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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...
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I suppose so but this article is about why it didn't happen.
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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.
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.
Or maybe we ARE plummeting into sun, but at a rate that is too slow to be observable.
Except for the fact that if something is falling slowly, it ain't a plummet. From the Oxford American Dictionary:
This ain't rocket surgery.
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
No, the facts don't fit that bogus model.
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."
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.
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.)
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.
Are we learning that?
I thought things were heading in the opposite direction. Considering that we've been finding exoplanets basically as fast as our capability allows, and every time we enhance our ability to find smaller planets farther from their star, we almost immediately find such a planet. We've found quite a few planets that are earth-like in mass already, closer to their parent star, not to mention tons of other things we didn't even think possible (like gas giants orbiting in earth-like orbits). So the evidence seems to be pointing at a ubiquity of planets, and a wider variety than we imagined.
Even this story is covering an improved model that seems to make earth-like planets in earth-like orbits more likely, not less. At least, if we figure that accretion disks of non-uniform temperature is more likely than uniform.
So I think the jury is still out on earth being a "perfect" scenario of extremely unlikely happenstance. But it wasn't that long ago that it was possible that planetary systems of any kind were a rarity, so at least the current trend is clear.
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Now, from what we've been seeing, a huge portion of the planetary systems consist of one or more "hot Jupiters". Massive gas giants orbiting extremely close to their parent star.
You mean a huge portion of planets we've found, and the reason for that is because they are by far the easiest exoplanets to find -- massive planets close to their sun create the most obvious wobble in the star and the shortest period over which to see it. These are the first exoplanets we were able to find, and we've been looking for them the longest, so it's no surprise we've found more of them than anything else. Given that they exist, that is. Before finding them, it was thought that gas giants couldn't exist that close to their star.
But then once we got more powerful instruments and refined our techniques, we gained the capability to find gas giants farther from their star, or rocky planets within a few multiples of earth mass very close to the star. And now we're finding those as fast as we are able. Fewer than "hot jupiters" because we haven't been looking for as long, and they take longer to find. But the very fact that as soon as we are able to detect a certain class of planet, we do, should be a hint.
We're only just barely reaching the edge of being able to detect earth-mass planets in the habital zone. So you can't determine from this data that such planets are rare.
On the contrary. Before we started finding exoplanets, we weren't sure if planetary systems were common at all. Now it's starting to look like they are essentially ubiquitous. And so far there's nothing to indicate that our particular type of system is rare, only that there exist more kinds of systems than we previously thought. But your estimate of number of sol-like systems in the galaxy or possible earth-like planets should only have gone up based on our findings.
As far as the rest of components of the equation for calculating the number of habitable worlds, I'm not going to say much. Yes our moon is fortunate, but the question is what range of stability in rotation is necessary, and how common such moons are. We definitely can't see those yet. Water is essential for life as we know it, but is hardly rare even in our own solar system. Mars is fairly stable, so if it were large enough to have held on to its atmosphere, it may still have the large quantity of surface water that we now know it used to have, and we'd have two candidates for life in our own solar system.
It could still be that the conditions necessary for life, much less life itself, is incredibly rare. However, stars like ours are not rare, and the jury is still out on the rarity of earth-like planets but the probability has only gone up since we started hunting for exoplanets. Again, this article itself is about evidence that the creation of our planet was not a freakishly improbable act in defiance of typical planet formation.
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