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The Diversity of Exoplanets – Introduction: History and context
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Introduction by Michel Mayor
Only 20 years ago, the first exoplanet has been discovered, 51 Pegasus. Today, lots of them
have been found with a lot of diversity amongst the planetary systems.
Until the early 20th century common thought was that planets outside the Solar System would
be very rare, and only in 1943 ideas changed when theories concerning the formation of
planetary systems changed to stipulate a lot of planets in other systems. And that is what we
observe these days.
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General introduction
Since the first exoplanets discovery in 1995, more than & 1000 have been confirmed by now,
which we know the masses of (very diverse architecture and systems).
This course will give a general view of the field, what we have learned until now and the limits
of the models that have been developed. This week will deal with early cosmology, Kepler’s
laws, star formation, protoplanetary disks and planet formation.
Astronomy is the oldest profession in the world as any amateur could see and think about the
heaven and the stars.
Cosmogony = what the universe is = religion/philosophy. The motion of the sun, moon and
stars were obvious and raised the interest of the people very soon in humanity’s history. That
knowledge was used for rituals like burials etc. Example: Carnac, Brittany. Others: Tassili
Tumulus (Sahara, about 10.000 BC). Ireland: corridor grave, both had an orientation towards
the east.
Apart from the sun, 5 other objects were moving and were discovered by early astronomers:
Mercury, Venus, Mars, Jupiter and Saturn. The stars are fixed, the planets are not. They move
across the sky independently, sometimes they move backwards (retrograde). A model was
needed to describe why this happens.
This leads to cosmology = how the universe works. Why do we see motions like this? Lots of
knowledge was accumulated by the Greeks, notably in Alexandria at the end of the Greek
period. They found that the earth (a sphere itself) is motionless surrounded by two spheres.
This model allowed to understand eclipses, deduce that the Earth was a sphere herself, and
even measure the size of our planet.
This model still left some questions: how to explain non constant velocity and retrograde
motion. At the time, Apollonius de Perga (262-190 BC) introduced epicycle theory. Main
circle = deferent, small circle = epicycle. This could reproduce the loop of the planets. Even
epicycles could not explain the change in speed, though.
Apollonius then introduced the “eccentric theory”: the Earth is not in the center of the
deferent cycle. This reproduces the speed differences.
Ptolemy (90-168 AD) wrote the Almagest and popularised the geocentric system. First
catalogue of starts (48 constellations). He adds more levels to the epicycle theory to
reproduce small variations. This approach was still in vogue until the 18th century.
Problem: the center of the epicycle is virtual! This is perceived as a difficult stickler. Only
Copernicus (1473-1543) brought a simpler model. He brought the heliocentric system with the
planets revolving on circular orbits. This explains a lot of aspects like retrograde motions,
speed depends on distance from the star. He still uses an eccentric aspect to account for
minute differences.
Without proper observation of the planets, none of this could be made clear. This is where
Tycho Brahé comes into play (1546-1601): he develops his own instruments to measure, he
had a star catalog as well (more than 1000), 10x more accurate than before. He also
systematically observed the planets (Mars for over 16 years!). His research led later to
Kepler’s laws. Good measurements and lots of them are needed for astronomy.
Lesson: he was proud of his observations, but he could not find the parallax of stars because
his observations were just not exact enough.
He ended up putting the earth back in the center, with the sun going around and all the other
planets going around the sun. Weird.
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Kepler’s laws
Observations had become more and more important for proper theories and development.
Tycho Brahé made a lot of very precise and long-time observations and noted them. He hired
Johannes Kepler to work on his data for planet Mars.
He tried to derive the geometry of Mars’ orbit.
With Earth and Mars positions, using the angles with the sun, there was a way to calculate the
position of the two planets over time. The distance was unknown, but the shape of the
motion could be derived. Ultimately, this led to Kepler’s laws. He proposed a model based on
the observations.
Law 1: motion of the planets is in the form of an ellipse.
Law 2: the area of the segment per unit of time is constant: far from focus slowly, near the
focus fast.
This means that a planet’s speed depends on its distance from the star, making sure that the
angular momentum stays the same.
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Keplers’s third law
Which force is responsible for this motion = see Law 3: this includes the mass of the planet.
All this was only fully understood after Newton introduced his law of gravitation about 50
years later. The attraction force of two bodies depends on the mass of both bodies and their
distance from each other. This can be expressed in terms of the ellipse and its area.
Newton stipulated that the force behind the gravity is based on mass and distance. This leads
to the idea of celestial mechanics = how planets move.
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[Planet &] Stellar Formation
The idea of stellar formation is based on our understanding of the formation of planets. All
planets are going around the sun in the same direction. The orbits are quasi circular, they spin
in the same direction. Kant and Laplace proposed the “solar nebula” model as being the
starting point of the formation of both.
The age of the sun and all the bodies in the solar system are approximately the same > this
means that planets are to be expected everywhere around stars.
Sketch: collapse under its own gravitation, the angular momentum is conserved, the rotation
increases, a disk is formed, and the disk leads to the creation of planets.
Observation of the molecular cloud. Gaseous pillars, clouds are made mainly of hydrogen and
helium, about 10-50 K temperature. Those clouds could lead to 100.000s of stars.
