The only catch isthat the recent work on planets around other nearby stars is showing ussomething that is not nearly so neat. Ifind it much easier to blame Jupiter for all of it.
Jupiter acts asthe sun’s binary and it happens to be spinning close to gravitationalinstability. That allows it to be aplanet pump quite able to produce the inner planets while leaving debris in theasteroid belt.
In an earlierposting, I pointed out that Venus is best explained as a recent emergent thatis has only begun to cool and that the red spot is the scar so caused. All others such events took place early inthe solar system’s formation. This lastevent was likely triggered by a large planetoid impacting into Jupiter sendingit over the limit of stability.
The rest of thesolar system is a handful of gas giants that may be explained by thismodel. However, I expect to find it muchmore messy than all that.
Elegant New Theory Explains Origin Of AsteroidBelt
The Solar System consists of distant gas giants and inner rocky planetsseparated by an asteroid belt. Now an elegant new theory explains how thisstructure arises
12/10/2010
When it comes to planetformation, the conventional thinking has been with us for over 40 years. Itgoes like this: bits of rock and dust clump together to form rocky planetswhich then attract the gases that form their atmospheres. The gas giants formwhen these rocky cores grow to at least ten times the size of Earth and so canattract huge gaseous envelopes.
There are numerous problemswith this model, not least of which is explaining how metre-sized lumps of rockend up sticking together after smashing into each other at random. Then thereis the problem of planetary rotation. If the planets form from the randomaggregation of rock and dust, why do almost all of them rotate in the samedirection? Surely, their rotations should be randomly distributed.
But in the last few monthsvarious astrophysicists have begun discussing another idea that solves theseproblems. Today, Sergei Nayakshin at the University of Leicester in the UK
The new approach turns theconventional model on its head. Planet formation begins at distances in excessof 50 AU from the mother star, when random variations in the density of theprotoplanetary gas cloud begin to attract more gas and so grow under the forceof gravity.
Inside these loose clumps,called giant planet embryos, any rocky material aggregates at the centreforming a rocky core. These cores all rotate in the same direction as theoriginal gas cloud because they from by the gravitational collapse of the cloudrather than by random collisions.
As the cores are forming, theembryonic planets interact with the mother star's gas cloud causing them tospiral inwards. Astronomers have long known that huge gaseous atmospheres areunstable at distances closer than a critical radius because of various factors,such as tidal forces and irradiation from the Sun. So when the embryonicplanets get closer than this critical radius, they loose their gas envelopesleaving behind terrestrial rocky planets like ours.
Incidentally, at the criticalradius, the inspiralling planets discard not only gas but any solids stillmixed up in their outer atmospheres. This radius corresponds to the asteroidbelt in our system. This new thinking explains for the first time how the beltformed and why it separates the gas giants from the terrestrial planets.
The gas giants like Jupiterare planetary embryos that simply hadn't made it this far towards the Sun whenthe orbital dynamics settled into the relatively stable system we have now.
One impressive feature ofthis 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 anasteroid belt. No other model does this so elegantly. It is this elegance thathas focused so much attention on it so quickly.
What's curious about this newthinking is that none of the mechanisms it relies on are new ideas. But in thepast, each has been suggested and then discarded.
For example, the idea thatterrestrial planets are gas giants that have lost their gas envelopes was firstput forward over 30 years ago. Astronomers abandoned it after variouscalculations showed that gas giants couldn't form close to a star where we findrocky planets today.
And the idea that planets canmigrate great distances in a planetary system has also been around for years.
What's new is the re-orderingof these processes so that the gas giants form first and then migrate, losingtheir atmospheres as they get closer to the mother star. All of a sudden, itlooks obvious.
There's still work to bedone, of course. Nayakshin points out that the new model doesn't yet accountfor structures such as the Kuiper Belt, the Oort Cloud not can it explain thecomposition of comets.
But there's a sense ofexcitement about this idea that is giving it considerable momentum in thecommunity. You can be sure that astronomers will be poring over the details asI write. Expect to hear more about it in the coming months.
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