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Drifting Planets


Learned a nifty bit of paleo-astronomy on Tuesday, thanks to a seminar given at my work.

It's about the early formation of the solar system. As I knew even before the seminar, the current logic states that the solar system, including the Sun, Earth, and all other bodies, was created from a spinning glob of dust and gas. Conservation of momentum flattened the disk, and most of the mass collapsed into a proto-Sun at the center. The rest started forming smallish lumps, where gravity and collisions pulled bits and pieces together. Eventually, enough of these bits and pieces combined to form the planets (or rather protoplanets, or planetessimals). Four little rocky ones near the Sun (they lost their atmospheres due to the immense solar wind that was emitted when the Sun "turned on") and four big gasious ones further out.

That's where my astronomy book discussion ended, but the seminar continued: After the major planetessimals were formed, there was still a lot of junk orbiting the Sun. Due to their large masses, the planetessimals were able to "quickly" (~20 million years) clear their orbits of debris. In the outer solar system, the huge gas giants had so much mass that when a smaller asteroid/comet came by, they either spun it into the Sun or else out of the solar system entirely. From these rejected bodies, the Oort Cloud and Kuiper Belt arose.

That made sense and fit with what I knew. But here was what surprised me: it turns out (at least according to all sorts of simulations), that the outer three planets throw much more mass inwards than outwards. That means that there's a net influx of mass toward the Sun -- and thus, conservation of energy requires that the outer three planets are pushed outwards! That means that Neptune, for example, was not always located at ~30AU as it now. It used to be considerably closer -- more like 20 AU. Saturn and Uranus were similarly closer. (It turns out that Jupiter, due to its huge mass and location, hardly moves at all in the models).

It was an interesting bit of info that I couldn't believe I hadn't learned before -- and, from the responses of others in the room, neither had most of my colleagues. I figure that means that most of you didn't know it either. emoticon So consider yourself informed now. Oh -- and by the way, this is one more bit of "proof" that Pluto isn't a planet. It's just a large resident of the Kuiper Belt.

It's expected (from views of extrasolar systems) that this is a fairly typical way for a solar system to evolve. Of course, those extrasolar observations are a bit ragged and disperse, so that could be wrong. Our guesses about our own solar system are somewhat more likely.

Lecture over! Feel free to apply to any scifi you write. emoticon

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10/25/2007, 6:45 pm Link to this post Email Reythia   PM Reythia AIM MSN
 
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Re: Drifting Planets


Very interesting indeed! Thanks for sharing! That could be quite useful in fact.

Though on the Pluto thing, I never did quite understand the logic behind it not being a planet. I know, I know. Old news. Get over it Loud G. I looked at the rules and even then they didn't make much sense and seemed pretty arbitrary. I don't get how your new info adds another nail in the coffin, so to speak, either.

The reason for my complaints on this matter is Neptune. For a planet to be a planet it must clear its orbit of other stuff. Right?

This is the thing that failed Pluto.

But all the data I've seen, in diagrams and such has indicated that Neptune has other junk that cross its orbit all the time.

Pluto itself crosses Neptunes path (I'm not sure how those two haven't collided yet.) and a bunch of the K-belt you mentioned also gets in Neptune's way quite often. This case is unique in our solar system. So why isn't Neptune being booted from the neighborhood too?


Last edited by Loud G, 10/25/2007, 7:21 pm


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Re: Drifting Planets


Ha! Never mind. Their orbits are offset from eachother. That's what I get for not getting my info from NASA. emoticon

So Neptune and Pluto don't conflict, but it does appear that Neptune and the K-belt do still have issues.

(A little research is a wonderful thing) emoticon

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Re: Drifting Planets


Thanks, Reythia, that bit about the outer planets drifting out because they kick stuff inside was new to me as well.

Good to know! emoticon

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Re: Drifting Planets


quote:

Loud G wrote:
Though on the Pluto thing, I never did quite understand the logic behind it not being a planet.



