TNOs: could some have formed elsewhere? Options

[ngunn]

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.

[Guest_Richard Trigaux_*]

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.

[ngunn]

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!

[Bob Shaw]

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

[Guest_Richard Trigaux_*]

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!

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.

[The Messenger]

It's all too easy to picture the sun's stellar environment as a stable...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.

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

"The major things that are still unknown: inside the classical Kuiper belt, there are two entirely distinct populations. One is a low-inclination population, and one is a more extended-inclination population. These are in exactly the same place in space. It's very difficult to do that mechanically; it's like trying to heat up only half of a cup of coffee. The other big one is the radial distribution in the Kuiper belt. You get a big peak right around 43 AU, then there is a big dropoff right around 50 AU...
We really know that the Kuiper belt doesn't extend out from 50 to 80 AU. But you can't see anything out beyond 100, because most surveys don't look for anything moving that slowly.

"One other population that I like to talk about, are objects that are really out beyond the Kuiper belt. Sedna -- when we first found it we thought it would be circular or scattered. We were really hoping it would be circular. We were shocked when we figured out what it was, and it's THAT." His initial graph was a plot of the current locations of objects in the Kuiper belt, projected on the plane of the solar system; Sedna sat just beyond the belt, but not far from it. Then he dropped in Sedna's orbit. The Kuiper belt shrank to cover less than a tenth of the slide, and the orbit of Sedna swept way out in a gigantic ellipse that always remained far outside the Kuiper belt, as far or farther than its current position beyond the Kuiper belt. "It never comes close to an outer planet; there has to be something out there beyond the Kuiper belt. This thing is probably ½ or ¾ the size of Pluto. If you have the same size distribution of objects out there as you have in the Kuiper belt, then you have a substantially larger population." There was some quibbling about this; it's kind of poor practice to create statitstical arguments from very small numbers.

[Guest_Richard Trigaux_*]

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.

[Guest_BruceMoomaw_*]

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."

[ngunn]

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.

[Guest_Richard Trigaux_*]

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.

[Bob Shaw]

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

Richard:

Absence of evidence is not evidence of absence!

Bob Shaw

[dvandorn]

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

-the other Doug

[Rob Pinnegar]

Not looking for things that you don't expect may make sense in oil exploration but it's a cardinal sin in astronomy.

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

[dvandorn]

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

[ngunn]

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!