Geoscience Reference
In-Depth Information
For the task of remotely observing the presence
of a magnetic field, there are two lines of evidence
in the solar system: through the detection of radio
waves emitted from the interaction of the plane-
tary magnetic field and the solar wind, and from
the splitting of specific spectral lines (Zeeman
effect) due to the additional atomic quantum
states possible in such presence. Observing the
Zeeman effect requires a precision that presents
a technological challenge for exoplanets, thus the
efforts have been concentrated on radio emission
signals. The important parameters governing ra-
dio emissions are the magnetic field strength and
morphology, the solar wind power, the plane-
tary rotation rate, and if there are any satel-
lites that could be altering the signal (e.g. Io
and Jupiter). In the solar system, radio-emissions
have been detected from Earth, Jupiter, Saturn,
Uranus and Neptune. Because the radio emis-
sion strength is proportional to solar wind power,
close-in extrasolar planets may have detectable
magnetic fields. However, this raises an impor-
tant issue in detection, which is resolving the
signal from the planet (if one exists) and that of the
star. Planets that are close to their parent star may
experience interactions with the stellar magnetic
field that require complicated analysis to disen-
tangle the signal (one example is shown by Jardine
& Collier Cameron, 2008). For extra-solar plan-
ets, all efforts so far have been directed towards
giant planets, that are expected to be convective
at depth, and to have the conductivities suffi-
cient for dynamo action (Stevenson, 2003), with
indirect evidence for a few from the modulation
of emission lines of their host stars phased with
the planetary orbits, likely due to interactions
between the stellar and planetary magnetic fields.
It is worth asking how would the magnetic field
strength of a super-Earth compare to that of Earth
and Jupiter to find out how plausible a detec-
tion may be. Christensen
et al
. (2009) proposed
that the magnetic field strength of planets (in-
cluding rocky and gaseous) and stars depends on
the energy flux, expressed in a relationship where
the magnetic field strength depends on the cubic
root of the core heat flux times the convective
length scale (among other quantities that may
not vary much among rocky super-Earths). As a
first approximation take the convective length
scale to be the size of the core, which would be
a factor of 2 larger than Earth on a 10 M
E
-planet.
The heat flux would have to be 15 times that of
the Earth for the magnetic field to be an order
of magnitude larger than on Earth and similar
to that of Jupiter. It is unclear if this value is
plausible, as it implies very fast cooling from the
mantle while at the same time a sustained en-
ergy source to prevent complete freezing of the
core. Furthermore, specific studies suggest that
magnetic fields for rocky super-Earths, even if
strong, are short-lived (Gaidos
et al
., 2010; Tachi-
nami
et al
., 2011). Therefore, it is unclear if such
a detection may happen in the near or mid fu-
ture. On the other hand, another avenue not yet
explored, is if atmospheric composition is shaped
in a detectable way via escape by the presence or
absence of a magnetic field.
9.5
Challenges for the Future
In summary, the data available for exoplanets
are masses, radii, age, and orbital parameters.
For the gas giants, it also comprises information
on the composition and dynamics of their atmo-
spheres. Some studies have proposed additional
techniques to yield more information, such as the
Love number of giant exoplanets (Batygin
et al
.,
2009), the discovery of exo-moons via transit
timing variations and durations (Kipping, 2009),
and spectroscopic information about individual
regions of the planetary surface by first deter-
mining the rotation period from the scattered
light, which is possible since the ocean currents
and continents result in relatively stable averaged
global cloud patterns (Pall e
et al
., 2008), among
other perspectives. The near and mid-future for
the super-Earths is to emulate what has already
been done for the gaseous exoplanets, discover
many more and measure their masses and radii,
and study their atmospheres. The challenge is
that super-Earths are much more difficult to de-
tect than the giant exoplanets. However, they
are more numerous. Three different groups have