The thing about Pluto is that when you compare it to the other eight planets, both in terms of formation (as above) or orbit, it is not very similar. However, it IS similar to a large group of objects called Kuiper Belt Objects. These KBOs live in the "Kuiper Belt", a doughnut-shaped region of space starting at about Neptune's orbit and continuing outward some 10s of AUs. In that region are thousands of asteroid- and comet-like objects (maybe even many millions -- we can't see the smallest ones). These KBOs are often on the size of several km across, but there are a bunch of larger ones. The largest of them all is Pluto. At the time that Pluto was first observed, the Kuiper Belt had not been theorized, and telescopes weren't good enough to spot any smaller objects at that range. So astronomers (understandably) assumed that Pluto was the only thing out there, and thus named it our ninth planet. Decades later, once telescopes improved, they realized they had erred. Pluto is not the only object out there: it's just the biggest object in a whole jumble of them. Calling it a planet would be like calling Ceres (the biggest asteroid in the asteroid belt) a planet -- which hasn't been seriously considered by scientists. The only reason there's been such confusion about Pluto is because it was taught one way for so long. For some reason, it's hard to convince people that scientists make mistakes, too!

About the clearing of the orbit thing. First off, I want to refer you to this Wikipedia page:
]Wikipedia: Dwarf Planet
Scroll down a ways until you see the table on "Planetary discriminants". Under that, there's a column headed "mu", which is the ratio of the mass of the "planet" divided by the total mass in the orbital region. All eight of the recognized planets have mu's between 10^4 and 10^6: in other words, the planet contains 1,000 to 100,000 times as much stuff as there is anywhere else near it. Pluto, Ceres, and Eris (the three "dwarf planets") are all less than one -- only [sign in to see URL] for Pluto (less than the others!). That means that there is 14 times more mass AROUND Pluto than there is IN Pluto. Clearly, it's not the only important object in the area, even though it's the biggest by a large margin.

quote:

Pluto itself crosses Neptunes path (I'm not sure how those two haven't collided yet.) and a bunch of the K-belt you mentioned also gets in Neptune's way quite often. This case is unique in our solar system. So why isn't Neptune being booted from the neighborhood too?



Pluto (like many other KBOs) is in a resonance orbit of Neptune. It makes two orbits around the Sun in the exact same time that Neptune makes three. What this means is that even though their orbits overlap, Pluto NEVER comes near enough to Neptune for Neptune to clear it out of the way. If it wasn't in such a resonance, then Pluto would occasionally near Neptune -- and when it did, Neptune's much larger mass would either send Pluto sailing out into the Oort Cloud or in towards the Sun. This is what happened to almost all of the other planetessimals that surrounded Neptune 4-5 billions of years ago. Pluto and almost all of the other KBOs that cross Neptune's orbit are kept there simply because they were lucky enough to be in resonance with the planet. What Neptune can't see, it can't boot. Nothing more. There's nothing special about Pluto except it's resonance (and hundreds of other KBOs have that too).

Pluto's a cool object -- the whole set of KBOs is interesting. But it's not a planet.

Make sense? Holler if you're still confused or if I explained things badly! And don't worry about being unhappy with the new classification. It's going to take all of us some time to get used to it. I recently tutored a ten-year old on space and the planets and I had to watch myself to keep from tacking "Pluto" on at the end of my list of planets. It's just habit, you know? And my friend Robert, though he fully understands the reasoning behind the "demotion", keeps complaining that he can't use his "My Very Excellent Mother Just Served Us Nine Pizzas" memory tool anymore. It's kind of funny to hear him complain about it. emoticon

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Re: Drifting Planets


quote:

Reythia wrote:
All eight of the recognized planets have mu's between 10^4 and 10^6: in other words, the planet contains 1,000 to 100,000 times as much stuff as there is anywhere else near it.



