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Author Topic: Where do comets come from?  (Read 2807 times)

Offline chris

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Where do comets come from?
« on: 14/03/2013 04:01:58 »
Comets have a different composition to other inner solar system bodies, and asteroids, and also wildly eccentric deep-space orbits. So they must have a different origin to the rest of the inner solar system. But what is that origin, and what brings comets in towards the inner solar system so we can see them?


 

Offline CliffordK

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Re: Where do comets come from?
« Reply #1 on: 14/03/2013 05:30:15 »
My thoughts is that at low pressures, vapor sublimes from ice near the sun.  But, around Pluto and the Kupier Belt, water vapor will condense onto anything, and thus the growth of comets.

According to this article, the material returned by Stardust included calcium-aluminum rich inclusions (CAIs), which were believed to have formed during the very young solar system.  Indicating that at least comet Wild 2 included material from the solar system formation.  The whole comet?  Or just part of it?

Hopefully someone else has an idea why they would have elliptical orbits rather than round orbits.  Perhaps the influence of Jupiter and the outer planets.  Perhaps some wander around the galaxy and may get occasionally captured by our sun and planets, but that would indicate that at least some of them would have been formed elsewhere.  Could that be identified?
 

Offline Don_1

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Re: Where do comets come from?
« Reply #2 on: 14/03/2013 11:04:58 »
As an expert in matters astronomical.... ....... Alright, alright! As a complete and utter idiot, here's my take on this fenom..... phenoon...... phonem... happening.

If it can be shown that Mars was once covered in oceans, then it must suggest that other planets, not just in our solar system, were also blessed with water. Solar heat and/or asteroid impact generated heat vapourised the surface water. Earth has a gravity field and atmosphere strong enough to bring that vapour back to the surface, but smaller planets without these benefits may have lost the vapour as it rose in the thin atmosphere where it was susceptible to solar winds.

Blown into spaace, the vapour begins to condense as the sun's rays become too weak to maintain its vapourous state. In the absence of any gravitational field strong enough to have an effect on the vapour, the natural attraction of water molecule to water molecule may be effective over very considerable distances, thus the condensing vapour 'clouds' and as it cools forms blocks of ice. If the ice block remains fairly small, it may wander off into deep space, until it comes close to some gravity field which ensnares it. But a large block may be just big enough for the Sun (or some other source of gravity) to counter the effect of the solar wind which sent it off in the first place, drawning it back into an eccentric orbit. Perhaps on their travels, comets may gain more water as they pass through vapour clouds which have not yet had the chance to form into a new individual comet. They would certainly sweep up dust and rock as they wander through space.
 

Offline David Cooper

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Re: Where do comets come from?
« Reply #3 on: 14/03/2013 20:28:38 »
Comets have a different composition to other inner solar system bodies, and asteroids, and also wildly eccentric deep-space orbits. So they must have a different origin to the rest of the inner solar system. But what is that origin, and what brings comets in towards the inner solar system so we can see them?

I think the answer is that they aren't really part of the solar system, although many may have been built with water thrown off into space when the sun and other stars formed (there was a story recently about a star forming and lots of water being jetted off into space in the process, though I forget the details).

The most important word is "cloud" - we have an asteroid belt, then the Kuiper belt (of comet-like objects), and then the Oort cloud (more comets), and the cloud part of that is a crucial distinction. A belt is formed out of an acretion disc, but a cloud was never part of that disc. That means that there's a spherical cloud of these icy objects way out beyond the solar system. This cloud spreads all the way between the stars, so in reality it's more a case of there being spherical holes in this cloud centered on stars which have cleared out most of the icy objects from the cloud around them.

So, we have lots of icy objects way out in the interstellar spaces, falling through space in random directions, and some of them inevitably head towards a star from time to time. If they're close enough and slow enough moving, they naturally fall into eliptical orbits, but others are moving too fast and will simply make one pass of a star before vanishing off back into deep space forever (or until their next brush with another star).

There will probably be quite a few of these objects in fairly circular orbits around the sun which are too far away for us to detect, but few of them will be moving in the same plane as the planets, and half of the ones which are in the same plane will be going the wrong way. I'm guessing that some of these could have their orbits affected over time through resonance with the planets, so their orbits may become more eliptical and cause them to start to come closer in - this certainly can happen with asteroids in the asteroid belt, but I don't know if it works the same way with distant comets.
 

