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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.'"
For me the most amazing aspect of planetary migration is the probable exchange of order for Neptune and Uranus, with Neptune being thrown out to the position of outer planet; without it being ejected from the system, plunging into the Sun or colliding with other big body. Though who knows, perhaps some planet was doomed that way; certainly wild axial tilt of Uranus isn't a testament of calm times.
http://en.wikipedia.org/wiki/Nice_model
PS. There's some joke here, with Uranus ending up closer to the Sun, about total asses always ending the race in better place...
One that hath name thou can not otter
I suppose so but this article is about why it didn't happen.
http://michaelsmith.id.au
Morbo: Orbital mechanics do not work that way.
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This would seem to suggest the inner planets formed first and swept the disk of hard derbies, leaving nothing but the gas, which was ultimately blown outward by the pressure of the sun as the disk was swept clear of big chunks.
The gas giants would accumulate at a much slower rate, and almost by definition must be far younger than the rocky planets.
Then there are the oddball moons of the outer planets. Captured planetoids forming late, almost falling into the sun because the disk was pretty much cleared by that time, but being slung outward and captured by chance?
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How Earth Avoided a Fiery Premature Death
The dinosaurs were smart (especially the Velociraptors). They stopped driving SUVs. That's why we're here.
There's no place like
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.
If I'm reading the article right, it says that the gravity of a gas/rock disk around a star will cause the whole thing to migrate inward until it is consumed by the sun. However, account for temperature differences due to varying cooling rates across the disk, then this causes a different force which can be shown to balance out the inward migration.
My question is. Why does the gravitational effects of a gas disk around a star cause inward migration? The only thing I would expect to cause inward migration would be friction resulting in the loss of kinetic energy. I haven't the foggiest idea how a temperature gradient can cause matter to climb out of a gravity well. Maybe I should go looking for the original paper.
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
I probably am the only one who misread the title as "How to avoid a fiery premature death."
Destiny doesn't really factor into it. What we're learning is that essentially our planet is rare. Rocky planet of about the right size, at about the right distance, where our planet didn't fall into the sun, nor did a gas giant falling inwards destroy us, and with a very large moon serving to stabilize the planet's wobble.
All those things coming together for our perfect scenario seem like being very, very against the odds, but the reality is that there's an effing huge number of stars in the universe, and repeat their formation process enough times and you're bound to get our scenario play out from time to time (it obviously happened here or we wouldn't be here).
Only downside is that with all these specific things we're learning that make Earth like planets so rare, it may just be the case that such planets are rare enough that we might as well be the only one. The reality is that if they were rare enough that there were only say, 1 such planet per galaxy, then while the universe itself would be pretty much swimming in Earth-like planets (billions of them), but we'd never be able to detect them, much less contact any possible civilizations on them.
"People who think they know everything are very annoying to those of us who do."-Mark Twain
> 4.6 billion years ago
I like the way it's just a bit bigger than 2^32 to stop you using 32 bit variables for the year.
echo -e 'global _start\n _start:\n mov eax, 2\n int 80h\n jmp _start' > a.asm; nasm a.asm -f elf; ld a.o -o a;
From the viewpoint of the lottery winner, it always look like destiny: "if my birthdate is the winning numbers, I must be special in some way".
From an outside viewpoint, some random guy won lottery because when millions of tickets are bought, there's a high probability that someone checked the winning numbers.
Difference is, in the case of a planet not forming, there's no exterior viewpoint: losers and non-players simply don't exist.
I just want to point out 1 more important factor in contacting, or meeting other civilizations in the universe: Time.
The age of our sun is a blink of an eye in the cosmological time scale. It's like tiny little lightbulbs going on and off and on and off. We might not reach an "on" one before ours turns "off", the destination is simply not turned on yet. It's a very lonely picture, but highly probable.