The locked migration of giant protoplanets (Forwarded)

Journal Astronomy & Astrophysics
Paris, France

Contact persons:

Science:

Mr. Paul Cresswell
Phone: +44 (0) 20 78 82 70 30

Press office:

Dr. Jennifer Martin
Journal Astronomy & Astrophysics
61, avenue de l'Observatoire
75014 Paris, France
Phone: +33 1 43 29 05 41

Press Release: March 21st, 2006

The locked migration of giant protoplanets

Two British astronomers, Paul Cresswell and Richard Nelson present new
numerical simulations in the framework of the challenging studies of
planetary system formation. They find that, in the early stages of
planetary formation, giant protoplanets migrate inward in lockstep into
the central star. Their results will soon be published in Astronomy &
Astrophysics.

In an article to be published in Astronomy & Astrophysics, two British
astronomers present new numerical simulations of how planetary systems
form. They find that, in the early stages of planetary formation, giant
protoplanets migrate inward in lockstep into the central star.

The current picture of how planetary systems form is as follows:

i) dust grains coagulate to form planetesimals of up to 1 km in diameter;
ii) the runaway growth of planetesimals leads to the formation of ~100 –
1000 km-sized planetary embryos;
iii) these embryos grow in an "oligarchic" manner, where a few large
bodies dominate the formation process, and accrete the surrounding and
much smaller planetesimals. These "oligarchs" form terrestrial planets
near the central star and planetary cores of ten terrestrial masses in
the giant planet region beyond 3 astronomical units (AU).

However, these theories fail to describe the formation of gas giant
planets in a satisfactory way. Gravitational interaction between the
gaseous protoplanetary disc and the massive planetary cores causes them
to move rapidly inward over about 100,000 years in what we call the
"migration" of the planet in the disc. The prediction of this rapid
inward migration of giant protoplanets is a major problem, since this
timescale is much shorter than the time needed for gas to accrete onto
the forming giant planet. Theories predict that the giant protoplanets
will merge into the central star before planets have time to form. This
makes it very difficult to understand how they can form at all.

For the first time, Paul Cresswell and Richard Nelson examined what
happens to a cluster of forming planets embedded in a gaseous
protoplanetary disc. Previous numerical models have included only one or
two planets in a disc. But our own solar system, and over 10% of the
known extrasolar planetary systems, are multiple-planet systems. The
number of such systems is expected to increase as observational
techniques of extrasolar systems improve. Cresswell and Nelson's work is
the first time numerical simulations have included such a large number
of protoplanets, thus taking into account the gravitational interaction
between the protoplanets and the disc, and among the protoplanets
themselves.

The primary motivation for their work is to examine the orbits of
protoplanets and whether some planets could survive in the disc for
extended periods of time. Their simulations show that, in very few cases
(about 2%), a lone protoplanet is ejected far from the central star,
thus lengthening its lifetime. But in most cases (98%), many of the
protoplanets are trapped into a series of orbital resonances and migrate
inward in lockstep, sometimes even merging with the central star. Figure
1 illustrates the migration of a swarm of protoplanets.

Cresswell and Nelson thus claim that gravitational interactions within a
swarm of protoplanets embedded in a disc cannot stop the inward
migration of the protoplanets. The "problem" of migration remains and
requires more investigation, although the astronomers propose several
possible solutions. One may be that several generations of planets form
and that only the ones that form as the disc dissipates survive the
formation process. This may make it harder to form gas giants, as the
disc is depleted of the material from which gas giant planets form. (Gas
giant formation may still be possible though, if enough gas lies outside
the planets' orbits, since new material may sweep inward to be accreted
by the forming planet). Another solution might be related to the
physical properties of the protoplanetary disc. In their simulations,
the astronomers assumed that the protoplanetary disc is smooth and
non-turbulent, but of course this might not be the case. Large parts of
the disc could be more turbulent (as a consequence of instabilities
caused by magnetic fields), which may prevent inward migration over long
time periods.

This work joins other studies of planetary system formation that are
currently being done by a European network of scientists. Our view of
how planets form has drastically changed in the last few years as the
number of newly discovered planetary systems has increased.
Understanding the formation of giant planets is currently one of the
major challenges for astronomers.

On the evolution of multiple protoplanets embedded in a protostellar disc
by P. Cresswell and R.P. Nelson
To be published in Astronomy & Astrophysics (DOI number:
10.1051/0004-6361:20054551)

Full article available in PDF format,

http://www.edpsciences.org/articles/...PRAA200607.pdf

Movies are available in AVI and MOV format (each file is about 60 MB):
http://www.maths.qmul.ac.uk/~pc/down...1full_long.avi
http://www.maths.qmul.ac.uk/~pc/down...1full_long.mov

For more information about the evolution of multiple protoplanets, see
http://www.maths.qmul.ac.uk/~pc/home/planets.html

IMAGE CAPTION:
[Fig. 1:
http://www.edpsciences.org/papers/aa...r4551_fig1.gif
(141KB)]
Inward migration of a swarm of protoplanets. The protoplanets are
represented by white circles, with size proportional to mass. The disc
is coloured according to density: the brighter part is the denser region
of the disc.