Photochemistry of Terrestrial Exoplanet Atmospheres - Planetary Exploration and Science: Recent Results and Advances - page 287

exoplanets are important objects to study for at least three reasons: (1) the
detection of terrestrial exoplanets provides a large ensemble of planets and planetary
systems that enable comparative planetology of Earth-like planets beyond the Solar
System; (2) terrestrial exoplanets may themselves be habitable if they receive
appropriate radiative heat from their parent stars; (3) characterization of atmo-
spheres and surfaces of terrestrial exoplanets, starting from planets that are larger
than Earths (referred to as “super-Earths”), and the expertise of instrumentation,
observation, and data interpretation techniques gained will serve as indispensable
stepping stones for eventually characterizing Earth-sized planets that are potentially
habitable.

12.2 Observations of Exoplanet Atmospheres

The atmosphere on an exoplanet can be analyzed by spectroscopy. If an exoplanet
could be directly imaged, the light from the planet's atmosphere, either planetary
thermal emission or reflection of the stellar light, could be analyzed via spectroscopy
to determine the composition of its atmosphere. Such observations are extremely
challenging due to the existence of a much stronger radiation source at close angular
proximity (i.e., the host star). Therefore, direct spectroscopy studies of exoplanets
have only been performed for Jupiter-sized or even larger giant planets. The first
high-resolution spectrum of a directly imaged exoplanet (a nascent gas giant 40 AU
from its host star) has recently been reported (Konopacky et al. 2013 ).
A newly developed method to mitigate the weak signal of an exoplanet without
spatially resolving the planet or nulling the stellar light is to make use the
information of the planet's orbital motion. A correlation between the star's radial
velocity and the radial velocity of a certain group of molecular lines (e.g., CO) was
used to establish the existence of the molecule in several giant planets' atmosphere
(Brogi et al. 2012 ; Rodler et al. 2012 , 2013 ; de Kok et al. 2013 ; Birkby et al. 2013 ;
Lockwood et al. 2014 ).
At current stage and in the near future, however, characterization of the atmo-
spheres of terrestrial exoplanets focuses on the planets that transit. The predictable
on-and-off features of a planet's radiation when the planet passes behind its host
star (referred to as “occultation”) can be observed by monitoring the total light from
the star-planet system in and out of transits (e.g., Seager and Sasselov 1998 ; Seager
et al. 2000 ). In addition, when the planet passes in front of its host star (referred
to as “transit”), parts of the stellar radiation may transmit through the planet's
atmosphere and carry the information of the atmospheric composition (Seager
and Sasselov 2000 ). Soon after the first detection of an exoplanet atmosphere
via transit (Charbonneau et al. 2002 ) and the first detection of thermal emission
from an exoplanet atmosphere via occultation (Charbonneau et al. 2005 ;Deming
et al. 2005 ), both methods have been successful in characterizing extrasolar giant
planets (e.g., Seager and Deming 2010 and references therein). Recently, attempts
to observe super-Earth atmospheres are growing (e.g., Demory et al. 2012 ; Knutson