I dispute that. The Moon has a mass a little over 1% that of the Earth and shares the same orbit as the Earth, giving Earth a mu in the region of 10^2.



Last edited by David Meadows, 10/26/2007, 8:06 am


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Re: Drifting Planets


quote:

David Meadows wrote:
I dispute that. The Moon has a mass a little over 1% that of the Earth and shares the same orbit as the Earth, giving Earth a mu in the region of 10^2.



I suspect that they counted the Earth-system as a whole, yes. Presumably they did the same with Saturns moons and Pluto's satelite Charon, too. And even if you count the Earth as having a mu of 100, that's a big difference from [sign in to see URL].

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posticon Re: Drifting Planets


I watched the entire discussion take place and seriously the cleared the orbit idea was stupid and is going to come back to haunt them over and over again as they are almost for sure going to encounter a massive amount of exceptions.

Also they are leaving a bunch of already known objects in a lurch right now, what do you call for instance Jupiters and Super Jupiters in deep space?

For theoretical ones, how about an object of that mass in the L3 or L4 of a binary star system? Then we have mars sized objects that might have been flung into the Oort region, which would make them actually similar to the inner system and thus make the history portion completely moot. And this skips past the possibility of a super massive Oort object, something made mostly of water perhaps.

And of course we should also note, that in the first 20 million years of a system, even a Jupiter sized planet might not be called a planet but a Dwarf Planet, because it just hasn't cleared its orbit yet. Which really in my eyes is ridiculous, in all honesty, I liked the mass definition better, at least it described an unerringly simple to measure thing as well as leaving no exceptions. The only drawback to it is, is that seemingly quite a few Astronomers for some reason or the other think there shouldn't be a lot of planet classed objects. I have no idea why they think tis, but considering the overall definition of planet, you can not logically argue it is based on their origin really, considering that can be very diverse.

It isn't really further a problem I suppose, it just means that eventually the term that defines an object that has the mass to make itself spherical will take over in the end, because planet will become a problem for all the exceptions. Or maybe they'll eventually retcon the additions away. I suppose in the end I'm mostly annoyed because they are ruining a perfectly usable term.


PS Pluto is not the largest object out there, though I forgot what they exactly named the even larger one. emoticon
10/30/2007, 10:49 pm Link to this post Email QS2   PM QS2
 
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Oh, absolutely, QS. The definition of "planet" is rather arbitrary -- and I agree that once we find more large objects, the definition is likely to change again (or a new name will be introduced). That still doesn't mean that calling Pluto and planet but Ceres not a planet makes any sense at all. For OUR solar system (possibly excluding the Oort cloud, which we know relatively little about), the "cleared orbit" definition makes a great deal of sense. If, in a hundred years, we find that it doesn't make sense when we apply it to other systems, then I have no problem with them changing the definition again. It makes sense to me that, as our understanding improves, or "rules" and definitions should change.

quote:

QS2 wrote:
And of course we should also note, that in the first 20 million years of a system, even a Jupiter sized planet might not be called a planet but a Dwarf Planet, because it just hasn't cleared its orbit yet.


I believe there is a stability check in place, whereby if the planetary system is rapidly changing, NOTHING is called a planet yet. The objects (large OR small) are "proto-planets" or "planetessimals" instead.

quote:

Which really in my eyes is ridiculous, in all honesty, I liked the mass definition better, at least it described an unerringly simple to measure thing as well as leaving no exceptions.


Which mass definition? That's an honest question, not a sarcastic one. I've actually heard several mass definitions proposed -- including the current "sweeping out the orbit" one, which is, after all, just a ratio of masses. Which one do you prefer and why?

quote:

...considering the overall definition of planet, you can not logically argue it is based on their origin really, considering that can be very diverse.


How so? Again, this is something that might very well change once we understand extra-solar planets better, but for now, the origin argument isn't too bad. What diversities do you see? There are some, but there's an even broader diversity if you include KBOs and other celestial objects.