Offline Ophiolite

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Re: Where do comets come from?
« Reply #4 on: 16/03/2013 17:00:00 »
David, your inventive explanation does not match current thinking. While there is still a great deal to be learned about comets the view is that they are a product of the same processes which produced the rest of the solar system. They condensed beyond the ice line and were then thrown into more distant orbits by planetary perturbation. Further disruption was a consequence of close stellar approahces and the Galactic tide.

This is a simplistic model and researchers are uncovering evidence that the history of at least some comets is much more complex than this. CliffordK noted the important fact that they contain CAIs.
 

Offline David Cooper

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Re: Where do comets come from?
« Reply #5 on: 16/03/2013 19:36:45 »
David, your inventive explanation does not match current thinking.

What makes you think that? I can't link to my sources, but they are all professional astronomers who have discussed this subject many times on the Sky at Night.

Quote
While there is still a great deal to be learned about comets the view is that they are a product of the same processes which produced the rest of the solar system.

But don't misunderstand that as meaning they were created by the sun - they were created during the formation of all the stars in our vicinity. Many of the comets which dumped water on the Earth will have been bringing back water that was flung out from the sun when the sun formed, but others will have been formed from water flung out during the formation of other stars.

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... CliffordK noted the important fact that they contain CAIs.

Which in no way conflicts with anything I said. Is there any specific part of what I said which you take issue with and which is actually wrong?
 

Offline Ophiolite

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Re: Where do comets come from?
« Reply #6 on: 17/03/2013 05:02:28 »
Is there any specific part of what I said which you take issue with and which is actually wrong?
Yes. You say this:

1. I think the answer is that they aren't really part of the solar system,

Yes, they are1,2. As I noted, they condensed beyond the ice line from the same materials that formed the planets, in particular Neptune and Uranus. (Minority views have them condensing much further out, but still from that portion of the GMC directly associated with the formation of the rest of the system3.)

Might some of them have come from beyond the solar system? Almost certainly, but this would be a very tiny minority.

2. Many of the comets which dumped water on the Earth will have been bringing back water that was flung out from the sun when the sun formed, but others will have been formed from water flung out during the formation of other stars.

You may be correct, but I should like to see the research that asserts this. True, Levison4 proposed, on dynamical grounds, that up to 90% of Oort cloud comets have been captured from other stars in the sun's birth cluster. The view is contradicted by other work covering a wide time span5,6,7. Moreover, in a quick review of around 25% of the papers which cite Levison's work I find no instances where his hypothesis is supported.

In addition, Levison appears to view the formation of comets in other systems to be occuring beyond the ice line in the conventional way.

3. Which (the detection of CAIs in cometary ejecta) in no way conflicts with anything I said.

The CAIs were the first major solid materials to form in accretionary disc. If the CAIs from Wilde 2 (?) have been dated, then that dating would corroborate their direct association with the formation of the sun and the planetary system. If not, it would support your belief in Levison's hypothesis. I strongly suspect you will turn out to be mistaken on this point.

References
1. Oort, J. H. (1950). Bull. Astron. Inst. Neth. 11, 91.
2. Kuiper, G. P. (1951). In Astrophysics, edited by J. A. Hynek (McGraw-Hill, New York), p. 357.
3. Fernandez, J. A. (1985). In Dynamics of Comets: Their Origin and Evolution, edited by A. Carusi and G. B. Valsecchi (Reidel, Dordrecht), p. 45.
4. Levison, H.F. et al (2010) Science 329 (5988): 187-190.
5. Whipple, F. L. (1964) Proc. Natl Acad. Sci. USA 51, 711–715.
6. Stern, S.A. (2003) Nature 424, 639-642.
7. Morbidelli, A. et al  (2005) Nature 435:462.
 

Offline Ophiolite

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Re: Where do comets come from?
« Reply #7 on: 17/03/2013 19:45:18 »
In regard to point 3, I have found two further interesting items.

For example,
Brownlee, D.  et al (2012), Meteoritics & Planetary Science, 47: 453–470.

They say, "It appears that the majority of the analyzed Wild 2 solids were produced in high-temperature “rock forming” environments, and they were then transported past the orbit of Neptune, where they accreted along with ice and organic components to form comet Wild 2." They base these conclusions on isotopic, mineralogic and petrologic studies, including comparison with chondrite compositions.

Clearly, their view remains the consensus view I originally commented on, and which runs counter to the view you were aware of through the Sky at Night programme and that seems to be that proposed by Levison.

However, there is this, relating to Wild 2 CAIs and chondrules:
Ogliore, R.C.  et al The Astrophysical Journal Letters, 745:L19.

