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D
The Diversity of Exoplanets – Statistical results I: mass-separation-radius
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Introduction by Michel Mayor
Michel Mayor talks about the use of the telescope and the spectrograph in the detection of
Pegasus 51b. The spectrograph allowed to see a lot of information that was previously
impossible to accumulate. The amazing thing about the data was that the planet they found
had Jupiter mass, but orbited its star in only 4,2 days, completely overthrowing all previous
assumptions about the creation of gas giants. The understanding that planets move inwards
while gobbling up the accretion disk helped to understand this strange phenomenon
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Early result on the occurrence of extra-solar planets
Radial velocity can only determine the MINIMUM mass of a detected planet, not the real
mass! Due to the inclination of the orbit. This leads to a skewed view on the situation.
This is based on the assumption that the orbits are distributed uniformly with respect to the
line of sight.
Higher metallicity means higher probability of stars.
Average gives 7% and metal-rich stars tend towards gas giants.
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The variety of systems
1: one planet
5: one very excentic planet.
systems are not just compact, bus also dynamically stable.
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Distribution, mass, separation – from RV Technique
Very important for statistics = Radial Velocities: numbers, mass and orbital elements of
planets.
51 Peg b proved the existence of extrasolar planets, but this planet had half the mass of Jupiter
and was much heavier than expected Many of the later planets had often similar
characteristics.
µAra c: only had 10 times the mass of Earth, due to a new program with the HARP
spectrograph, contributing many other discoveries since 2004, espcialy lower mass
distribution.
2011: very low mass planet around Alpha Centaury Bb, about 2 Earth masses! Not only a
planet around our nearest neighbour This gives a lot of interest to statistics.
2007; first paper about high precision RV measurements. 30-40 planets were demonstrated to
have masses probably below Neptune mass. Probabulit of such planets could be extrapolated:
at least 30% of solar type stars should have low mass planets.
Red curve: fraction of stars that have at least one planet?
Red curve = real corrected expectation. 15% or more of all the planets larger than 50 earth
masses for any kind of orbit.
Good for low mass and high mass planets.
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Distribution, mass, separation – from Transit Technique
Most prominent contributor is the Kepler spacecraft. A large percentage of those discoveries
actually live in a multi-planet system. Rather than mass-distribution, this is a radiusdistribution. Information about small and large, rather than mass.
Small planets are much more frequent than large ones. Left, by size, right, by orbital periods.
If planets are detected in both techniques, that gives a baseline for the mass/radius
distribution that must be general.
Stars with high metallicity tend to have giant planets, while the lower mass planets appear to
be more uniformly distributed.
Summary of the HARPS-KEPLER surveys:
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Mass-radius relation
RV gives a minimum mass of a planet, transit gives a radius.
Sun, red dwarf, planet! Radial velocity is important to separate small suns from giant planets.
We need mass measurements, not just the radius!
Left: many giant planets with mass and radius estimations. Lower left part: small radius – low
mass planets. Very precise instruments are needed for those, especially KEPLER. Most of
those candidates orbit faint stars and mass is difficult to measure.
For bright stars, eg Kepler 78b, this is possible. Smallest radius and mass planet known by this
time.
Mass is measured by observing the amplitude.
Must be rocky and same size as the Earth. However, very short orbit > several thousand
degrees on the surface. Pity.
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Observation vs Theory
Theory tries to understand the physics behind the observations Does the theory correspond to
the observations?
Model:
Grey right corresponds to the full area to the left. Theory predicts the same as the assumptions
from the practical constraints: there must be many more planet with lower mass.
Another parameter space: mass distribution. How many planets of a given mass is observed?
Gas giants decrease about 50-100 earth masses, then increase again. Just like the observatins
Theory and model correspond to observations again. Lower left = rocky planets, upper right =
gas giants.
Now that the theory and observations seem to fit together, it becomes possible to make
predictions: this is of course an ongoing process. With more observations the theory can be
fine-tuned.
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We need more planets to support better statistics and compare on a finer scale
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We need more precise instruments and different techniques to complement the work
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Once the statistics are understood, we need to characterise peculiar systems: orbital
elements, inner structure an atmosphere > habitability
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Comparison of observations with the theory.