I got a question today for Planetary Radio Q&A that I couldn't answer, being pretty ignorant about solar system formation research. Can anybody help?



--Emily

That's not your run-of-the-mill Q&A inquiry, Emily. Is this, by chance, a grad student asking?

I'm working off dim memory but I believe Jane Luu and David Jewitt did some work a few years ago on the survival of Kuiper belts over the main sequence lifetime of central stars. I believe they (or someone else) came up with estimates on the sizes of given belts and the amount of material that would survive the clearing phases of stellar evolution.

I'll try and track down a reference.

Hi Emily,

I personally have worked on this problem and can help you out. First, comets in the KB cannot survive the red giant phase. This work was published in Nature (1990, by Stern, Shull, and Brandt). OC comets can survive this phase of stellar evolution because they are so far out.

As to the capture of interstellar comets, various dynamycists have looked at this and the capture efficiency is dismal-- <10^-12 per interloper comet. This indicates that no KB comets and only a handful of the OC comets could be captured. As to larger (KBO-sized) bodies-- forget it, the odds ar there isn't even one object that is from a passing star UNLESS the Sun was born as a double or in a cluster-- which is possible but controversial.

-Alan

For the cognoscenti:

Evolution and detectability of comet clouds during post-main-sequence stellar evolution
S. Alan Stern, J. Michael Shull and John C. Brandt
Nature 345, 305-308 (1990).
doi:10.1038/345305a0
Abstract & References

I would also recommend:

Delayed Gratification Habitable Zones: When Deep Outer Solar System Regions Become Balmy During Post-Main Sequence Stellar Evolution
S. Alan Stern
Astrobiology 3, 317-321 (2003).
Abstract

Thanks, Alan, Alex!

Emily

Could cometary interlopers 'aerobrake' into orbit around a star with a proto-planetary disk and in doing so kick start planet formation or is the gas/dust density of such a disk too low?

Perhaps, but I see three difficulties: First, the typical speed of interlopers is near 30 km/sec, so that's one heck of a lot of
braking. It's more like aerocapture. Second, comets are fragile, so you'd have to do it very slowly, over a great path lentgth
with only a small drag. There is also the problem that the disk is transient-- it's only in place for <1% the age of the solar
system.

My guess is that this won't work out.

-Alan

A very different (and more technical) abstract of this paper than the one in Nature is available via the "Bulletin of the American Astronomical Society" here.

======
Stephen

Another angle on Emily's query. The recent paper on the capture of Triton begs the question: could the sun have captured objects in a similar way? If there is an interstellar population of planetary sized bodies then perhaps some of them are double or multiple systems. This would greatly enhance the probability of solar capture. The orbits of any such captured objects would presumably be arbitrarily distributed in size, inclination and eccentricity - in fact rather Sedna-like.

Alan already dismissed this as highly improbable, see previous posts:

I saw that, and I can see why the probability of a solitary object getting captured by the sun is virtually nil, but I'm not convinced that it's true if there is a significant population of dark, loosely-bound multiple systems floating about. Nobody knows whether or not there are, and if so how common, so I don't think we're in a position to calculate odds.

Somehow, I get the feeling that this binary capture scenario only works well when there's an appreciable gravity gradient across the binary object separation distance. Alternatively, you could state this as the two-body distance being not too small compared to the large 3rd body distance. How does this apply to the KBOs and OC? These are so far away from sun, that two components in a binary system would need to be very, very separated for this capture scenario to be feasible. At tens and hundreds of AU from the sun, both objects are likely to feel effectively the same amount of gravitational pull from the sun, with the net effect being virtually no disrupting force upon the binary pair.

I agree we can't know how many binary objects there are that wander around the interstellar space, but even if there are many (not likely), furthermore ones that are very loosely bound (even less likely), IMHO the Triton-like capture mechanism would be highly improbable. I'd be interested to hear if more knowledgeable people know more about this -- Alan et. al. ?

Me too! I was only speculating, not calculating. Relative velocity's important too. I'm guessing that for maximum disruption you would want the dark binary to be in the vicinity of the sun for about half of it's own mutual revolution period. Experts please come in! Computer simulations??