They comment that "We interpret the presence of this object in a Kuiper Belt body as evidence of late, large-scale transport of small objects between the inner and outer solar nebula."

There is a 3 million year, or greater gap between CAI formation and incorporation in Wild 2. You could argue that is consistent with formation in the ambience of another proto-star in the sun's birth group,  but I think the reasoning could get quite convoluted. What about you?

 

Offline David Cooper

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Re: Where do comets come from?
« Reply #8 on: 17/03/2013 20:33:33 »
While 100% of our samples of comet material back your argument, it is also the case that 100% of our samples come from Wild 2 - a periodic comet which clearly belongs to the solar system, its aphelion only being 5AU out from the sun (which is as far out as Jupiter). We have never sampled the kinds of comet that don't return, and it may be a long time before we can do so as they come with little warning, making it hard to organise a mission to colect material from any of them as a result. The CAI argument it based on a sample of one comet which is definitively a solar system object and which was easy to sample for that reason.

Quote
Might some of them have come from beyond the solar system? Almost certainly, but this would be a very tiny minority.

What proportion of comets are the kind that don't return? If it is only a tiny minority, then you are right. Most periodic comets have doubtless always belonged to the solar system, but the further out they go, the less certain that becomes - they could have been captured from other stars. Even so, the distances to other stars are so great that it may well be that hardly any comets that visit us have come from other stars, but if a comet comes in with such energy that it will never come back, I would stick my neck out and say that it almost certainly was not created from water and other materials ejected from the vicinity of the sun during the sun's formation.

Part of our disagreement may also hinge upon where the solar system ends. One of the Voyager spacecraft is said to be at the edge of the solar system now, but most of our comets come from far beyond there. This leads to possible communication difficulties where objects outside of the solar system by one definition are considered to be part of the solar system by another definition. Perhaps you can tell me where the solar system actually ends so that I don't spread misinformation about it based on what I hear in the media from experts - they often simplify the truth to get points across in a form that the public can understand, so it's often experts who unintentionally spread misinformation with their stamp of expertise on it: that commonly serves to create multitudes of misunderstandings.

Maybe their description of the Oort cloud as stretching between the stars is also misinformation? Or maybe it has a variable density which is highest nearer to the stars which formed the objects in their vicinity, with very low densities in deeper space where the only objects have become detatched from their original stars and now just do random fly-bys of any stars that are near their path. This is the thing: I've never seen statistics on the density of the cloud at different distances from the stars and proportions of comets that are moving too fast ever to return. I got the impression that a great many comets never return, but that may be wrong.
« Last Edit: 17/03/2013 20:36:11 by David Cooper »
 

Offline David Cooper

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Re: Where do comets come from?
« Reply #9 on: 17/03/2013 21:05:48 »
I see that Voyager 1 is now at 123AU away from the sun, and I also notice that Halley's Comet's aphelion is only 35AU out from the sun, though it is described as a short-period comet. Maybe most comets that visit the sun have their aphelion closer than the edge of the solar system (even when that edge is taken to be where Voyager 1 now is), in which case my picture of how far out they go has been distorted. Even so, the ones which are further out will visit massively less often and so the numbers of known ones will be a tiny fraction of the total.
 

Offline David Cooper

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Re: Where do comets come from?
« Reply #10 on: 17/03/2013 21:09:51 »
Okay: Hyakutake's aphelion is at 4310AU - that's well out of the solar system by most definitions, but it is periodic (apx. 70000 years) and is likely to have been formed out of materials ejected from the vicinity of the sun during the sun's formation, so that could still justify calling it a solar system object, even though it spends most of its time far outside the solar system.

Edit: Further thoughts. Wilde 2 probably hasn't been in its current orbit for very long or it would have lost all its water long ago, so I'm guessing it was captured by Jupiter in some way and lost a lot of its momentum on the way back out on that occasion. Even so, there must be a limit to how much momentum Jupiter can tap out of comets, so it was probably already a short-period comet with its origin still being fairly close in.

Edit2: On the issue of where the solar system ends, it's clearly not been sorted out yet: http://www.dailymail.co.uk/sciencetech/article-2296422/Nasas-Voyager-1-LEFT-solar-35-year-mission-explore-space-began.html
« Last Edit: 20/03/2013 17:48:06 by David Cooper »
 

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Re: Where do comets come from?
« Reply #10 on: 17/03/2013 21:09:51 »

 

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