Photochemistry of Terrestrial Exoplanet Atmospheres - Planetary Exploration and Science: Recent Results and Advances - page 286

planet has an N 2 -dominated atmosphere with variable levels of CO 2 to provide
appropriate levels of greenhouse effects. Such defined habitable zone around a
Sun-like star is evaluated most recently at 0.99-1.70 AU (Kopparapu et al. 2013 ).
Moreover, the range of the habitable zone can be considerably widened, if the
atmosphere is H 2 dominated (with H 2 -H 2 collision-induced absorption as the source
of the greenhouse effect; Pierrehumbert and Gaidos 2011 )orifthewatercontentin
the atmosphere is much lower than Earth (Zsom et al. 2013 ).
The search of habitable terrestrial exoplanets is difficult because both the radial
velocity method and the transit method are more sensitive to planets that are closer
to their parent stars. The amplitude of the radial velocity signal falls with the
semimajor axis a as a -1/2 ; the amplitude of transit signal does not depend on the
semimajor axis, but the probability of transit due to the alignment between the star,
the planet, and the observers is R * / a ,where R * is the radius of the star. Several
Jupiter- and Neptune-sized planets that are in the conventionally defined habitable
zones of their host stars have been discovered by the radial velocity method (Udry
et al. 2007 ; Vogt et al. 2010 ;Pepeetal. 2011 ; Bonfils et al. 2011 ; Tuomi et al. 2013 ).
Kepler has found several sub-Neptune-sized planets in the habitable zones by the
transit method (Borucki et al. 2012 , 2013 ; Barclay et al. 2013a , b ; Quintana et al.
2014 ). The frequency of terrestrial exoplanets in habitable zones of their host stars
is estimated using Kepler observations to be 15 % for cool stars (Dressing and
Charbonneau 2013 )and 20 % for FGK stars (Traub 2012 ; Petigura et al. 2013b ).
Therefore, it is reasonable to expect at least one potentially habitable terrestrial
planet in our interstellar neighborhood of a few tens of parsecs.
One sweet spot to look for habitable terrestrial exoplanets is around M dwarf
stars. M dwarf stars have sizes of a fraction of that of the Sun, and M dwarf
stars are the most common type of stars in the neighborhood of the Sun (Salpeter
1955 ). Recent surveys by Kepler have suggested that planets having radii within two
times Earth's radius are more frequent around small M dwarfs than around FGK
stars (Howard et al. 2012 ; Dressing and Charbonneau 2013 ). Moreover, because M
dwarfs are considerably sub-luminous compared with the Sun, the habitable zone
around an M dwarf is much closer to the star than the habitable zone around a
Sun-like star (Kasting et al. 1993 ). As a result, habitable planets around M dwarfs
would have higher transit probabilities and larger transit signals compared with their
counterparts around FGK stars. In fact, the first Earth-sized exoplanet in a star's
habitable zone was detected around an M dwarf (Quintana et al. 2014 ). However,
this planet is too far away from Earth to allow atmospheric characterization. In
parallel with Kepler , ground-based searches for terrestrial exoplanets around nearby
M dwarfs have been ongoing (e.g., Nutzman and Charbonneau 2008 ), which have
resulted in the discovery of a 2.7 Earth-radius planet orbiting an M4.5 star only 13
parsecs away (GJ 1214 b; Charbonneau et al. 2009 ).
The discovery of terrestrial exoplanets, especially those potentially habitable,
has impacted profoundly our inquiry of the Universe. It is the first time when
the human being can say for sure there are locations outside the Solar System
that may have rocky environment, widely accepted as a prerequisite for life to
emerge. As a milestone in the search for planets that might harbor life, terrestrial