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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 think this is just another case of if the Earth wasn't destined to exist, it wouldn't exist.
Read the Holy Bible.
Morbo: Orbital mechanics do not work that way.
Can you be Even More Awesome?!
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?
Sig Battery depleted. Reverting to safe mode.
Or maybe we ARE plummeting into sun, but at a rate that is too slow to be observable.
The largest prime factor of my UID is 263267.
Well then what part of orbital dynamics suggest the inner planets would have crashed into the sun?
After all, accretion would happen mostly from the "back" side (hemisphere opposite the orbital direction). The planetoid wouldn't "catch" anything in its orbit, but would be over taken by things on more elliptic orbits.
Therefore the impacts would be accellerative, and puhs the planetoid to a higher orbit.
So where did the original assumption that they would spiral into the sun come from?
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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
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.
> It's fairly easy for an object in orbit to catch up to an elliptically orbiting body.
Well, not really.
Elliptical orbiters are going much faster as the approach the orbits of the inner planets, and they exit faster too. Most of these are crossing paths.
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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, 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.
The incorrect use of periods to indicate emphasis is not linguistic evolution. It is just semantic stupidity. I wish it didn't catch on.
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
Actually, no. Original poster is right, the gas in the disk orbits slightly slower than the solids do. So there is drag. However, the gas is pretty tenuous, so the drag only really affects things that are small, say less than a meter or so. (Or so classical theory has argued.)
No, the facts don't fit that bogus model.
Well, yeah, I've wasted some youth at the pool table...
But of many thousands of hits by smaller objects one would expect it to sort of average out...
Sig Battery depleted. Reverting to safe mode.
Wow, thank you for that faith inspiring rhetoric; I can't think of how I could further prove that modern day theory is not much better then religion for the people who can't yet invent a new "God" or Genesis story.
I could come up with a much better theory, about how the sun's rotation speed caused all the slower particles to form planets much faster as a result of the suns unequal gravitational balance, however I fear that would be lost in the argument about why such circumstance would come to be when no evidence of such things suggest a theory even close to mine, much less this one...
The next time I pick a random thought out of my head, I'll be sure to post it on slashdot however... oh wait..
Indeed, it should largely cancel; the momentum transfer should be a bell-like curve centered near zero depending on where the material is in the nebula.
Sigs are too short to say anything truly profound so read the above post instead.
I probably am the only one who misread the title as "How to avoid a fiery premature death."
>Well then what part of orbital dynamics suggest the inner planets would have crashed into the sun?
Nothing. According to that theory, everything always gets sucked into everything else, and the universe would be one giant star. Obviously that's not the case, so anyone operating under that theory has a screw loose.
>Therefore the impacts would be accellerative, and puhs the planetoid to a higher orbit.
They don't need to. We could have started from a higher orbit and fallen inward to where we are now. Of course this contradicts the accepted theory that God created humans 6,000 years ago along with the dinosaurs.
How do we know if the death of Earth is premature? We have absolutely no relative data to compare an M-class planet's typical life.
> 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;
Premature? More like "long overdue" amirite.
o hai
Yes, but going slower... makes you go faster. From a certain point of view.
Can you be Even More Awesome?!
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.
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
Or, perhaps, gravity could be a consequence of temperature differences, so the "pull" and the "push" don't really happen.
Ask me about repetitive DNA
True, but I think what the OP meant was that it'd lose energy and move toward the Sun.
After all, accretion would happen mostly from the "back" side (hemisphere opposite the orbital direction).
Not really. Simulations show that the accretion happens pretty much symmetrically from both sides.
The planetoid wouldn't "catch" anything in its orbit, but would be over taken by things on more elliptic orbits.
In its precise orbit, no. But from nearby circular orbits? Yes. And the planets tend to feed on stuff from nearby like that. (They definitely have access, where is chance strikes from elliptical orbits are harder to engineer.)
Who knew?
Somebody better not tell global warming "scientists" lest the journal such "denial" is published in gets removed from the realm of "reputable".
"It turns out that portions of the disk are opaque " Maybe I'm off my rocker but the way this is stated, it sounds like a fact they observed rather than a model that they created. While this "fact" makes logical sense it is far too often that I see the statement "It turns out..."
And here I was all along believing it had something to do with Bruce Willis!
Astronomers have announced over 500 extra-solar planets and they have barely begun looking. So there are a lot of processes out there creating planets in spite of hypothetical process which may destroy them.
Excluding Copernicus and that chap the pope arrested etc. every one knows the earth stayed still and the sun moved.
Any statistic significantly skewed by adding or subtracting 1 to either your numerator or denominator is a statistic too fragile to support a conclusion.
The "we are here" argument is a functional celebration of innumeracy, which reminds me of Operation HUMBUG when Canada first introduced Metric: inference by nostalgia.
'Well, this contradicts basic observational evidence, like We. Are. Here,' says astronomer Moredecai-Mark Mac Low.
Well, this didn’t stop dark matter supporters, did it? ;)
Any sufficiently advanced intelligence is indistinguishable from stupidity.
I didn't read the article but I can only assume if Earth avoided a fiery premature death that Chuck Norris was certainly involved.
There is no better way to sum up some of the gaps between theoretical and applied science other than: "This contradicts basic observational evidence, like We. Are. Here." Did the proponents of the "classic" model not notice this minor flaw in their reasoning?
SirWired
The sun is around 4.5 billion years old, the universe as a whole - 14 billion years. So the sun has been around about 1/3 of the entire history of the universe. Human history, on the other hand, truly is an eyeblink when compared to the age of the universe, so your overall point may well be valid. We just don't really know how long human history will last, as it isn't quite over yet (Fukuyama notwithstanding).
Did anybody else notice the story about the prevalence of bad citations a few days ago on slashdot, and then notice the fact that the bottom of this article includes a "Need to cite this story in your essay, paper, or report? Use one of the following formats:" section at the bottom of this article? Not that Science Daily doesn't have their citation ducks in a row, but reading a "Science News" story is not a substitute for reading, understanding, and then elaborating or building upon research. "If any of my slashdot posts are read, it is because I stood on the shoulders of journalists, and became a scientist" -bda
Koalas. They're telepathic. Plus, they control the weather. -Margaret
Why? When I drive in the rain more raindrops hit the front of my car than the back.
Tempora mutantur, nos et mutamur in illis