[Paper Review] Atmospheric characterization of terrestrial exoplanets in the mid-infrared: biosignatures, habitability & diversity

Sascha P. Quanz, Olivier Absil+46 more|arXiv (Cornell University)|Jan 1, 2019

Stellar, planetary, and galactic studies3 references13 citations

TL;DR

This white paper proposes a large space-based mid-infrared (MIR) exoplanet imaging mission to directly detect and characterize thermal emission from terrestrial exoplanets, enabling the search for biosignatures, assessment of habitability, and study of atmospheric diversity. By leveraging MIR nulling interferometry, the mission would surpass current and planned missions in sensitivity and spectral resolution, offering a unique path to answer whether Earth-like planets are common in the galaxy.

ABSTRACT

<p>Exoplanet science is one of the most thriving fields of modern astrophysics. A major goal is the atmospheric characterization of dozens of small, terrestrial exoplanets in order to search for signatures in their atmospheres that indicate biological activity, assess their ability to provide conditions for life as we know it, and investigate their expected atmospheric diversity. None of the currently adopted projects or missions, from ground or in space, can address these goals. In this White Paper we argue that a large space-based mission designed to detect and investigate thermal emission spectra of terrestrial exoplanets in the MIR wavelength range provides unique scientific potential to address these goals and surpasses the capabilities of other approaches. While NASA might be focusing on large missions that aim to detect terrestrial planets in reflected light, ESA has the opportunity to take leadership and spearhead the development of a large MIR exoplanet mission within the scope of the "Voyage 2050" long-term plan establishing Europe at the forefront of exoplanet science for decades to come. Given the ambitious science goals of such a mission, additional international partners might be interested in participating and contributing to a roadmap that, in the long run, leads to a successful implementation. A new, dedicated development program funded by ESA to help reduce development and implementation cost and further push some of the required key technologies would be a first important step in this direction. Ultimately, a large MIR exoplanet imaging mission will be needed to help answer one of mankind's most fundamental questions: "How unique is our Earth?"<br /></p>

Motivation & Objective

• To address the lack of comprehensive atmospheric characterization of terrestrial exoplanets despite growing detection capabilities.
• To overcome the limitations of current ground-based and space-based missions in detecting thermal emission from small, rocky exoplanets in the mid-infrared.
• To enable the detection of biosignatures, assess habitability, and explore atmospheric diversity across a statistically significant sample of exoplanets.
• To establish Europe’s leadership in exoplanet science through a dedicated, large-scale MIR space mission under the ESA Voyage 2050 program.
• To catalyze international collaboration and technology development for a future mission capable of answering whether Earth-like planets are rare or common.

Proposed method

• Utilize mid-infrared (MIR) nulling interferometry to detect thermal emission from terrestrial exoplanets, minimizing starlight contamination.
• Employ long-baseline interferometric arrays to achieve high angular resolution and suppress stellar glare, enabling direct detection of planetary signals.
• Target exoplanets around nearby G-, K-, and M-type stars within 15 parsecs, focusing on those in the empirical habitable zone.
• Leverage prior data from Kepler, TESS, and radial velocity surveys (e.g., CARMENES, HARPS3) to pre-identify promising targets for follow-up.
• Integrate multi-wavelength data and geometric constraints (e.g., dust belt orientation) to optimize observation efficiency and reduce search time.
• Develop a dedicated ESA-funded technology roadmap to mature key instruments and reduce mission cost and risk.

Experimental results

Research questions

• RQ1 How many terrestrial exoplanets exhibit atmospheric signatures indicative of biological activity?
• RQ2 What fraction of terrestrial exoplanets possess surface conditions suitable for liquid water and life as we know it?
• RQ3 How diverse are the atmospheric compositions of terrestrial exoplanets across different planetary masses, radii, host star types, and orbital periods?
• RQ4 Can a mid-infrared space mission detect and characterize a statistically significant sample of rocky exoplanets not accessible to current or planned missions?
• RQ5 How can a dedicated MIR imaging mission surpass the capabilities of reflected-light missions and ground-based ELTs in probing planetary habitability and biosignatures?

Key findings

• A mid-infrared space mission using nulling interferometry can detect thermal emission from terrestrial exoplanets with high sensitivity and angular resolution, surpassing current capabilities.
• The mission would enable the detection of dozens of terrestrial exoplanets within 15 parsecs, including those around M-dwarfs, which are currently underexplored due to detection biases.
• Estimates suggest that 20 or more small exoplanets within 15 pc lie in the empirical habitable zone, providing a rich target pool for atmospheric characterization.
• The occurrence rate of Earth-sized planets in habitable zones is estimated at η⊕ ≈ 0.2–0.3 for Sun-like stars and potentially higher for M-dwarfs, supporting a statistically robust sample for study.
• The mission would provide unique access to atmospheric composition, temperature profiles, and surface conditions—critical for assessing habitability and biosignatures—beyond what is possible in reflected light or with current ELTs.
• A dedicated ESA development program could reduce mission costs and accelerate technology readiness, paving the way for international collaboration and long-term scientific leadership in exoplanet exploration.