My research focuses on the formation and evolution of (extrasolar)
planets. Below, I will describe some recent projects.
Planetesimal and gas dynamics in
binaries
Evolution of low-mass planets in non-isothermal
discs
Planetesimal and gas dynamics in binaries
The majority of stars in the Milky Way are part of a multiple system,
in which two or more stars orbit each other. This work focuses on
planet formation in a system with two stars on a relatively close
orbit. In particalar, we are interested in relative velocities between
planetesimals (the builing blocks of terrestrial planets) in a
circumprimary disc for such a system. If we want to grow planets from
these building blocks, relative velocities had better be small,
otherwise collisions between planetesimals would lead to their
destruction rather than growth. This is an interesting problem,
because we do find planets in these highly perturbed systems.
The gravitational perturbation of the (eccentric) binary puts the
planetesimals on eccentric orbits. At first, neighbouring
planetesimals will be on almost similar orbits, which leads to small
relative velocities. At a later stage, orbital crossing may occur. Gas
drag plays an important role in this problem, because the strength
of the drag depends on the size of the planetesimal. This leads to
differential orbital phasing: although at first, neigbouring
planetesimals of equal size are on almost similar orbits,
this will not be the case for different sized bodies. This leads to
high relative velocities even before orbital crossing occurs.
Previous studies treated the gas disc as being staic and circular. In
reality, the gas disc is also perturbed by the binary. We show that it
is the eccentricity of the gas disc that plays a major role in
determining the relative velocities between planetesimals. For a
favourable gas eccentricity, there may be no differential orbital
phasing at all. This would make planet formation in a close binary
system relatively easy. However, our hydrodynamical simulations show
that this situation does not occur in practice. In fact, the full
non-linear evolution of the gas disc leads to even higher encounter
velocities between planetesimals. It is therefore not yet clear how
planets can form in close binary systems.
Further reading:
Paardekooper, S.-J., Thebault, P. and Mellema, G.
Planetesimal and gas dynamics in binaries
2008, MNRAS, in press
Thebault, P., Marzari, F. and Scholl, H.
Relative velocities among accreting planetesimals in binary
systems: The circumprimary case
2006, Icarus, 183, 193
Evolution of low-mass planets in non-isothermal
discs
Planets are formed in circumstellar discs of gas and dust around young
stars. During the early stages of their lifes, these discs are still
around and planets interact gravitationally with them. This
interaction may lead to large changes in the mass and orbit of the
planet, and is therefore crucial in linking the observed, present-day
orbital distribution of planets to the process of planet formation.
For low-mass planets, the interaction between planet and disc is
linear, and it is usually found that in this case planets move inward,
at a rate proportional to their mass. This is called Type I migration,
and may well prove fatal for planets of a few times the mass of the
Earth, because Type I migration rapidly takes them into the central
star.
One aspect that is usually ignored is the heating and cooling balance
in the disc. A locally isothermal equation of state is then adopted,
which makes the temperature a constant function of the distance to the
central star. This equation of state is appropriate when the disc is
able to radiate away all excess heat very efficiently. However, this
assumption may not hold in the region where planets are supposed to
form, because the disc is very optically thick from typically 1-10
AU. This makes it hard for the disc to cool efficiently.
If we want to release the locally isothermal assumption, it becomes
necessary to perform radiation-hydrodynamical simulations in order to
account for the heating and cooling balance. Surprisingly, simulations
then show a torque reversal when cooling is inefficient. Planets start
to move outward! Only in regions of low density, where the disc can
cool efficiently, does one recover inward Type I migration.
Further reading:
Paardekooper, S.-J. and Mellema, G.
Growing and moving low-mass planets in non-isothermal
disks
2008, A&A, 478, 245
Paardekooper, S.-J. and Mellema, G.
Halting Type I migration in non-isothermal disks
2006, A&A, 459, L17