What about a close (in interstellar terms) encounter with another star-mass (or large Jupiter) object? You'd only need a few such spread over billions of years to have strange Oort Cloud effects up to the present day. If the intruder isn't gravitationally bound to our Sun then it could be halfway across the galaxy by now. I suppose it's a statistical thing - and the sort of area where Hipparcos and similar astrometry craft might be able to give pretty good answers by identifying anomalous proper motions of stars.

Bob Shaw

Well, such interstellar interactions and captures seem rare, if not impossible, even in assuming that there is a large interstellar population of small objects (Jupiter-size or smaller).


But there is a moment where it is much more possible: when the forming star already has its well individualized bok globule or accretion disk. At that stage, objects are not yet condensed (although they already exist individualy as gravitationally bound cloud structures) so they are much larger. In more, into star formating nebulae, the distances are much smaller, and relative velocities too.

The result of this is that interactions would mostly happen at this time, and be much easier at this time, to the point of having common and drastic effects.

For instance in the thread about the hypothetical Triton capture by Neptune, I imagined that it could result, not from an interaction between already formed moons, but between protoplanetary clouds. For instance the Pluto cloud would have passed through Neptune's disk. In doing so, it was captured by the Sun, but also it forbad any large satellite formation around Neptune, lefting only the two close satellites or the too far Nereïde. Of course in this hypothesis Triton would be a part of the Pluto cloud left around Neptune, but in a reverse orbit.

Recent thinkings came into a similar way, wondering if the chaotic outer KBO belt and its abrupt cutt-off would result simply from collisions between accretions disks, which would be relatively common in star formation zones. Many puzzling features could also result from a deeper interaction, for instance large planets into very excentric orbits, of very close from their star.

I think we should think of an accretion disk as something complex, formed of lumps of various materials and various speeds, with an history of interacting each other and with other disks, before being constrained to take the relatively regular position we see today.

Eventually it would not be astonishing if other planets of our sytem, or some KBO, would appear to come from different nebulae, with different composition. Pluto and KBOs are good candidates for this, as they may result from matter swaps between two colliding disks. For instance radioisotopic analysis show that Earth and common meteorites all formed from materials of a supernova some 20 millions years before accretion (I have not the exact figures in head). But similar radioisotopic analysis with comets or other far objetcs may show some with a different origin, perhaps another supernova, of another older source of matter.

It's all too easy to picture the sun's stellar environment as a stable, uneventful sort of place with the nearest neighbours that are big enough to be visible at a comfortable few light years' distance. But in fact it changes very fast on a geological timescale. The constellations are by no means as old as the hills! Almost certainly in its 20 or so circuits of the galaxy the solar system has passed through some very different environments, some more lonely, some more crowded than today. Furthermore we have simply no idea how many bodies of substellar mass our galaxy contains or how they are distributed spatially, dynamically or in terms of size distribution. I am not arguing for the existence of any particular class of object or any particular scenario for what has shaped the outer solar system, just for keeping an open mind. I think there is plenty of room out there for quite a lot more interesting discoveries, and plenty of time since the formation of the solar system for subsequent events to modify it in complex ways. I doubt if that's ended for good, even now.

Eventually if the solar system had crossed a zone where stars were forming, the probability of an interaction at this time was much larger at this time.

Indeed, and I like your multiple disc idea too. Would the multiple discs have to be contemporaneous or could new ones be acquired some time later than the one from which the major planets formed, say by the system passing slowly through a dense molecular cloud? These may sound like unlikely freak events, but we know that some galaxies go through fits of star formation, possibly triggered by galactic collisions. We don't know much about the history of our galaxy or how many others it has swallowed. There could have been times when the solar system had nowhere safe to hide. Just look at M82!

Hopefully, future astrometry missions will provide a sound statistical basis for this sort of speculation - if we see that some scenarios are happening at one instant in the history of the galaxy, then it surely implies that instant isn't special and that such incidents occur all the time. Do the arithmetic, and out pops (at least) ball-park figures! We need to look at large populations of stars similar to our own, in similar areas within the galaxy, and to see how many are moving in odd directions; that'll begin to put a set of limits on encounters. You may need to look for several decades, however...

