The locked migration of giant protoplanets (Forwarded)

Journal Astronomy & Astrophysics
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Mr. Paul Cresswell
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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.