Exoplanets: understanding the bulk properties and structures of Ice-giants and SuperEarth
extra-solar planets
Background
Since the first extrasolar planet was discovered around a sun-like star in 1995 we have found hundreds
more and begun to understand their most basic qualities. One of the surprises has been that planets with
masses ∼5–25 Earth masses and a few Earth radii seem to be by far the most common. This project is about
understanding the nature of these objects.
Most exoplanets have been discovered using spectroscopy. With this technique we look for motion from
the star as it orbits the common centre of mass in a system from which we can infer the existence of a
planetary companion. So for example an alien astronomer studying our solar system with spectroscopy
would not be able to see any planets but would be able to detect the sun ’wobbling’ around the centre of
mass in the solar system (which is close to the surface of the sun). Unfortunately, spectroscopy alone gives
us little information about the planets themselves as the inclination of the planetary orbit to the line of
sight is unknown. Hence, from spectroscopy alone all we can deduce is lower limit for the planetary mass.
Ironically therefore while we think we know of more than 700 exoplanets, most we know almost nothing
about (some may not even be planets at all).
A growing but still relatively small number of exoplanets have been discovered because they orbit their star
in a plane perpendicular to the sky with the result that periodically they pass between us and their host star,
blocking out a small amount of star light. Hence for a few hours the star appears slightly fainter. The depth
of this ’transit’ tells us about the relative size of the star and its planet. The transiting planets are therefore
important: these are the only objects that we can deduce an orbital inclination (it has to be close to 90
degree’s or the transit wouldn’t occur), and hence we can determine the mass, radius and density of these
planets. These are the very parameters needed to compare to theoretical models that attempt to understand
the characteristics (eg bulk density) and evolution of these newly discovered solar systems. There are only a
few hundred transiting planets known (but very few with host stars bright enough for detailed examination).
This project is about understanding the bulk properties of planets in the 5-25 Earth Mass regime. The nearest
equivalent in our solar system is the ice giants of Neptune and Uranus but the extrasolar planet data also
show objects that are significantly smaller in radius (and higher in density) which have been dubbed SuperEarths. These objects are thought to resemble massive terrestrial planets but their properties are only poorly
known and their relationship to the ice giants undetermined.
The Project
Our experience and heritage gained from the SuperWASP experiment (which is still the leading survey for
large planets) has allowed us to design a facility capable of detecting much smaller planets - it will be the
leading experiment capable of detecting planets in this size range. Most importantly and in contrast to all
other experiments in this area (including the space missions CoRoT and Kepler) NGTS candidate planets
will be bright making confirmation a tractable but still difficult process.
We will use a new facility, NGTS, to obtain the light curves of bright candidate exoplanets which will be
confirmed through spectroscopy on one of the ESO telescopes. Warwick is one of the founding members
of the NGTS experiment that will be built at Paranal (Chile) in early 2013. Paranal is one of the best
astronomical sites in the world and is home to the European Southern Observatories VLT and the future
39m diameter e-ELT telescopes.
We anticipate that the PhD student will be involved in all parts of this process and will be especially heavily
involved in the spectroscopic confirmation and subsequent followup observations and their interpretation.
Given the importance of the results (aspects of this work are likely to be in future text books) we expect the
student to quickly integrate into a hard working team. As is always the case while the work is difficult the
rewards are great.
More informations from: Prof Don Pollacco (Tel: 024765 74329, email: d.pollacco@warwick.ac.uk)
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