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How Earth Avoided a Fiery Premature Death
Hugh Pickens writes "Space.com has a piece about changing theories of planet migration. The classic picture suggests that planets like Earth should have plummeted into the sun while they were still planetesimals, asteroid-sized building blocks that eventually collide to form full-fledged planets. 'Well, this contradicts basic observational evidence, like We. Are. Here,' says astronomer Moredecai-Mark Mac Low. Researchers investigating this discrepancy came up with a new model that explains how planets can migrate as they're forming and still avoid a fiery premature death. One problem with the classic view of planet formation and migration is that it assumes that the temperature of the protoplanetary disk around a star is constant across its whole span. It turns out that portions of the disk are opaque and so cannot cool quickly by radiating heat out to space. So in the new model, temperature differences in the space around the sun, 4.6 billion years ago, caused Earth to migrate outward as much as gravity was trying to pull it inward, and so the fledgling world found equilibrium in its current, habitable, orbit. 'We are trying to understand how planets interact with the gas disks from which they form as the disk evolves over its lifetime,' adds Mac Low. 'We show that the planetoids from which the Earth formed can survive their immersion in the gas disk without falling into the Sun.'"
ok neat, But how did the main asteroid belt form again,
Roche Limit fail? Jupiter was nearby, relatively speaking, could have been a disruptive influence.
Do not mock my vision of impractical footwear
Or maybe we ARE plummeting into sun, but at a rate that is too slow to be observable.
Al is that you?
A transfer of angular momentum from one region of the disk to another would cause some section of the disk to migrate toward the sun while another set migrated outward. However, it probably isn't caused by a drag force through the residual gas in the disk as most of it is orbiting the same direction as the debris its self. As for accretion, it depends on the distribution of close encounters with objects in a more elliptical orbit. It's fairly easy for an object in orbit to catch up to an elliptically orbiting body.
Sigs are too short to say anything truly profound so read the above post instead.
So, the writer of the space.com article got a wee bit confused, understandably so given that it's quite a tricky topic.
The orbital migration is driven by three effects, one of which was neglected in the original calculations showing inspiral. The main one that was treated was the *imbalance* in the shapes of the spiral arms produced in the disk gas by the orbiting planet. Each spiral arm exerts a gravitational torque on the planet, and the negative torque (removing angular momentum, causing inward migration) turns out to be consistently larger than the positive torque -- in the locally isothermal case. Similar calculations show a lesser contribution from gas in the same orbit as the planet.
However, including 1) the effect of gas on "horseshoe orbits" that overtake the planet, get slingshotted outward (to a slower orbit) then are overtaken by the planet and slingshotted back to the inner, faster orbit, and 2) the actual, local compressibility of gas in the opaque midplane of the disk, reveals that if there is a negative temperature gradient outward, migration will also be outward (positive torques outweigh negative torques).
Hard to capture all that in a soundbite to be sure. The paper should be out in a few weeks, and meanwhile, if you want more, Paardekooper's papers on arXiv.org are the technical foundation for this work.
Take a look at the velocity vectors; not all of that velocity is effectively directed in the same direction as the object it's colliding with that has a lower eccentricity.
Sigs are too short to say anything truly profound so read the above post instead.
So look for more reports of this sort over the next few years. Still, it looks like a big jump forward for our early-solar-system models.
--Greg
Why does the gravitational effects of a gas disk around a star cause inward migration?
Throw a ball up... it comes down. This is gravity. The "base state" for gravity is everything sticking in the centre. Now when something has the right velocity this acceleration towards the centre just causes it to form an orbit around the body.
However given that gasses expand to fill up available space its very hard to have a stable orbit of gas moving at a constant velocity and thus obtaining an orbit. Gasses just don't behave like solids so it doesn't work like that. The expectation would be that as a gas spreads some of it will get pulled in and over time this "some" would become "all".
An Eye for an Eye will make the whole world blind - Gandhi
Not exactly. The body that caused formation of the Moon likely formed in Earth L4 or L5 point; technically making it not a planet. Coming from there also gives less chance for axial tilt such wild as in the case of Uranus...
Since it was already gravitationally bound with Earth, I don't think it changed its orbit in significant way.
One that hath name thou can not otter