Low temperature, high density: atoms will exist in the form of molecules: H² as the main
molecule. Luminosity: it radiates at a long wavelength. Higher temperature, higher emission
of the visible light, lower temperature > shift of the maximum emission to long wavelengths.
This can be calculated:
In this mid-infrared range, the molecules have spectral features that we can identify. From the
distribution of the energy and the spectral features we can guess the physical state, density,
kinematics through Doppler shift.
What is happening in this cloud. It is in equilibrium between gravity and gas pressure. If this is
interrupted, the cloud starts to collapse. If it is too big, this will happen. The mass limit is the
Jeans mass. The hotter inside, the bigger the cloud can be. If the density is increasing, gravity
will win > collapse, or breakup. This will lead to many smaller mass stars rather than one huge
star.
When the collapse starts, a denser part of the cloud will start sending out charged particles.
The shell of the forming star is shielding from UV = ambipolar diffusion.
When the core is dense enough, a disk develops, a central star is surrounded by the disk. The
viscosity will lead to dissipation (loss of kinetic energy), light in outer regions will be redder, UV
is much higher near the star.
Accretion-ejection phase > bipolar jets throwing out material perpendicular to the disk.
Slowdown of accretion = T Tauri star.
4 categories: class 0, 1, 2 and 3. They differ in the relative proportion of the mass of envelope,
the disk and the star.
T-Tauri > classical (CTT) and weak-line (WTT)
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Protoplanetary disks
What is happening in those disks? Lots of those disks in the Orion nebula. These disks only
became visible about 20 years ago because of new satellites.
From the shape of the radiation graph it is possible to see if the star has a disk or if it doesn’t
by reading the access infra-red radiation (red line below).
The disk emits a large variation of radiation along a whole range of wavelengths. IRAS
telescope (IR), Hubble, Spitzer, Herschel telescopes, etc.
Imaging capacity has grown over time as well. Also by combining images from several
telescopes at different wavelengths.
This can be mapped. If there is a hole, this can be seen:
Infrared access can show lots of strange behaviour:
From models and observations, we can reconstruct the structure of the disk. The disks are
made from dust and gas.
Dust: about 1% of the mass, most is gas. The dust influences the opacity of the disk and will be
the raw material for planets: silicates, graphite and ice mantles. The grain size will grow over
time.
Gas: 99% of the mass, mostly H² and He. H² is very difficult to detect, one of the tracers is CO
which can be easily detected (rotational transitions). CO < 1% of the gas. Most of this CO is
missing, but it is most likely frozen for T<20 K. But they are present in warm layer close to the
star.
CO can be detected with Doppler shift (most are in Keplerian rotation). From the dynamic of
the disk, the central star mass can be estimated.
How common are protoplanetary disks? They are only available with young stars, as they
gradually dissipate.
They can be found through Infrared probing of a zone near the star (contribution of the star
and the disk). What part is coming from the star? Once this is determined, the disk can now
be modelled. IRAS, groundbased.
Further out, where usually terrestrial planets are formed, mid-IR is measured. Spitzer and
Herschel.
Submm continuum (50-100 AU). Submm, Radio, ALMA
If this is done in a star cluster, the family are about the same age. For each of these stars, it is
estimated how many have disks.
The disks disappear within about 10 million years for most of the stars observed in these
studies: inner disk half-life is about 2-3 million years. > time for gaseous planet formation.
What about the structure of the disk? Full observation of all wavelengths, full 3D modelling
(grain size, etc.) Most disks are flaring disks:
The range of disk mass is very wide within one family. Massive disks are more likely to
produce massive planets.
The mass of the disk depends on the mass of the star. The form of the disk also depends on
the age of the star:
Mass of disk will decrease with time (and class). Planet formation might be the reason for a
hole in the disk = transition disks.
This is being checked: looking for planets around very young stars. If this is true, this will allow
us to compare observed systems and determine an evolution.
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Planet formation
Good model has to reproduce what we observe:
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Known sample of exoplanets
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Protoplanetary disks
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Solar system
A good model should explain the diversity of exoplanets and the detailed knowledge about the
solar system.
Planets will form from the disk materials: gas and dust.
First approach: core accretion (Safronov 1969 and Pollack 1996). Small particles collide and
accrete to greater and greater objects. Bottom up.
Gravitational instability: clumps could be seeds for the final planets.
Those two are running in parallel, because they were answering questions of different types.
Gravitational instability model: The older stars become, the less disks can be found. Gas must
be cold and the disk has to be massive. Extreme initial conditions. Then clumps will form and
massive planets will develop. This type of planet has been found (10x Jupiter mass). They
would be difficult to form with the accretion model.
Accretion model: takes time, and this is unlikely in the outer region of the disk. First,
planetesimals will collect stuff in the feeding zone. Slow accretion of solids and gas.
First, quickly building up the core. Long phase of accretion of gas. Runaway accretion of gas.
Formation is defined by phase II. This phase has to be finished by the end of the lifetime of the
disk.
Because the planet moves, there is more material available. A planet can be built up in a few
million years.
Phase III is very fast as well. Either a planet stays mainly rocky, or it grows VERY quickly.
Type I: small planet, no gap opening. Spiral waves in disk.
If there is turbulents, random movement of planets. Gravitation can push the small new
planet into the sun.
Type II: the planet creates a hole in the disk, but it will still be moved towards the center.
Migration rate is slower (about ½ million years), but the planet will still be killed.