Bob Shaw

Why not, a star already having a disk can get another load of matter, eventually not in the same plane. But mandatorily there will be interactions between the two disks, which will more or less collapse into one.

But if there are already planets, it is very likely that they will direct further flows of matter, and will forbid the formation of new planets where there is already a Titus-Bode series.

http://planetary.org/blog/article/00000574/

Two populations of objects sharing the same space? A configuration mechanically difficult to obtain, as with heating half of a cup of tea?

This could result from the mechanisms discussed above: interactino with the disk of a neighbouring star, of a more excentred flow of matter over our regular disk.

The most interesting part of Michael Brown's interview in the May 2006 "Discover" is his discussion of the scientific importance of Sedna:

"Quaoar is about half the size of Pluto. Everybody was really excited and wanted to hear about it. This was June 2002. Now when I look back, it's 'Hmmmm. Quaoar was big, but not that big compared to what came afterward.'

Sedna was completely unexpected. It's 8 billion miles [13.3 billion km] from the Sun -- Pluto is 3.6 billion miles [6 billion km] -- and in 2004 we had no idea that things in that very outer region of the Solar System existed. The fact that they do is going to tell us an incredible amount about the birth of the Sun and the earliest history of the Solar System.

"Sedna shouldn't be there. There's no way to put Sedna where it is. It never comes close enough to be affected by the Sun, but it never goes far enough away from the Sun to be affected by other stars, which is the case with comets that have been observed in the Kuiper Belt. [He may mean the Oort Cloud -- Moomaw.] Sedna is stuck, frozen in place; there's no way to move it. And if there's no way to move it, basically there's no way to put it there -- unless it formed there. But it's in a very elliptical orbit, and there's no way to form anything in an elliptical orbit like that. It simply can't be there. There's no possible way -- except it is. So how, then?

"I'm thinking it was placed there in the earliest history of the Solar System. I'm thinking it could have gotten there if there used to be stars a lot closer than they are now, and those stars affected Sedna on the outer part of its orbit, and later on they moved away. So I call Sedna a fossil record of the earliest Solar System. Eventually, when other fossil records are found, Sedna will help tell us how the Sun formed and the number of stars that were close to the Sun when it formed.

"Sedna is incredibly far away, and we never would have seen it if it weren't as close as it ever gets on its orbit. In fact, there's only about a 200-year period when we can see it, and it has a 12,000-year orbit. So what does that mean? If we see it for 200 years out of 12,000, that means there's only a 1 in 60 chance that we could have seen it, which means to me that there may be 60 of these things out there. And if there are 60 of these things, then there are probably 20 of these things just a little bigger, and maybe a couple the size of Mercury or Mars. We're trying very hard to find the whole population. Once it's done, we'll be able to read the entire fossil record and learn incredible things.

"Even though we went on to discover Xena, which is bigger than Pluto and could be called a planet, that is not particularly profound in and of itself. We've known all along that there was likely to be something bigger than Pluto out there, and we finally found it. Scientifically, without question, the most important object we've discovered is Sedna."

A marvellous quote, Bruce, music to my ears. It's good to know that someone is not making conservative assumptions. As I read the history of astronomy they have a pretty poor track record. It took the newer science of geology to give us the right timescale for a start, and the spatial extent of the visible universe was only properly recognised in the 20th century.

Even geology has had its blind spots however: continental drift comes to mind. The problem here was refusal to see something because there was no ready mechanism to explain it - exactly the reason why no one was looking for Sedna. Not looking for things that you don't expect may make sense in oil exploration but it's a cardinal sin in astronomy.

Eventually Sedna is the best candidate for a body formed from another accretion disk, having different isotopes ratio and even formed from another supernova.

It is also an evidence that our solar system never had any deep encounter with another one since.

The only alternative explanation would be that Sedna resulted from such an interaction, eventually more recently than the sun formation.

Richard:

Absence of evidence is not evidence of absence!

Bob Shaw

'Is not' is not 'not is'...

-the other Doug

Or in any other not-completely-applied science, for that matter.

