The only catch is
that the recent work on planets around other nearby stars is showing us
something that is not nearly so neat. I
find it much easier to blame Jupiter for all of it.
Jupiter acts as
the sun’s binary and it happens to be spinning close to gravitational
instability. That allows it to be a
planet pump quite able to produce the inner planets while leaving debris in the
asteroid belt.
In an earlier
posting, I pointed out that Venus is best explained as a recent emergent that
is has only begun to cool and that the red spot is the scar so caused. All others such events took place early in
the solar system’s formation. This last
event was likely triggered by a large planetoid impacting into Jupiter sending
it over the limit of stability.
The rest of the
solar system is a handful of gas giants that may be explained by this
model. However, I expect to find it much
more messy than all that.
Elegant New Theory Explains Origin Of Asteroid
Belt
The Solar System consists of distant gas giants and inner rocky planets
separated by an asteroid belt. Now an elegant new theory explains how this
structure arises
12/10/2010
When it comes to planet
formation, the conventional thinking has been with us for over 40 years. It
goes like this: bits of rock and dust clump together to form rocky planets
which then attract the gases that form their atmospheres. The gas giants form
when these rocky cores grow to at least ten times the size of Earth and so can
attract huge gaseous envelopes.
There are numerous problems
with this model, not least of which is explaining how metre-sized lumps of rock
end up sticking together after smashing into each other at random. Then there
is the problem of planetary rotation. If the planets form from the random
aggregation of rock and dust, why do almost all of them rotate in the same
direction? Surely, their rotations should be randomly distributed.
But in the last few months
various astrophysicists have begun discussing another idea that solves these
problems. Today, Sergei Nayakshin at the University
of Leicester in the UK
The new approach turns the
conventional model on its head. Planet formation begins at distances in excess
of 50 AU from the mother star, when random variations in the density of the
protoplanetary gas cloud begin to attract more gas and so grow under the force
of gravity.
Inside these loose clumps,
called giant planet embryos, any rocky material aggregates at the centre
forming a rocky core. These cores all rotate in the same direction as the
original gas cloud because they from by the gravitational collapse of the cloud
rather than by random collisions.
As the cores are forming, the
embryonic planets interact with the mother star's gas cloud causing them to
spiral inwards. Astronomers have long known that huge gaseous atmospheres are
unstable at distances closer than a critical radius because of various factors,
such as tidal forces and irradiation from the Sun. So when the embryonic
planets get closer than this critical radius, they loose their gas envelopes
leaving behind terrestrial rocky planets like ours.
Incidentally, at the critical
radius, the inspiralling planets discard not only gas but any solids still
mixed up in their outer atmospheres. This radius corresponds to the asteroid
belt in our system. This new thinking explains for the first time how the belt
formed and why it separates the gas giants from the terrestrial planets.
The gas giants like Jupiter
are planetary embryos that simply hadn't made it this far towards the Sun when
the orbital dynamics settled into the relatively stable system we have now.
One impressive feature of
this model is that it naturally accounts for the structure of the Solar System,
with the distant gas giants separated from the inner rocky planets by an
asteroid belt. No other model does this so elegantly. It is this elegance that
has focused so much attention on it so quickly.
What's curious about this new
thinking is that none of the mechanisms it relies on are new ideas. But in the
past, each has been suggested and then discarded.
For example, the idea that
terrestrial planets are gas giants that have lost their gas envelopes was first
put forward over 30 years ago. Astronomers abandoned it after various
calculations showed that gas giants couldn't form close to a star where we find
rocky planets today.
And the idea that planets can
migrate great distances in a planetary system has also been around for years.
What's new is the re-ordering
of these processes so that the gas giants form first and then migrate, losing
their atmospheres as they get closer to the mother star. All of a sudden, it
looks obvious.
There's still work to be
done, of course. Nayakshin points out that the new model doesn't yet account
for structures such as the Kuiper Belt, the Oort Cloud not can it explain the
composition of comets.
But there's a sense of
excitement about this idea that is giving it considerable momentum in the
community. You can be sure that astronomers will be poring over the details as
I write. Expect to hear more about it in the coming months.
No comments:
Post a Comment