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￿ 55 Cnc-e . This is the fifth planet discovered
around 55 Cnc (with RV by Fischer et al ., 2008),
the only naked-eye star so far hosting planets.
In 2011, transit searches measured its radius
(Demory et al ., 2011; Winn et al ., 2011; Gillon
et al ., 2012). 55 Cnc-e shares a mass (i.e. M
region smaller than the orbit of Mercury. With an
average density of 6.9 g / cm 3 it is the most similar
planet to Earth in terms of composition, although
somewhat lighter (see Figure 9.8). Given the large
error bars it is difficult to say if this planet has
a volatile envelope, or if it has a composition
depleted in iron with respect to Earth. The Kepler-
20b system is unique in two ways: it harbors two
planets that have a radius smaller than Earth
(although their masses are not measured, thus
technically they are planet candidates), and the
configuration is such that the smaller ones are in
between the three larger planets. This contrasts
the configuration of the solar system where the
smaller rocky planets are inside the outer gaseous
planets.
In a relatively small range of masses (between
2and9M E ), the variety in composition shows
that low-mass planets are diverse. In the near
future, as more transiting super-Earths are found
and characterized any trends in their composition
and properties should become clearer, yielding
knowledge about their formation and evolution.
8M E ) and period similar to that of Mercury-
like CoRoT-7b, but its size is larger (i.e. R
=
2R E ), making this planet much lighter. It sits at
the boundary between planets that are possibly
rocky and those that necessarily have substantial
amounts of volatiles (the no-iron curve). If in-
deed 55 Cnc-e is rocky, it would be considerably
depleted in iron with respect to Earth and even
more so with respect to CoRoT-7b and Kepler-
10b. The calculated volatile amount is less than
10% by mass of supercritical H 2 Oorafewparts
per 10,000 of H-He (Demory et al ., 2011). Hydro-
gen and helium may be ruled out on the basis that
they would have escaped hydrodynamically from
the high UV flux received from the star within a
few million years. On the other hand,
10% of
H 2 O has an evaporation timescale in the order
of billions of years, thus making it possible for
55 Cnc-e to have a few percent of water by mass
(Valencia et al ., 2010).
￿ Kepler-11 system . This is a remarkable sys-
tem hosting six low-density planets with masses
smaller than Uranus in a compact architecture
(Lissauer et al ., 2011). Even the densest and
hottest of the system, Kepler-11b requires an
envelope to match its radius for its given mass, in-
dicating that all the other planets have envelopes
as well. Their envelopes are most likely composed
of a mixture of H-He and water, in proportions
that vary depending on the equilibrium temper-
ature of the planet and bulk density. The hotter
and more compact the planet, the less likely it
has H-He.
￿ Kepler 18-b . This is the innermost of three
planets around star Kepler-18. The other two are
Neptune-sized planets and are in a 2:1 resonance,
meaning that the outermost planet has a period
twice that of the middle planet. Kepler-18b has a
volatile envelope most likely dominated by water.
￿ Kepler 20-b . This planet is the closest one of
5 planets to host star Kepler-20, which orbit in a
9.4
Interior Dynamics
Earth's thermal evolution has had a large impact
on its present state. By analogy, we can expect
the same for super-Earths. Three aspects derived
from its internal dynamics are unique to Earth: its
atmospheric composition, the operation of plate
tectonics, and the presence of a magnetic field.
They are related to habitability and remote de-
tection. As mentioned before, the composition
of the tenuous atmospheres of super-Earths, rela-
tive to the Jovian planets with H-He dominated
atmospheres, is largely connected to the interior
dynamics by outgassing and ingassing. This is
important because in the future it may be pos-
sible to observe the atmosphere of super-Earth
planets, and infer their composition from spec-
troscopic observations, with telescopes like the
James Webb Space Telescope (JWST, Deming
et al ., 2009), or the Extreme Large Telescopes
(ELTs, Kissler-Patig, 2009) planned for the next
decade. Thus the motivation to understand the
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