by Bruce Moomaw
Cameron Park - May 2, 2000 - It still seems very likely that giant planets form in the cold outer reaches of their stellar nebulas -- but that many of them migrate considerably inwards afterward.
At the Conference, Alan Boss delivered a talk on gas giant formation in which he discussed the possible mechanisms for this migration.
Sometimes it may be due to the fact that two or more giant planets form relatively close to each other and have a series of close encounters which cause them to tug on each other gravitationally and steer each other into wildly new orbits.
Some of them may be flung to escape velocity and leave their stars completely; others are put into new, highly eccentric orbits -- and some of those eccentric orbits are considerably closer to the star.
(As for those planets which end up in eccentric orbits farther from their sun, our detection techniques can't sense them yet.)
But there are many cases where this theory is not adequate.
In particular, it has trouble explaining how the "hot Jupiters" managed to get so extremely close to their stars.
So other theories have been developed, in which the gravitational tuggings of a giant planet with the remaining gas, dust and small forming "planetesimals" of the nebula disk itself cause it to migrate inwards.
In one theory, the planet's gravity plows a debris-free ring-shaped wake in the nebula at the planet's orbital distance -- after which, as the planet and that disk debris which is farther from the star (and thus orbits the star more slowly) pull at each other, the outer debris is accelerated and moves farther from the star, while the planet is slowed and spirals inward.
Meanwhile, the planet's pull at the disk debris which is closer to the star and orbiting it more rapidly works against this, tending to accelerate the planet and nudge it back outward at the same time that the inner debris is slowed and spirals closer to the star.
And there is one more key ingredient: it's been known for a long time that the loose disk material as a whole has a strong separate tendency to slow down and spiral in toward the central star, simply because the friction of the nebula's gas and dust particles against each other turns much of their orbital momentum into heat energy which is then radiated into space and lost.
And as most of the material of the disk migrates inward for this reason, the gravitational "linkages" that we've just talked about pull the planet in toward the star with it, while keeping its orbit fairly stable. Some giant planets may actually end up crashing into their central stars; others may have their deadly inward movement finally halted when they come so close to the star that the tidal tuggings of the planet on the star's gas tend to drag the star around the planet faster in the wake of the star's own rapid rotation, giving the planet a new centrifugal speed boost.
(Boss mentioned one variant of this model in which the planet's tuggings on the nebular cloud create spiral-shaped "density waves" -- literal traffic jams of nebular particles -- like the spiral arms of a galaxy; and the complex gravitational tuggings of the planet on these spiral arms may cause its orbit to become more eccentric at the same time that it spirals inward, until it is very close to the star and its tidal pull on the star's surface re-circularizes its orbit.)
Why hasn't this happened to our own giant planets? Well, it may have; it's quite possible that Jupiter and Saturn formed considerably farther from the Sun and then migrated inwards to their present orbits, ceasing their migration only when most of the remaining small disk debris had been cleaned out of the Solar System.
It may be that the only reason that they're still fairly far from the Sun is that they initially formed at such a great distance from it.
We are just starting to analyze the very complex orbital-mechanics interactions that go into the formation of a solar system.
But, in any case, this new model of planetary systems has grim implications for habitable planets.
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