Not looking for the unexpected is a slippery slope. If you have the time and luxury to develop and use a multi-approach instrument suite, designed to gather as much possible information about as many characteristics of a phenomenon as possible, then it is, indeed, a cardinal sin to ignore all results except the ones that you wanted to see.

But planetary probes, in specific, are so mass-limited that you have to design your instruments carefully. You inevitably design your instruments to constrain existing theories, or to look for a very small subset of the available information that directly relates to what's seen as a pivotal prediction of a given theory.

We have two fabulous little robotic geological explorers on Mars right now, and yet they are incapable of analyzing the oxidation properties of the soils. They couldn't find organics if they were strewn over the surface liberally. They are designed almost solely to identify hydration effects on the rocks and to identify a *limited* range of minerals in the rocks and soils. Because they were designed to constrain current theories on the effects of water on the Martian surface.

So, with the MERs, we're not ignoring unexpected information -- we designed them to return *only* information about expected conditions. At least on several levels. (I admit freely that the Pancam returns a wide variety of data, and we see in its images not only what we expected but much that we didn't. I'm really speaking only of the non-imaging experiments, here. But that really does show you the value of imagery...)

This is not a condemnation of the process. Planetary probes are so mass-limited that you *must* tailor their instrumentation suites to gather that subset of the available information you think is going to be the most valuable and worthwhile. You just can't afford to put every sensor you can think of on such probes.

The trap here is in the phrase "information you think is going to be the most valuable." The only path to that kind of judgment is illuminated by best theories. So, we get trapped into designing our probes to constrain, prove or disprove best current theory. Which works against looking for the unexpected.

-the other Doug

My comment was about astronomy, as in studying celestial phenomena from afar, not about planetary probes to places where there has already been some reconnaissance on which to base mission objectives. In that case the oil exploration analogy applies and I fully accept what you say. Even so, wherever this kind of selectivity is unavoidable we need to be continuously alert to the implications of the observational selection thus introduced. Good lateral thinking and an open mind are needed to avoid the risk of circularity and the missing of potentially fruitful lines of inquiry. Thank goodness we sometimes get surprises despite the selectivity of our search!

Astrophysics, abstract
astro-ph/0605745

From: David Rabinowitz [view email]

Date: Wed, 31 May 2006 16:33:15 GMT (809kb)

The Diverse Solar Phase Curves of Distant Icy Bodies. Part I: Photometric Observations of 18 Trans-Neptunian Objects, 7 Centaurs, and Nereid

Authors: David L. Rabinowitz1, Bradley E. Schaefer2, Suzanne W. Tourtellotte3

Comments: 5 tables, 5 figures

We have measured the solar phase curves in B, V, and I for 18 Trans-Neptunian Objects, 7 Centaurs, and Nereid and determined the rotation curves for 10 of these targets. For each body, we have made ~100 observations uniformly spread over the entire visible range. We find that all the targets except Nereid have linear phase curves at small phase angles (< 2 deg) with widely varying phase coefficients (0.0 to 0.4 mag/deg). At phase angles > 3 deg, the Centaurs (54598) Bienor and (32532) Thereus have phase curves that flatten. The recently discovered Pluto-scale bodies (2003 UB313, 2005 FY9, and 2003 EL61), like Pluto, have neutral colors compared to most TNOs and small phase coefficients (< 0.1 mag/deg). Together these two properties are a likely indication for large TNOs of high-albedo, freshly coated icy surfaces. We find several bodies with significantly wavelength-dependent phase curves. The TNOs (50000) Quaoar, (120348) 2004 TY364 (47932), and 2000 GN171 have unusually high I-band phase coefficients (0.290+/-0.038, 0.413+/-0.064, 0.281+/-0.033 mag/deg, respectively) and much lower coefficients in the B and V bands. Their phase coefficients increase in proportion to wavelength by 0.5 - 0.8 mag/deg/um. The phase curves for TNOs with small B-band phase coefficients (< 0.1 mag/deg) have a similar but weaker wavelength dependence. Coherent backscatter is the likely cause for the wavelength dependence for all these bodies. We see no such dependence for the Centaurs, which have visual albedos ~0.05.

http://arxiv.org/abs/astro-ph/0605745