Annual Report 2011 Max Planck Institute for Astronomy

42 III. Selected Research Areas
low for direct falsifiability with future observations is a
strength of the method. Besides that, the predictions are
also useful to estimate the yield of future instruments
and surveys.
Planet formation models
On the observational side, usually only the initial conditions
(the protoplanetary disks) and the final outcomes
(the planets) are accessible to observations. With theoretical
formation models, it is possible to bridge this gap
at least on the theoretical side. The global, numerical
models of planet formation used in population synthesis
calculations try to cover the largest possible extent
of important mechanisms. The various mechanisms like
accretion or migration must be treated in an interlinked
way, since they happen on similar timescales and feed
back on each other. Ideally, the models would start with
a protoplanetary disk at a very early stage when the
solids are in the form of micrometer-sized dust grains,
and yield as an output full-blown planetary systems at
an age of several billions of years. This means that also
the evolution of the planets over long timescales must
be modeled, since we essentially observe planets a long
time after they have formed. It is clear that these glob-
Fig. III.1.3: Theoretical planetary formation tracks which show
how planetary seeds (initial mass 0.6 Earth masses) concurrently
grow and migrate. The colors indicate the different types
of orbital migration. The position of the planet at the moment
Mass [M Earth ]
10 4
10 3
10 2
10
1
0,1 1
Semimajor axis [AU]
al models of planet formation and evolution involve
important simplifications of the actual processes, and
cannot describe all physical effects at the same level
of detail as models dedicated to one single process. On
the other hand, only combined models allow to see the
interaction between different processes, and only they
allow for direct comparisons with many observational
constraints.
The global planet formation and evolution models
used in population synthesis calculations are based on
the so called core-accretion paradigm which states that
first, solid cores are formed, some of which later accrete
massive gaseous envelopes to become giant planets
(bottom-up process), while the remaining cores collide
to form both ice giants and terrestrial planets. The
models address the following processes in a number of
coupled computational modules:
1. A structure and evolution model for a gaseous protoplanetary
disk. The gaseous disk model yields the
ambient properties in which the planets form. The
ambient pressure and temperature serve as outer
boundary conditions for the calculation of the structure
of the gaseous envelope of the planets. The
structure of the disk is also very important for the
orbital migration of the protoplanets, since the di-
in time that is shown (4.9 Myrs) is indicated by black symbols.
Some planets have reached the inner border of the computational
domain at 0.1 AU.
10
t 4.9 Myr
Credit: C. Mordasini