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In-Depth Information
found that super-Earths exist in larger numbers
compared to their giant counterparts. The HARPS
search for southern exoplanets has found in their
radial velocity survey, that 30% of stars (and
even up to
the mass of Jupiter), and comet-like planets with
large eccentricities of up to
0.9, to name a few.
There have also been surprises in terms of compo-
sition of planets, such as Kepler-7b with a density
close to Styrofoam, or HD 149026b (Sato
et al
.,
2007) with a staggering 70 M
E
dense core (Latham
et al
., 2010). In case of the low-mass planets, al-
though suspected, it is only recently that the data
has begun to show how super-Earths and mini-
Neptunes overlap in terms of mass while differing
in other properties.
Among the discovered low-mass planets, three
super-Earths share very similar characteristics in
terms of mass, radius and effective temperature.
Two of them, CoRoT-7b and Kepler-10b, have a
composition similar to that of Mercury (enriched
in iron compared to Earth), while 55 Cnc-e, if
rocky is closer in composition to the Moon (very
depleted in iron). This information needs to be
reconciled with formation scenarios to produce
such planets at their orbital distances. The other
transiting low-mass planets are better described
as mini-Neptunes or vapor planets depending
on the composition of their non-negligible
envelopes. In particular, the composition of
GJ 1214b's envelope is under study. While
two observational studies suggests a H-He
atmosphere, the other five report a nondetection
consistent with either a high-molecular weight
atmosphere (such as water) or the presence of
hazes.
On the other hand, being more massive than
Earth, super-Earths have interiors under very high
pressures and temperatures. This shows a need to
understand the behavior of minerals under such
conditions. Super-Earths' interiors may be an ex-
tension of the Earth, or perhaps be in a completely
different state. It is important to know if there
are any other phases beyond post-perovskite, and
evaluate whether or not MgSiO
3
dissociates into
MgO and SiO
2
at the
∼
40%) have planets with masses less
than 50 M
E
, which include both super-Earths and
mini-Neptunes (Udry, 2009). Howard
et al
. (2011)
used the Keck Telescope to survey 166 sunlike
stars and found that the occurrence is
∼
12%
for planets with masses between 3-10 M
E
and
6.5% for masses between 10-20 M
E
On the
other hand,
Kepler
has reported within their can-
didates the following numbers (Borucki
et al
.,
2011): 5.4% for Earth-sized planets (radius be-
tween 1-1.25 R
E
), 6.8% for planets with radius
between 1.25-2 R
E
, and 19.3% for Neptune-sized
planets (size between 2-6 R
E
). In addition, core-
accretion formation models predict an increase
in planet occurrence with decreasing mass. Even
though the formation models and observational
surveys consider somewhat different populations
(different orbital periods of the planets, tempera-
ture of the stars, etc.), they all point to one simple
conclusion, super-Earths are common. Therefore,
despite the serious challenges to observe their
properties, there are more of them to discover.
This suggests that in the mid-future, as the data
builds up, characterization of these planets will
entail statistical approaches.
∼
9.6
Final Remarks
The wealth and diversity of extra-solar planets
have allowed for comparative planetology in a
broad and more complete sense. Previous biases
resulting from only having the solar system to
study are now being removed. For example, high
eccentricities, orbital short periods or masses
above that of Jupiter were not expected. This
is why planet HD 114762b with its 10 M
Jup
mass,
84-day orbital period and 0.33 eccentricity was
dismissed as a brown-dwarf instead of a planet in
1989 (Latham
et al
., 1989). We now know that
formation models need to explain Jupiter-sized
planets at periods of 3 days and close-in plan-
ets in general, very massive planets (
1 Tpa pressure, or if
indeed there is a virtually incompressible ox-
ide phase with an M
3
N
5
O
12
structure, where
M and N are a combination of Mg, Fe and Si.
While knowing more about high-pressure phases
has an effect on the planetary radius and thus
on the compositional inference from mass and
∼
∼
10 times
