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a model of the star spots behavior to uncover the
planet. The result depends highly on how well
the star is known, and might explain why this
is the only study that suggests a very low mass
of 1-4 M
E
(implying a nonrocky composition)
for CoRoT-7b. With the mass value proposed by
Hatzes
et al
. (2011), CoRoT-7b has a composition
enriched in iron with respect to Earth (close to
that of Mercury with a Fe/Si
it is relatively bright; and the planet to star ra-
dius ratio is large, so that the fraction of starlight
transmitted through the optically thin part of
the atmosphere around the planet may be large
enough to be detected. Where there are detectable
absorption lines, it means the optically thin part
of the atmosphere is extended enough, implying a
relatively low molecular weight compared to one
that has no features. So far there have been eight
spectroscopic studies of GJ 1214b with somewhat
conflicting results. By probing the upper atmo-
sphere at the
6 times higher then
Earth's). Such a high density is a good indication
that the planet has negligible amount of volatiles
in terms of its structure. For it to have any, the
trade-off would entail an even higher content
of iron for the rocky interior below the enve-
lope, an unlikely scenario. On the other hand, it
is quite likely that this planet is tidally locked
with permanent day and night sides. At 2000 K,
the dayside surface would be partially molten
resulting in a permanent magma ocean (Leger
et al
., 2011b), while the night-side would be per-
manently cold in the absence of an atmosphere
capable of redistributing heat.
Kepler-10b
. This planet is very similar in prop-
erties to CoRoT-7b, except for it being much older
(
>
9 Gy, Batalha
et al
., 2011). It is the first rocky
planet discovered by
Kepler
. The exquisite preci-
sion in radius measurement (R
∼
millibar level, Bean
et al
. (2010,
2011), D esert
et al
., 2011), Crossfield
et al
. (2011)
and Berta
et al
. (2012) propose a high-molecular
weight composition, while Croll
et al
. (2011) and
de Mooij (2012) find a nonflat spectrum that sug-
gests an atmosphere rich in hydrogen. Recently,
Howe and Burrows (2012) put forward a theoret-
ical atmospheric composition model and fit all
the available observations. Their analysis favors
a hydrogen-rich atmosphere with a cloud or haze
layer, although they cannot rule out a hydrogen-
poor model with 10% water. Moreover, these
findings need to be understood in the context of
the results from internal structure models, which
are sensitive to pressures of
∼
1 bar and above, and
seem to indicate the presence of a light compo-
nent in the atmosphere (H or H-He). These studies
consider four different compositional scenarios:
an H
2
O envelope,
1
a H-He envelope, a mixture of
H
2
O and H-He (Rogers
et al
., 2010b; Nettelmann
et al
., 2011; Valencia, 2011), or outgassed H from
the interior (Rogers
et al
., 2010b). One argument is
that while the M-R discovery data (from Charbon-
neau
et al
., 2009) is consistent with a planet made
of 100% H
2
O (see Figure 9.6), formation models
would predict the presence of refractory material,
which would imply the presence of a lighter com-
ponent H or H-He to offset the trade-off in density
(Valencia, 2011).
∼
1
.
416
0.033
−
0.036
R
E
)
was possible thanks to the tight constraints im-
posed by astroseismology on the stellar radius.
Remarkably, this planet has the same bulk com-
position as CoRoT-7b, perhaps suggesting a fam-
ily of iron-enriched short-period super-Earths.
GJ 1214b
. This was the second low-mass planet
detected by the ground-project MEarth, which
surveys the nearest M Dwarves for companions.
With a mass of 6.55
=
0.98 M
E
, its large radius
of 2.7 R
E
(Charbonneau
et al
., 2009) implies un-
doubtly that GJ 1214b has a volatile envelope. Its
size is about 1000 km larger than if it was made
of pure ice, the lightest composition of a solid
planet. Figure 9.6 shows the data for GJ 1214b
and the M-R relationships for compositions of
different water vapor amounts. The important
question, which is still unresolved, is the nature
of this envelope. Interestingly, this planet is suit-
able to observation by transmission spectroscopy
because of two reasons: the star is nearby and thus
±
1
Because GJ 1214 is a low-mass star (1/3 of the Sun),
despite the orbital period of 1.94 days, the planet's
effective temperature (
500 K) is considerably cooler
compared to the other transiting super-Earths. This
translates into an atmospheric temperature of
∼
1000 K
at the 1 bar level (Miller-Ricci and Fortney 2010), which
implies that the water is in supercritical form.
∼
