Brief description of the images:

The images named "formation*" refer to the standard planet formation
model, i.e. without any external perturbation. A protostellar cloud
(formation1; of the size of e.g. the central region of the Orion
Nebula, or some of these dense clouds in the Carina Nebula) fragments,
i.e. breaks apart into several gravitationally bound sub-clouds or
clumps (formation2).

formation3: Such a clump continues to collapse under gravity, and
since it always has some initial spin the rotation becomes faster and
faster -- just like the water in a bathtub when it rushes towards the
drain. This leads to a flattening in shape (formation4).

formation5+6: In the central region forms a clump containing most of
the mass -- the protostar. The surrounding disc contains typically a
few per cent of the total mass (initially up to a few tens of percent,
but most of this gets lost by stellar winds, brown dwarf formation
[Thies et al. 2010], close encounters etc.), but it contains most of
the angular momentum -- the protoplanetary disc.

formation7-9 show the formation of protoplanets in the disc and,
finally, the planetary system, like the Solar System (although this
simple model already fails to explain the observed 7 degree
inclination between the planetary orbits and the solar equator).

Images "encounter*" shows the scenario of a star+disc encountering a
dense clump of gas, and capturing material from it.

"annulus1" depicts the situation directly after the encounter, while
"annulus2" shows the situation about 30,000 years after the encounter
when disc and annulus have aligned to each other.

"inclined*" illustrate the formation of a planetary system out of this
combined disc in three steps: 1. the disc resulting from the merger of
the initial disc and the captured gas annulus, 2. clumping of dust (or
gravitational instability in massive discs) leads to protoplanets, and
3. the resulting planetary system. Note that (inclined4) the star
spins still in (almost) the same direction as before the encounter.

"ejection*" and "kozai*" show two possible scenarios of a hot Jupiter
formation event in their simplest form.

I. Ejection: 1. two (or more) planets interact gravitationally, 2. one
(typically the least massive one) gets ejected while the other remains
on a now eccentric orbit with less energy than initially. 3. Over
time, the tidal interaction with the star itself reduce the orbit to a
circular orbit with a radius similar to the former peri-astron
radius. The henceforth very close orbit (several stellar radii) gets
tidally locked to the stellar rotation (like the Moon's rotation is
tidally locked with its orbit around the Earth). Such orbits can be
very close to the star, i.e. we have a "hot Jupiter".

II. Kozai: 1. the two planets do not interact that strongly as in the
ejection scenario, (2.) but over a long time their orbits are in
resonance and oscillate between high mutual inclination and high
eccentricity. This effect is strongest for the inner planet, since due
to the smaller angular momentum of its smaller orbit the orbit is
easier deformed. 3. As above, tidal damping leads to a "hot Jupiter"
again.

Finally, "system*" are example cartoons for a 1. regular planetary
system like ours and 2. a system with mutually inclined planetary
orbits as a possible extreme case for the outcome of our scenario
(which may render the system unstable over time).

Additional images: rmleffect1 and 2 describe the
Rossiter-McLaughlin-Effect which allows the detection of spin-orbit
misalignments for transiting exoplanets. When a planet passes in front
of its host star it obscures a small area of the star along its
(projected) path. Due to the stellar rotation one side of the stellar
disc (i.e. the 2-D projection of the star as it appears to an
observer) is moving towards us and is therefore blue-shifted by the
Doppler effect. The opposite side moves away from us and is
red-shifted. A prograde orbiting planet first obstructs the
blue-shifted part, so that the sum colour is biased slightly to the
not obstructed red-shifted side. Then, the planet continues to hide
progressively red-shifted areas of the star, making its remaining
light be biased towards blue-shift. This can be detected by sensitive
spectrographs.

If this sequence occurs the other way round, i.e. first a blue bias,
then a red bias, the planet has a retrograde orbit. Inclined orbits
can be detected this way as well, although the analysis is a bit more
complex than for mere detection of retrograde orbits.

There are JPG and PNG versions of all images.