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F
The Diversity of Exoplanets – Statistical results II: orbital characteristics
and architecture of systems
1
Introduction by Michel Mayor
What about the internal structure and the atmosphere of these exoplanets? We have no
pictures of low-mass planets, only some of very large planets.
We have information about some atmospheres, though, gathered while the planet is in transit
– it filters some info from the star, and that can lead to technical analysis of the planet’s
atmosphere.
2
From bulk composition to atmospheric properties
The density helps in determining the very basic idea we have, but it tells us only so much. The
high-quality spectra will help us further determine the physical parameters.
Even with the spectra, we will only ever get partial views of exoplanet properties and we need
to retrieve other methods of data retrieval and interpretation.
3
Basics of atmospheric physics
3.1
Interpreting emergent spectra
Thermodynamic equilibrium =
If we know the temperature, we can theorize about the properties of energy levels, ionisation
of atoms, how are the electrons populating the orbitals of an atom.
Examples:
3.2
Chemical composition
Stellar atmospheres: mostly atoms and ions. Molecules dissipate due to the heat.
Chemical equilibrium under certain temperature and pressure give an idea of what to expect
and look for.
The elements in the solar composition determine the abundance of certain elements also in
the planets.
Above are more a theoretical idea along the x-Axis.
H² and He have to have enough mass to accrete H² and He or they will simply dissipate. The
form of carbon molecules depends on the temperature!
3.3
Pressure-temperature profile
How about the distribution of elements vertically within an atmosphere?
LEFT: Gravity pushes down, pressure gradient pushes up. RIGHT: pressure = density *
temperature * mean molecular weight.
Combined, those two equations show the typical pressure per altitude.
Assumptions: constant temperature!
Higher flux = higher gradient. Mean opacity has an influence on the gradient as well.
When does either case apply?
Stratosphere: inversion mode: sunlight heats up the atmosphere and leads to inversion.
PT profiles depend heavily on the location of the photosphere. The altitude has a specific
temperature and this changes the shape of the spectrum.
4
Observational strategies: how to separate star and planet spectra
Freeze turbulence needed. Large telescope for infrared. Extreme contrasts of planet and star
> Things are difficult: narrow angle, small visibility of variation, small light point for planets.
Angular separation = visualisation of star systems by optical means. Large intrinsic emissions
of young stars facilitate this option in the infrared range.
Info comes from stellar light filtering through the planet’s atmosphere during primary eclipse.
The secondary eclipse, eliminates the light from the planet, while transit entry and exit will
bring info on STAR + PLANET. By calculating the star’s contribution can be deducted and that
leaves us with the light from the planet.
Telluric absorption = water vapour absorbing light.
Molecular bands are made of thousands of atomic transitions > high spectral resolution is
needed to change the big band into a comb of absorbing lights.
Doppler shift allows to separate spectra of star and its planets.
5
Transmission spectroscopy (2 parts)
Allows to study exoplanet atmospheres.
Examples of transmission spectroscopy results:
First planet to be found transiting. Radius of planet increases at the wavelength of water
vapour. There is water vapour, sodium and something else, maybe a hazy atmosphere.
Flat spectrum with a opacity extending quite far. Cloud layer obscuring the absorption of
gases lower down. Blue curve is a theoretical model without haze. Lots of sodium above the
haze layer.
Part of the atmosphere is evaporating, NOT transiting so far. The planet appears to be
surrounding by a large extended atmosphere.
The spectrum is extremely flat. But it constrains the composition of the atmosphere: it has to
be a cloud layer. We can rule out water vapour, CO² or methane.
High µ compresses the features in the compression spectrum.
6
Thermal emission (2 parts)
This allows to detect emissions from the planet, based on its temperature that is lower than
the sun’s.
Further away from the star, the colder the star will be. Duh.
The difference of flux between the planet and the star.
Things are much better the higher the spectrum is in the red.
Examples:
Day side just before and after secondary eclipse, night side before and after primary eclipse.
Stellar energy is distributed unequally due to tidal lock to the star = dayside and nightside stay
the same. Here, the heat is only 300 K difference. If there is wind circulation, isothermal
planet is possible, though.
You can see the primary transit in the drop.
The minimum is not exactly at transit, but indicates some kind of convection, probably strong
winds.
7
Reflected light
This is another technique to characterise exoplanet atmospheres.
We have no clue about the planet: theoretical construct, we use a disk rather than a sphere.
This leads to
This is valid for perfectly diffusive disks. Now we replace the disk with the actual planet
through introduction of the geometric albedo.
Examples:
Mainly molecular absorption in all spectrums. This level of knowledge is NOT available for
exoplanets.
Again, flux minima/maxima do not occur in transit. This makes a case for changing cloud
patterns … ?
8
Future prospects
Field is expanding very quickly. New telescope will be launched in 2018: James Webb Space
Telescope. Whole wavelength range allows to explore exoplanets of all kinds. 6.5 meter
telescope. Direct imaging and transitions. Also has spectrographs.
Extremely large telescopes:
Ground-based, but huge mirrors (up to 40 meters for E-ELT). Advantage of large mirror: better
angular resolution, better separation in planetary systems. Also, photo-collecting power is
much better than older ones. Good for high-resolution of transiting systems.
Potential of ELT techniques:
Good indication of potentiality of life as we know it.