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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
Don't. It's a work of fiction and a pretty boring one at that.
All that begatting and not a bare breast in sight.
Morbo: Orbital mechanics do not work that way.
Can you be Even More Awesome?!
Surely the mindless violence makes up for that.
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?
Sig Battery depleted. Reverting to safe mode.
My haiku is poor!
Each line must end with p-tags!
I am mortified.
Do not mock my vision of impractical footwear
All that begatting and not a bare breast in sight.
You should check out R. Crumb's "Book of Genesis".
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.
I first read that as "designed to exist." Was gonna mod you funny.
This post is LAW where prohibited by VOID. Prosecutors will be violated.
> 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.
All that begatting and not a bare breast in sight.
Plenty of sex (check out the Psalms sometime) and violence, though. Especially violence.
This ain't rocket surgery.
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.)
I wasn't aware of sex playing any major role in the Psalms, which are holy liturgical songs, though admittedly I've only read about half of them. I believe you are thinking of the erotic Song of Solomon, various sexual imagery in the prophesies of Ezekiel and Hosea, and historical/mythological narrative in Genesis, 2 Samuel, etc.
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...
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--Greg (No, I'm not one of the folks who moderated it!)
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.
Mindless violence? To a modern reader, it may seem like that - but if you live in a society where blood vengeance is sworn by any survivors, you kind of have to wipe out any people group you attack. ... and from a purely human perspective (which would appear to be your own perspective), you kind of have to attack someone if you are fleeing THE aggressive super-power of the era and have to go through antagonistic locals to escape the super-power. Compare it to the infighting in Africa within countries... people flee the larger powers and sometimes have to fight through lesser powers to ensure they have safe distance from the main aggressor. It isn't pretty, but it is understandable. You again see this even today in less developed countries (though I recall we recently had a discussion over whether we can actually say 'less developed' given all the stuff the 'developed' world does to others).
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
>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?!
I believe you are thinking of the erotic Song of Solomon
Yes, you're right. What little I know about the Bible was learned many years ago (and against my will) so it's sometimes a little sketchy.
The violence part still stands, though. Plenty of that all through the Old Testament.
This ain't rocket surgery.
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.
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
Whatever next - suggesting that the kosher/halal rules for food make sense for avoiding food poisoning in a hot climate with no refrigeration? That's crazy talk!
Confucius say, "Find worm in apple - bad. Find half a worm - worse."
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.)
"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.
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 think it's more about how it didn't happen. Why is left up to philosophers, theologians and the like.
Give me Classic Slashdot or give me 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.
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.
The enemies of Democracy are
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).
Seeing how the development of life in general, and our technology in particular, seems to follow an exponential curve, I don't think that Sun dying really has much to do with that probability, unless it's going to die next millenia or so.
Forget magic. Any technology distinguishable from divine power is insufficiently advanced.
Part of Earth being rare is linked to those discoveries though.
We know basically this: gas giants will form outside the orbits of rocky planets. The star is going to blow the gas outwards and so a gradient is established.
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. They almost certainly had to have formed farther out and migrated inwards - their orbits might not even be stable, in that way may just be seeing their final "days" (which could actually be millions of years) before they plunge into their parent star. A stable orbit in and of itself could a rarity - much less a stable orbit at the right distance. Any gas giant migrating inward almost certainly will sweep up any rocky planets within that orbit along it's way.
Also, there is the issue of the moon. The moon acts as a gravitational anchor. It keeps the Earth relatively stable on it's axis of rotation. This ensures that geographically the Earth maintains relatively similar climates in particular regions from year to year. Without that anchor you could see areas shifting from arctic to tropic temperature levels within a matter of centuries - not conducive to life developing. With that in mind, we have to look at the requirements to get that huge honking moon out there in orbit. Forming along with a planet this close to it's own mass looks unlikely. It had to be a huge impact event, and it had to be at a very specific angle (essentially a glancing blow). Too direct a hit and they bodies just merge.
There's also another nagging little fact - the vast majority of stars are red dwarfs. For a planet to be within the habitable zone of such a star would require that it be tidally locked to the star, which means that you're not going to get an Earth-like planet. Maybe some simple life might evolved near the day/night boundary, but it won't look anything like our system. Of the remaining stars, you can't go too big as the really massive ones are too short lived for life to develop in that time (some of the really big guys have lifespans of less than 50 million years). The most likely stars for life (as we know it) are going to be yellow dwarfs like our sun, and orange dwarfs. These two groups together don't make up too large a percentage of the stars in the galaxy.
You also need tons of water, which our planet happens to have.
As such, it's looking like planets are common, and even rocky planets might be common. However, finding a rocky planet of similar mass, with tons of water, around Class G or Class K star (yellow and orange dwarves), at the right STABLE orbital distance, that ALSO happened to have a huge impact at the right angle to form a large moon, and that didn't have a gas giant drift inwards and eat it up . . . it's looking more and more like a tall order to fill.
Now, as I said, this whole planetary formation thing has happened enough times that obviously it's possible, and it's likely happened just perfectly numerous times, but the universe is a really, really big place. Getting a planet like ours might well be like winning the lottery. Obviously people do win the lottery, and on an astronomical scale Earth did won too (along with I'm sure tons of other planets scattered about the universe), but having ANOTHER habitable planet within any reasonable travel distance of yours may well be like winning the lottery two weeks in a row. Mathematically possible, but very, very unlikely.
"People who think they know everything are very annoying to those of us who do."-Mark Twain
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.
The enemies of Democracy are
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
compact and clear. Great.
Herve S.