Interferometric Space Missions for Exoplanet Science: Legacy of Darwin/TPF

DARWIN/TPF is a project of an infrared space-based interferometer designed to directly detect and characterize terrestrial exoplanets around nearby stars. Unlike spectro-photometric instruments observing planetary transits, an interferometer does not rely on any particular geometric constraints and could characterize exoplanets with any orbital configuration around nearby stars. The idea to use an infrared nulling interferometer to characterize exoplanets dates back to Bracewell (1978), and was extensively studied in the 1990s and 2000s by both ESA and NASA. The project focuses on the mid-infrared regime (5-20 μm), which provides access to key features of exoplanets, such as their size, their temperature, the presence of an atmosphere, their climate structure, as well as the presence of important atmospheric molecules such as H2O, CO2, O3, NH3, and CH4. This wavelength regime also provides a favorable planet/star contrast to detect the thermal emission of temperate (∼ 300 K) exoplanets (107 vs 1010 in the visible). In this chapter, we first review the scientific rationale of a mid-infrared nulling interferometer and present how it would provide an essential context for interpreting detections of possible biosignatures. Then, we present the main technological challenges identified during the ESA and NASA studies, and how they have progressed over the last 10 years. Finally, we discuss which technologies remain to be developed before flying such an instrument, and possible ways to make DARWIN/TPF a reality in the mid-term future.