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Already with the discovery of the first extra-solar giant planet
(exoplanet) in 1995, we learned that some stars have giant
planets orbiting at distances that are 10 times closer than the
distance from Mercury to the Sun (57 million km). While not all of the
70 or so exoplanets so far
found are that close, a significant number of them move in orbits
within 15 million km of their central star! Moreover, with the
exception of very close planets whose orbits have become circular
because of gravitational effects ("tides"), the eccentricity of the
orbits of most exoplanets is rather large. There are two major reasons why the existence of giant exoplanets
at close orbital distances requires that the standard theory for the
formation of planets must be significantly modified and/or
extended. First, the mass of a typical proto-planetary disk within the orbit
of the closest exoplanets observed is less than one Jupiter mass by a
large factor. Thus there is simply not enough matter in the disk so
close to the star to form a giant planet there. Second, even if sufficient mass were available, a young planet like
the one discovered near the star 51 Peg would be torn apart by the
strong gravitational force of the star at its current
location. To reconcile theory and observations, different mechanisms have
been studied which allow planets to migrate from their birth place
to where they are observed today. Planetary migration is not a new
idea, but it was never before considered as an essential ingredient in
planet formation. Most migration scenarios consider the gravitational interactions
between the growing planet and the gaseous disk. When a massive
objects orbits inside a gaseous disk, gravitational interactions
between the two give rise to significant changes ("perturbations") in
the disk. In particular, if the planet is massive enough, a gap
opens in the disk. The tides raised in the disk by the planet result in changes in the
density distribution in the disk (structures like the spokes of a
wheel may result) which in turn exerts a force on the planet. Complicated
computations show that the net result is a transfer of angular
momentum from the disk inside the planet's orbit to the planet, as well
as a transfer of angular momentum from the planet to the disk outside
its orbit. The planet then "opens" a gap in the disk and slowly
spirals inwards towards the star. While migration appears to solve some of the problems raised by the
planetary systems recently discovered, other issues remain puzzling
and apparently hint of more fundamental problems in our
understanding. For example, the migration timescale appears to be quite short (a
few hundred thousand years). A central question is therefore,
why the planets do not "fall" into their star, but stop just in
time to avoid this sad fate, after having travelled 99% of the
distance? However, in this connection, the recent observation of
one
such case in which it appears that "the star ate a planet" has
now become of special interest. Even more puzzling is the fact that there are no signs of extensive
inward migration in our own solar system. In particular, Jupiter does
not appear to have migrated significantly - it is still located at
about the same distance from the sun as where it was formed. In summary, while a few years ago it was believed that we had
reasonably well understood the formation of planetary systems, today we
are left with pieces of theories that no longer provide a
physically coherent picture! So there is much work to be done in this exciting research
field. In this, we will be supported by new observations of more
planetary systems, as these are discovered by means of steadily
improving observational techniques.
Life in the Universe |
Last updated September 3, 2001
Planet Migration