Photochemistry of Terrestrial Exoplanet Atmospheres - Planetary Exploration and Science: Recent Results and Advances - page 284

Chapter 12
Photochemistry of Terrestrial Exoplanet
Atmospheres

Renyu Hu

Abstract Terrestrial exoplanets are exciting objects to study because they could be
potential habitats for extraterrestrial life. Both the search and the characterization
of terrestrial exoplanets are flourishing. Particularly, NASA's Kepler spacecraft has
discovered Earth-sized planets receiving similar amount of radiative heat as Earth.
Central in the studies of terrestrial exoplanets is to characterize their atmospheres
and to search for potential biosignature gases (the atmospheric components that
indicate biogenic surface emissions). To achieve this goal, a deep understanding of
the key physical and chemical processes that control the atmospheric composition
and the atmosphere-surface interaction is pivotal.

Keywords Terrestrial exoplanets ￿ Astrobiology ￿ Photochemistry ￿ Radiative
transfer

12.1 Terrestrial Exoplanets in Our Interstellar

Neighborhood

One of the most exciting progresses in planetary exploration in the past decade is
the discovery of terrestrial exoplanets. These celestial objects are planetary bodies
outside the Solar System with masses within ten times Earth's mass or with radii
within two times Earth's radius. Terrestrial exoplanets were first discovered by the
“radial velocity” method (Rivera et al. 2005 ), i.e., by measuring the wobbling of
a star via the Doppler shift of the stellar spectrum induced by an orbiting planet's
gravitational force. The precision required for detecting an Earth-mass planet at the
1-AU orbit to a Sun-like star (referred to as a “true Earth analogue”) is on the order
of 0.1 m s 1 . The precision of the radial velocity measurements of bright stars is
getting close to this requirement, and as a result, the radial velocity search has been
finding terrestrial exoplanets that have sizes closer and closer to Earth (Udry et al.