Also, as an aerospace engineer, I can assure you that there's a practical reason to separate the planets from most (but not all, it's true!) of the remaining rubble: we have to use totally different tactics for rendezvous and even orbiting with, say, a moderate-sized asteroid than we would any planet.

quote:

PS Pluto is not the largest object out there, though I forgot what they exactly named the even larger one.


It's Eris -- another KBO, located in the scattered disk.


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Re: Drifting Planets


quote:

Reythia wrote:
Which mass definition? That's an honest question, not a sarcastic one. I've actually heard several mass definitions proposed -- including the current "sweeping out the orbit" one, which is, after all, just a ratio of masses. Which one do you prefer and why?



Oops..., did I forget to mention that? Maybe I deleted it while redacting my response. emoticon

Anyway, I'm proposing using the one where you have sufficient mass to pull an object into a spherical form, this is in part significant because such objects typically also have some material fractionation for I believe somewhat similar reasons. As such you at least have a somewhat useful difference as far as practical or astronomical applications go.


To respond to the rest of your comments, diversities seen are for instance the extremely obvious, gas giant and rocky planet types. Though even with the gas giants we have some significant differences, Saturn is for instance quite different from Neptune density and composition wise I believe. Which casts some doubt on whether these two planets really have similar histories.

As such origin is a might doubtful, because the planets don't have a really common heritage, instead there are several fairly unique backgrounds to them. Not to mention that you will simply continue to exclude many very large objects with otherwise all the properties of a 'planet' but because it fails a somewhat bothersome to check orbital test (Which it is if it isn't our own solar system) then lo and behold, it isn't a planet after all... because well... there is some 'debris' around... though in all honesty any debris that large should probably be called a planet as well.

Also to make a clear example, Ceres in the future may well be called a planet in the future under current rules, why? Because we'll probably start mining out asteroids in the asteroid belt in the future and Ceres will probably be left for last, because it would be a bit more difficult to work with. Thus leading to a situation where Ceres orbit might be cleared and it fulfills the mass criteria as given in the current rules. But..., if all that is needed to make it a planet was getting rid of a few rocks? Then what kind of a distinction is that? Ceres is no different then it was before... emoticon


PS I just thought of this, but we don't start calling a white dwarf a planet because it is orbiting another star, is no longer a star itself and obviously would have a clear orbit, would we? And I'm guessing that is because it is different from mass aggregated objects, not?
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Re: Drifting Planets


QS2; I believe a planet has to clear its own path. Then again, there are many factors that come into play, even with man's possible (or eventual) mining of asteroids.

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It is nice to say it has to clear its own path, but if human civilization collapsed after we did so and then revived later on but had forgotten all about our last space development, we'd never know the difference, so why make the distinction? Hmmm, I guess the entire planet definition really kind of annoys me in a sense, sorry if I sometimes sound a bit angry. emoticon
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Re: Drifting Planets


quote:

QS2 wrote:
It is nice to say it has to clear its own path, but if human civilization collapsed after we did so and then revived later on but had forgotten all about our last space development, we'd never know the difference, so why make the distinction? Hmmm, I guess the entire planet definition really kind of annoys me in a sense, sorry if I sometimes sound a bit angry. emoticon



Actually, we WOULD know the difference, or at least we should. Because when we took Newton's laws and propogated them forward in time, we would find that Ceres doesn't have the mass to be alone in an orbit where it is. It just doesn't. Such simulations are classic Monte Carlo-type models, based on fundamental equations where a lot of random small masses collide into each other and slowly form into larger planets, sweeping out clear-ish volumes of space where they pass by. There is no way that such simulations would repeatedly (ie: with non-negligable frequency) arrive at a solution where an asteroid like Ceres existed alone at that radius from the sun. So, while future humans might not know why Ceres was there alone, they should realize that it's something strange.

