Geoscience Reference
In-Depth Information
9.1
Introduction
and primitive high-temperature melts) that can
be studied in the laboratory, and seismological
data that map the Earth's interior. Because of the
few rocks acquired on the Moon and the SNC
meteorites identified as coming from Mars, there
is also some direct sampling, albeit limited, on
the composition of these two bodies. For the rest
of the planets and satellites in the Solar System
there are no samples. We know about their inte-
riors from remote sensing data collected through
observation from the Earth and spacecrafts. This
data comprises the masses, radii, moments of in-
ertia (of most objects), images of their surface,
orbital properties and interactions (such as libra-
tions), measurements of magnetic fields, to name
a few. In contrast, there is very little data on extra-
solar planets. Moreover the observations entail
the star
The quest for finding planets around other stars
(hereafter exoplanets) has been a long-standing
human pursuit. However, it is only in the last
20 years that there has been the technology nec-
essary to detect them. The first discovery came in
1995, when Mayor and Queloz reported a planet
with a mass close to that of Jupiter, orbiting a
Sun-like star in a 3-day orbital period: 51 Pegasi-b.
Since then, the interest and success of detecting
other exoplanets has continuously been on the
rise (see database on www.exoplanets.eu). Today,
the number of confirmed planets amounts to
more than 700 - most of them gaseous giants -
plus more than 2000 candidates detected by space
mission
Kepler
. As the technology and data inter-
pretation have improved, smaller harder-to-find
planets have become detectable. These include
the so-called super-Earths.
They are planets that are mostly solid, and
therefore devoid of a massive gaseous atmosphere.
As such, their properties are expected to resem-
ble better the terrestrial planets and icy satellites
than the Jovian planets in our Solar System. As
a consequence, they constitute a new laboratory
where ideas developed to explain the Earth's prop-
erties can be tested. In particular, these are more
massive planets and it is unclear if their charac-
teristics are an extension of the Earth's or they are
considerably different. This chapter will explain
what we have learned about super-Earths in terms
of their interior properties and some aspects of
what we expect to learn in the future.
planet combined system, which condi-
tions the information extracted on the planet to
how well the star is characterized.
+
9.2.1 Detection methods
To understand the type of data available, it is
useful to start with a brief description of the two
most successful detection methods, the radial
velocity and the transit method. Except for a
few cases, all exoplanets have been detected by
observing the effects they exert on their host
stars. The few exceptions, which are HR8799 b, c,
d(Marois
et al
., 2008), Formalhault b (Kalas
et al
.,
2008) and Beta Pic b (Lagrange
et al
., 2010), are
planets that are very bright - from being massive
and young - and distant from their star, which
makes the formidable task of directly observing
them possible.
The radial velocity (RV) method or ''wobble''
technique exploits the fact that both planet and
star orbit around the system's center of mass. This
causes the star's spectral lines to shift to shorter
(blue-shifted) and longer (red-shifted) wavelengths
periodically as the star moves towards and away
from the observer respectively. Figure 9.1 depicts
the basic characteristics of this Doppler effect.
The shift is greater for close-in and massive plan-
ets that exert a larger gravitational force on the
star. Aside from measuring the orbital period of
9.2
A Primer on the Data
The extent of what can be learned of a planet
depends on the data available and reliable theoret-
ical models. The wealth of data diminishes from
studying the Earth, to the Moon and Mars, to
the other bodies of the solar system, to extrasolar
planets. Two main sources of information make it
possible to know more about the Earth's interior
than of any other planet: mantle-derived rocks
(such as peridotite xenoliths, massif peridotites
