as H2O, CH4 and CO2 as well as helium in these atmospheres in order to constrain their composition and structure. A NCCR PlanetS-funded PhD student is currently working on this topic.
Work Package 2: High-resolution spectra of habitable exoplanets in reflected light.
Simulating the vast diversity of possible atmospheres and surfaces on terrestrial planets observed at high spectral resolution is the main goal of this WP. We are focusing on the coupling of 3D global climate model (GCM) simulations of terrestrial planets to a line-by-line radiative transfer code to produce realistic high-resolution reflected-light spectra. These in turn will be used to simulate observations with the future instruments VLT-RISTRETTO, ELT-ANDES and ELT-PCS. We have hired a NCCR PlanetS-funded PhD student to work on this work package (WP), as a shared position with Project C1. This is the continuation of activities started in Phase II bonus Project 4.2.
Work Package 3: Science with the RISTRETTO instrument.
The high-contrast, high-resolution technique is a novel approach that needs to be demonstrated on sky before it can be deployed at the ELT. For this purpose we are currently building the RISTRETTO instrument, a compact, AO-fed spectrograph to be installed at the VLT. RISTRETTO will be able, for the first time, to study nearby exoplanets in reflected light. Accessible targets include the temperate giant planets orbiting GJ 876, the super-Earth GJ 887c, and even the habitable-zone planet Proxima b. Moreover, RISTRETTO will be able to study accreting protoplanets like PDS 70 b/c through their H-alpha emission. It will also be a unique ground-based facility to obtain spatially-resolved spectra of solar system objects. This WP provides scientific support to RISTRETTO by developing these various science cases, leveraging the diverse expertise available across the NCCR PlanetS. The PhD student from WP2 is contributing to this WP.
Work Package 4: Development of the RISTRETTO AO system.
A major challenge for RISTRETTO is the development of an extreme AO system able to reach the required Strehl ratio and contrast at the diffraction limit of the telescope. The goal of this WP is to provide support to these efforts through the hiring of a PhD student who is working in collaboration with the Laboratoire d’Astrophysique de Marseille (LAM, France). The PhD student is currently developing the wavefront sensor of RISTRETTO, a near-infrared Pyramid wavefront sensor based on the C-RED ONE camera. The long-term goal of this WP is to develop critical know-how in adaptive optics within the NCCR PlanetS for the future instruments ELT-ANDES and ELT-PCS.
Work Package 5: Probing habitable exoplanets with ELT-ANDES.
The study of a sizable sample of habitable-zone exoplanets is the main science goal of the ELT-ANDES spectrograph, planned for 2030, in which Switzerland intends to play a major role. Prof. Christophe Lovis is the Swiss representative in the ANDES Board and has obtained funding for the project from the SNF-FLARE programme. This WP will provide scientific support to the ANDES project by developing the exoplanet science cases, in particular the characterisation of habitable planets through transit and reflected-light spectroscopy. The PhD students from WP1 and WP2 directly contribute to this WP.
Work Package 6: The nearby stars exoplanet census.
An essential condition to characterise exoplanets with RISTRETTO, ANDES, and other ELT instruments is the availability of a large enough sample of known planets orbiting very nearby stars (< 10-15 parsecs), so that planets can be angularly separated from their host star. Such objects are usually discovered with the radial-velocity (RV) technique, as most of them are not transiting. It is thus crucial for the future of the field to build an exhaustive catalogue of nearby exoplanets using RV surveys with instruments like HARPS, HARPS-N, ESPRESSO, CARMENES and NIRPS. In Phase II of the NCCR PlanetS we started an observational effort in that direction, as a complement to existing surveys. We are continuing this programme in Phase III. This WP is a close collaboration with Project C2.
Collaborative component and interdisciplinarity within the NCCR PlanetS
WP1 is a participation in the NIRPS Consortium GTO programme, and a close collaboration with Project B3. WP2 is a joint effort with Project C1. WP2, WP3 and WP5 have close ties to Projects C1, C2, and potentially A2, B4, C3 and C5 for WP3. WP4 is a collaboration with LAM. WP5 is a contribution to the ANDES Consortium science team. WP6 is a joint effort with Project C2.
