  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?"