Also, I really, honestly, see no fundamental difference between declaring "Anything that has enough mass to pull itself into a sphere is a planet", and declaring "Anything that has enough mass to pull itself into a sphere and clears its orbit is a planet". Both are based upon the crude understanding that a planet is a body of "enough" mass. It's just a question of how MUCH mass -- so why should one decision be better than the other?

And also, both can be broken, if you really put some effort into them. For example, with the "round body" argument, you have to ask: "How round is round?" The Earth is certainly not perfectly spherical -- there are differences of some 22km between the radius at the poles and equator. Self-gravitation wasn't enough to prevent that. And let's imagine an asteroid smaller than Ceres, but big enough to sort of pull itself into a round shape. But not totally-round. Just slightly elliptical and a little lumpy. How round is round? Who decides what is counted as "round" versus "not round"?

Also, considering your asteroid construction project, what happens if humans start mining the "planet" Ceres. What if we dig in and take away a huge chunk from one end of it. Now Ceres looks like a sphere with a bite taken out of it. It's no longer round -- so should it stop being a planet, in your book? (For the record, I agree with Pink that such human-made changes are not really what the astronomers are talking about. Probably if that ever becomes an issue, they'll state so explicitely in their rules.)

quote:

To respond to the rest of your comments, diversities seen are for instance the extremely obvious, gas giant and rocky planet types. Though even with the gas giants we have some significant differences, Saturn is for instance quite different from Neptune density and composition wise I believe. Which casts some doubt on whether these two planets really have similar histories.


The rocky and gasseous planets WERE formed in exactly the same way. The difference was merely distance from the proto-sun -- and thus temperature. Modern theory states that before the sun started burning hydrogen, the inner planets were originally part-solid, part-gasseous, as were the outer planets. The difference was that the inner planets were hotter than the outer ones, and as such, volatile molecules like water and most room-temperature gasses could not condense around the inner planets -- only the heavier elements could. The lighter elements formed a think atmosphere (much thicker than now). But out farther from the proto-sun, the environment was much colder, so everything could condense (at least into liquids). This, by the way, is why a lot of astronomers believe the gas giants have ices at their cores.

Then the proto-sun condensed enough to start burning hydrogen. When it did so, an immense solar wind blew across the entire solar system. The inner planets caught the brunt of it and their huge atmospheres where blown away, leaving only the small rocky parts remaining. The outer planets atmospheres were also blown away, but they had less to blow, since most of their mass was then in solid or liquid form.

Once the sun started up, things warmed up significantly in the solar system. The inner planets slowly exuded thin atmospheres, based on the gasses trapped within their centers. The outer planets thawed out, and their vast oceans of liquids evaporated to form huge atmospheres -- thus the name "gas giants".

These changes happened AFTER the planets were already formed. But there was still some leftover stuff remaining that, over the millions of years, kept getting thrown either into the sun or out to the Kuiper Belt / Oort Cloud. One of the bigger pieces smashed right into Earth, before Earth's crust had hardened. It blew a big chunk of rock out into space, which was then caught by the Earth's gravity and pulled into orbit. This became our own Moon. (That's the reason why rocks on Earth look very much like rocks on the Moon, with the exception of oxidizing effects.)

My point is, the eight planets WERE formed in the same way, even though they look different now. To be fair, so were Pluto and Ceres -- except that they weren't big enough to throw debris out of their way as they went around. Actually, Pluto probably WAS thrown around itself! That's the reigning hypothesis for Pluto's highly inclined orbit (17 degrees) and high eccentricity ([sign in to see URL]). Why should we count a thrown-out asteroid as a planet?

Actually, Ceres has a better claim than Pluto does. There's no evidence that Ceres was ever moved around by the gravitation of the other planets. Also, the only reason there's an asteroid belt instead of a planet between Mars and Jupiter is because Jupiter is so dang big that its gravity pulls against the sun's, causing anything too big in that region to be ripped apart!

