Photochemistry of Terrestrial Exoplanet Atmospheres - Planetary Exploration and Science: Recent Results and Advances - page 285

2007 ; Mayor et al. 2009 ; Vogt et al. 2010 ;Riveraetal. 2010 ; Dawson and Fabrycky
2010 ;Howardetal. 2011 ; Bonfils et al. 2011 ). Recently, an Earth-mass terrestrial
exoplanet was reported around Alpha Centauri B, one of the closest stellar systems
from Earth (Dumusque et al. 2012 ).
Another method to search for terrestrial exoplanets is to observe the dimming
of a star when a planet passes in front of the star as viewed from Earth (i.e., the
transit method). The signal of the transit is proportional to the ratio between the
size of the planet and the size of its parent star. Earth transiting the Sun as viewed
from another planetary system would have a transit signal of 80 parts per million
(ppm). Modern photometry technique has been able to provide this level of precision
and therefore enabled the detection of terrestrial exoplanets via transits (Leger et al.
2009 ; Charbonneau et al. 2009 ; Winn et al. 2011 ;Demoryetal. 2011 ; Dragomir
et al. 2012 ; Van Grootel et al. 2014 ). In recent years, the exoplanet community has
witnessed an explosive increase of the number of terrestrial exoplanets discovered
by transits as a result of the Kepler mission that monitored 160,000 stars in the sky
(Batalha et al. 2011 ; Lissauer et al. 2011 ; Cochran et al. 2011 ; Fressin et al. 2012 ;
Gautier et al. 2012 ; Borucki et al. 2012 ; Muirhead et al. 2012 ; Borucki et al. 2013 ;
Gilliland et al. 2013 ; Swift et al. 2013 ;Roweetal. 2014 ). The smallest transiting
planet that has been confirmed is only slightly larger than the Moon (Barclay et al.
2013a , b ).
Based on the discoveries made by Kepler , statistically, we now know that a
large number of stars in our interstellar neighborhood have terrestrial exoplanets.
The occurrence rate of terrestrial exoplanets can be estimated based on the Kepler
observations, with correction of the geometric effect (due to the fact that the transit
technique is only sensitive to those planets that pass in front of their host stars
periodically), the incompleteness of detection, and the false positive of signals
(Howard et al. 2012 ; Fressin et al. 2013 ). It has been estimated that 30 % of stars
in our interstellar neighborhood have terrestrial exoplanets that have radii within
two times Earth's radius and orbital periods within 85 days (Fressin et al. 2013 ).
It also turns out that the planets below twice the size of Earth are more populous
than the planets above (Fressin et al. 2013 ; Petigura et al. 2013a , b ). The occurrence
rate of exoplanets found by the Kepler transit survey is also consistent with the
finding of radial velocity surveys that are sensitive to very different observational
biases, supporting the fidelity of this result (Figueira et al. 2012 ). When calculating
the occurrence rate, terrestrial exoplanets are usually defined by their radii, because
the transit technique can only measure the radii but not the masses. Measuring the
masses of these small planets and confirming their rocky nature are ongoing and
have been successful for a number of close-in objects (Pepe et al. 2013 ;Howard
et al. 2013 ; Marcy et al. 2014 ).
A handful of the discovered terrestrial exoplanets are potentially habitable. A
habitable planet is defined as a planet on the surface of which liquid water is stable.
As the stellar radiation is the major heat source for a terrestrial exoplanet, the
conventional habitable zone, the range of semimajor axes in which planets could
be habitable, has been studied for main-sequence stars (Kasting et al. 1993 ). The
conventional definition of the habitable zone relies upon the assumption that the