Large Interferometer For Exoplanets

The LIFE space observatory concept is very different from previous space missions which covered a similar wavelength regime in the mid-infrared (MIR). This includes for instance the Spitzer Space Telescope, but also even older missions such as ISO, IRAS, AKARI. Also, the recently launched James Webb Space Telescope (JWST) has a powerful MIR instrument (MIRI) that covers the LIFE wavelength range. However, all of these missions were designed with different primary scientific objectives in mind. Most importantly, they all lack the spatial resolution to directly detect large numbers of planets in the habitable zones around nearby stars. JWST, with it’s 6.5-m primary mirror, might be able to characterize a few rocky planets around nearby stars, but will not provide a large sample of directly detected planets.

In the early 2000s the idea of a space-based mid-infrared nulling interferometer for exoplanet science was already studied by both ESA (the Darwin mission) and NASA (the TPF-I mission (TPF-I = Terrestrial Planet Finder - Interferometer)). LIFE is built on the heritage of these efforts and takes into account all their results and learnings and continues from where they stopped.

Nulling interferometry is the name given to an instrumental technique based on interference between several telescopes / apertures and aiming at detecting directly exoplanets. The principle is to create a virtual “blind spot” at the exact location of a bright source, a star, in order to reveal the much fainter source, which is a planet orbiting it. Designing a nulling interferometer supposes that the beams from two (or more) large telescopes, separated by baselines of several meters to hundreds of meters, are combined coherently and that a special device, called an achromatic phase shifter, is introduced in some of the beams before recombination. This phase shifter will create a phase shift of , so that when 2 beams are combined, light from any on-axis source (i.e., a π source that has an optical path difference (OPD) of zero) interferes destructively. This on-axis source is the star. Because a nearby exoplanet will not have an OPD of zero, its light is - at least partially - transmitted. In addition, since the expected brightness of the exoplanet is extremely low, the usually strong thermal background emission that characterizes observations in the mid-infrared has to be reduced: this means that the telescopes must be at a low temperature, typically below 100 K. If we add that the Earth atmosphere absorbs a large part of the mid-infrared wavelength range and that it also contributes to the thermal background, one understands that the space environment is mandatory. Finally, a very precise control of wavefront defects and a precise matching of the interferometer arms are required to effectively suppress the star light and achieve the required “null depth”.

• Exoplanet science with a space-based mid-infrared nulling interferometer, Sascha P. Quanz, Jens Kammerer, Denis Defrère, Olivier Absil, Adrian M. Glauser, Daniel Kitzmann, 9 Aug 2018

• Astrobiology
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