Large Interferometer for Exoplanets (LIFE): characterizing the mid-infrared thermal emission of terrestrial exoplanet atmospheres

LIFE (www.life-space-mission.com) is an initiative to develop the science, technology and a roadmap for an ambitious space mission that will allow humankind to detect dozens of warm, terrestrial exoplanets and hundreds of exoplanets overall at mid-infrared (MIR) wavelengths (Quanz et al., 2018, 2022). For most of the detected exoplanets direct estimates of their effective temperature and radius will be available, and a for a significant subset the atmospheric composition will be investigated including the search for potential biosignatures (Des Marais et al., 2002; Léger et al., 2019). Characterizing exoplanet atmospheres using their thermal emission at MIR wavelengths — compared to studies at optical/near-infrared wavelength looking at planets in reflected light — offers the possibility to study a broader set of molecular features (Schwieterman et al., 2018) and get a better understanding of the atmospheric structure (Line et al., 2019). Hence, in particular for questions related to the habitability of exoplanets, a mission like LIFE offers unprecedented scientific potential.

The current baseline design of LIFE features a 4-aperture interferometer array with a 6:1 baseline ratio to reduce the impact of instability noise (Lay, 2006). A beam combiner spacecraft is located at the center of the array. The size of the individual apertures is currently under study, but based on detection yield simulations including all relevant astrophysical noise sources, diameters of 2–3.5 m are under consideration. The aperture size is primarily driven by the number of detectable planets and the time-on-target required for in-depth atmospheric characterization. The current wavelength range requirement is 4–18.5 μm, but additional studies are underway for further verification. A spectral resolution of at least R=30, but better R=50, seems required in order to reliably quantify the abundance ratios of main molecular species in the atmosphere of an Earth-twin planet at several pc distance. The minimum mission lifetime is 5-6 years in order to have sufficient time for both a dedicated search phase, to identify the most interesting and promising targets, and a characterization phase for in-depth investigations of a subset of those. LIFE shall be launched to the Earth-Sun L2 point.

While the general feasibility of the required null-depth and stability was demonstrated in the context of Darwin and TPF-I (Martin et al., 2012), a corresponding experiment under cryogenic conditions is underway in the form of the Nulling Interferometric Cryogenic Experiment (NICE) at ETH Zurich (Gheorghe et al., in prep.). A more general overview of the readiness of key technologies for a space mission like LIFE was presented in Defrère et al. (2018).

 

Defrère, D., et al. 2018, Exp. Astron., 46, 475

Des Marais, D. J.,  et al. 2002, Astrobiology, 2, 153

Lay, O. P. 2006, in SPIE Astronomical Telescopes + Instrumentation (SPIE), 62681A-14

Léger, A., et al. 2019, Astrobiology, 19, 797

Line, M., et al. 2019, BAAS, 51, 271

Martin, S., et al. 2012, Appl. Opt., 51, 3907

Quanz, S. P., et al. 2018, SPIE Conf. Ser., 10701, 107011I

Quanz, S. P., et al. 2022, A&A, 664, A21

Schwieterman, E. W., et al. 2018, Astrobiology, 18, 663