Project C2 – Towards the direct detection of terrestrial exoplanets
One of the long-term objectives of exoplanet science (and as such of the NCCR PlanetS and its anticipated successor SIPS) is the direct detection of a sizeable sample of small, terrestrial exoplanets in order to investigate their atmospheric properties and search for indications of habitability and – possibly – biological activity. In this project we aim at taking some important steps towards this goal by combining various activities that directly inform and prepare future exoplanets instruments and missions. Being directly involved in the development of instruments for the ESO ELT and pushing for future exoplanet space missions of significant scale, these activities are timely and critically important. At the end of this project, we will have a better understanding of, 1) what future instruments / missions should be able to measure, and 2) what nearby systems (within 10-20 pc) are the primary targets for the direct search for small, potentially habitable worlds.
The project is split into the following five work packages (partially funded through in-kind contributions from ETH Zurich). Some of them are a direct continuation of ongoing projects from Phase II; some are strongly interlinked.
WP1: The nearby stars exoplanet census. We will continue our efforts to systematically search for ex- oplanets around nearby stars (cf. Boehle et al. 2019). We will re-analyse data from the ESO archive obtain new data with, e.g., ERIS and ESPRESSO, and combine datasets from the various detection techniques (in particular RV, high-contrast imaging and GAIA) to effectively probe a broader exoplanet parameter space than provided by each technique alone. While the detection of new exoplanets is a primary goal of these efforts, also non-detections and the resulting upper limits on the existence of certain types of planets in nearby systems is of great importance, in particular for planning future observations with ELTs and space missions. This WP is a continuation from Phase II.
A nice result was the discovery of L 363-38 b, a planet newly discovered with ESPRESSO orbiting a nearby M dwarf star. The figures below illustrate the detection and some of the planet’s properties. The corresponding paper can be found here.
WP2: Earth as an exoplanet. The Earth observing Aqua satellite provides a unique dataset covering the optical and infrared wavelength regime of Earth’s emission and has been taking data for more than 15 years by now (cf. Mettler et al. 2020). We will continue our efforts to investigate Earth’s appearance as an exoplanet by generating disc integrated views of the Earth at various wavelengths and from different viewing angles and search for temporal and spatial variations of key (atmospheric) signatures (including bio-signatures). This will be compared to the capabilities of future instruments. In addition, these data provide vital reference points for atmospheric models and retrieval frameworks as well as 3D global circulation models. This WP will be developed with Project A5 and is a continuation from Phase II.
New results are summarized in a paper entitled “Earth as an Exoplanet: II. Earth’s Time-Variable Thermal Emission and its Atmospheric Seasonality of Bio-Indicators”, which can be found here.
WP3: From stellar abundances to planetary properties. One of the key questions for future instruments (in particular if space-based) is which stars to observe in order to maximise the chances of finding terrestrial and potentially habitable exoplanets. Available observing time will be limited and providing information that can help prioritise objects, will be important. In this WP, which has a direct link to WP1 above and with Project A5, we will use the elemental abundance information obtained from high-precision optical spectra of nearby stars in order to constrain the “ingredients” and internal properties of (potential) terrestrial exoplanets orbiting these stars (cf. Wang et al. 2019, 2020). We will continue the work started in Phase II and seek to expand the analyses to stars within 10-20 pc and apply it to known exoplanets with well-measured mass and radius.
WP4: The information content of terrestrial exoplanet spectra. Obtaining spectra of terrestrial exoplanets will be a top priority of any future exoplanet instrument and is driving the requirements for future space missions such as LIFE (Quanz et al. 2021, 2022) or NASA’s Habitable World Observatory. To (1) derive quantitative science requirements and (2) understand the scientific characterisation potential of new instruments/missions, state-of-the-art simulations are required. In this WP, we will con- tinue the development and expansion of the petitRADTRANS package (Mollière et al. 2019) for terrestrial exoplanet atmospheres and coupled with powerful atmospheric retrieval frameworks. We will simulate a suite of terrestrial exoplanet atmospheres starting with Earth-twins (cf. WP2 above) and couple these with instrument simulators for ELT instruments (e.g., such as METIS and PCS) and future space missions in order to add relevant (instrumental) noise terms. These mock-observations will then be fed into our atmospheric retrieval framework in order to derive constraints on key atmospheric properties. By varying the signal-to-noise ratio, spectral resolution and wavelength coverage, we will be able to assess how well terrestrial exoplanets can be characterised by future observations.
First results have been published as part of the LIFE paper series here, here and here.
WP5: Preparing for the LIFE mission. In addition to working on the science relevant for (and driving!) the LIFE mission, ETH Zurich is leading laboratory efforts to further develop the “nulling interferometry” technique as it would be applied in future mid-infrared interferometry missions, such as LIFE. With the outcome of ESA’s Voyage 2050 process and with LIFE being selected as one of the NCCR flagships, significant support for these activities is required. Specifically, we seek to demonstrate in the lab that “nulling” to the required level of flux suppression is feasible (1) under cryogenic conditions, (2) with flux levels representative of the ones from astrophysical sources, and (3) over sufficiently long time periods and broad enough wavelength ranges.
We will continue to report on the status and progress of these lab efforts via papers in the SPIE conference series, such as here and here.
An overview of the LIFE mission and the related community effort can be found on the LIFE webpages.