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Re: Drifting Planets


Just to be clear, my definition of mass is directly taken from the official IAU definition of a planet as you can find it #IAU_debate]here
. Which is admittedly somewhat controversial, but close enough for general usage, if need be you could create a transitioning class between asteroids and planets to resolve problem cases. (ie Dwarf Planet)

The current IAU definition is however filled with problems, including that there are no planets beyond our on solar system, you can only be a planet if you orbit the Sun. Objects free floating in space of several Jupiter masses are not planets, and if the Earth orbited Jupiter it would be a moon and not a planet, but then so is every small rock, so what makes a moon and a planet different. To make things worse yet, at current rates eventually the moon will spiral so far from Earth that under current IAU rules it will actually be a planet as well, but then a double planet, though obviously it has failed to clear its orbit. Given enough time Ceres will clear its orbit and then it would have to be called a planet. And the problems go on and on and on and on and on and so forth etc etc etc.

This isn't just some minor exceptions anymore, this is basically a definition that quite possibly works well in our solar system and others like it. Which is ridiculous, cause you aren't describing anything of any actual importance anymore, no true defining characteristic, just some fluke chances. This is why I favor throwing all the dross overboard and focusing on the only thing that actually makes a real difference, masses so great that they pull their objects into a spheroid to reach hydrostatic balance, which also tends to allow them to have some fractionation of materials, which makes them different from the smaller aggregated objects.


-----------------


As some further notes, the object that hit earth wasn't some small rock but an object the mass of Mars, nothing less would have sufficed it seems like according to simulation to make the moon. Thus, until it did hit Earth, Earth was not a planet but a Dwarf Planet.

It is true that Pluto probably got flung out a distance, but for all we know a Uranus mass object got pitched into the deep Oort cloud by Jupiter as well and that far out we wouldn't have found it yet. Of course if such an object exists it would have an irregular orbit and you could start worrying about if it had 'cleared' its orbit. In all seriousness, affecting objects significantly in the Oort cloud takes a long time, so it would technically not have had the time yet. Even if by luck it was the majority amount of mass in the orbital area it covers, you could wonder if that is really in the spirit of the rules.

Jupiters gravity field does not rip objects apart in the asteroid belt, its gravity is nowhere that strong, it merely makes it harder for them to clump together, but that is just delaying the matter, Ceres will eventually clean the place out, but when exactly is it cleared out enough, mass factor 1? 10? a 100?. Incidentally, this also means we would just find it odd that Ceres is the main mass there already if humans cleared it out, but there would be no real reason to assume it hadn't. You literally wouldn't be able to tell really.

There we are using the IAUs definition, it is only the mass that matters, not the actual shape. So even if you blow a chunk out of Ceres, it would under current rules still be called a dwarf Planet just like Pluto and not an asteroid.


-----------------------------


What this all comes down to is, is that under current rules that you are a planet just by the sheerest chance of where you started forming. So in other words your location determined that you are a planet or not, this is due to that if you are in the wrong spot, you will be flung out by one of the larger aggregates. And I must ask seriously, do we base the definition of a planet on where it started? And not on say perhaps some actual less randomly selecting characteristic of the object?
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Most people assume that the planets in our Solar System formed in the places where they are today. However, some astronomers now believe that planets like Jupiter & Saturn didn't form in their current positions, instead forming much closer to the Sun, then slowly moving further away. Another theory is that Jupiter formed further from the Sun, then began to move in towards it. The only thing that stopped Jupiter and the Sun colliding was the formation of Saturn. As Saturn formed, it too began to move in toward the Sun. When the two planets got close, they were affected by each other's gravity and, over time, returned to their original positions. Astronomers also believe that Jupiter's existence is the reason why Mars is so small. After it formed, as Jupiter moved toward the Sun, it pushed away a lot of rocky material. This prevented Mars from gathering the material it needed to grow and, therefore, Mars remained small.

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