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combination with a radial velocity measurement
it yields the planetary mass as well. These two
parameters are key in learning about the composi-
tion and interior structure of a planet. The preci-
sion in the measurement of the planet's radius is
directly connected to how well the stellar radius is
inferred. As with the RV method, large and close-
in transiting planets are easier to detect as they
block more photons from the star. Around a sun-
like star, only Jupiter-sized planets at short orbital
periods have a transit effect measurable from the
ground. In order to detect smaller planets, it is im-
perative to go to space, or look for planets around
smaller stars. This last opportunity is being ex-
ploited by the MEarth project (Irwin et al ., 2009),
which surveys the closest M Dwarves and de-
tected the second low-mass transiting planet, GJ
1214b. The first space mission to look for transit-
ing exoplanets was French-led CoRoT launched
in December 2006. Despite a detection capability
at the limit of what is needed to find super-Earth
planets, it discovered the first one, CoRoT-7b.
Remarkably, the playing field of planet de-
tection and characterization has dramatically
changed after the analysis of the first four
months of Kepler 's data (Borucki et al ., 2011).
This mission, launched in February 2009, was
designed to accomplish two objectives: deter-
mine the frequency of Earth and super-Earth
exoplanets, and discover the first Earth analog
(e.g. a one Earth-radius planet at a distance of 1
AU from a sunlike star) (Borucki et al ., 2003).
As of April 2012, it has yielded more than 2000
planet candidates. Not all of them are planets, as
false positives can arise from other astrophysical
scenarios, such as binary stars in the line of
sight (Torres et al ., 2011). Unfortunately, radial
velocity follow-ups for the Kepler candidates
are difficult. Despite this, Kepler has confirmed
planets and measured their masses via another
planetary effect present in multi-planet systems,
transit timing variations (TTV). The transit of a
planet does not happen strictly periodically (but
occurs a little early or a little late) in the presence
of other companions as they perturb its orbit
through gravitational interactions. To measure
this small effect (Holman & Murray, 2005; Agol
et al ., 2005), the photometric precision needs to
be extremely high and is the reason why the first
TTV was observed with Kepler (in August 2010
on the Kepler-9 system, Holman et al ., 2010).
Planets that transit offer other opportunities for
characterization, specifically of their atmospheres
(see Figure 9.2).
￿ During the primary transit it is possible to learn
about the atmospheric composition. As the planet
passes in front of the star, some of the light from
the star passes through the planet's atmosphere,
interacting with its molecules. The observation
is a low-resolution transmission spectrum that
carries information about a variety of absorption
lines tracing the composition of the planet's at-
mosphere.
￿ During the secondary transit (when the planet
passes behind the star) of tidally-locked planets,
which have a permanent day and night sides, it
is possible to learn about the dayside integrated
flux. As the planet moves behind its host star,
the dayside starts facing the observer. If seen in
the optical it is dominated by the reflection of the
starlight, and if seen in the infrared it is dominated
by the intrinsic luminosity of the planet.
￿ Monitoring the whole light-curve of a tidally
locked planet gives information about the atmo-
spheric circulation. It is possible to investigate
how efficiently the heat in the atmosphere is re-
distributed from the dayside to the night side.
Planets with very efficient day-night side redis-
tribution have a luminosity modulation along
their orbit as seen from Earth (i.e. phase curve)
with a reduced amplitude (as the temperature is
more equilibrated around the planet), compared
to planets with an inefficient atmospheric cir-
culation. In other words, planets with a more
homegenous atmospheric temperature (thanks to
efficient winds) will emit more uniformly as the
planet goes around the star, than planets that
have a high temperature contrast between their
permanent day and night sides.
In summary, for many planets only masses or
radii are known, but for a subset both quantities
are measured, which enables the inference of their
bulk composition. Additionally, for the gaseous
exoplanets there have already been observations
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