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(disclaimer: layman speaking) "We're pretty sure Neptune and Pluto pushed things out to form the Kuiper belt." Perhaps, but that's not the question - the ques

by sho 18y ago

(disclaimer: layman speaking) "We're pretty sure Neptune and Pluto pushed things out to form the Kuiper belt." Perhaps, but that's not the question - the question is why the sudden drop-off beyond a certain point? It's not about what is pushing objects out, it's about what is (apparently) pushing them back in. There is a large spike in density centred around 44 - 46 AU which abruptly trails off by 50, the sharpness of the edge implying something holding them in/clearing the zone/having done that in the past. An undiscovered massive object is as reasonable an explanation as any other at this point. Works for Saturn's rings, after all. And I don't think Pluto is large enough to do much "pushing" - the whole reason it was downgraded to sub-planet status was because it had failed to clear its orbit. Neptune, sure, but even that doesn't have much mojo beyond about 48AU. As well Mike Brown's work that found Xena, Santa, Easter Bunny etc. would have picked up on a Mars sized object past the Kuiper Belt but close enough to shepard things." Let's remember that these are all very recent discoveries; the ones you mentioned are from 2005, and they're relatively close compared to the "cliff". If we're still finding completely new objects that huge and that nearby just in the last few years, I wouldn't be so sure to rule out something pretty big hiding outside the Belt. As Brown himself said, "Could Lykawka's planet be out there and have been missed? Easily." Oh well, we'll know for sure soon enough. Pan-STARRS et al should definitively answer this and many other questions.

Avshalom 18y ago
Perhaps, but that's not the question - the question is why the sudden drop-off beyond a certain point? It's not about what is pushing objects out, it's about what is (apparently) pushing them back in. There is a large spike in density centred around 44 - 46 AU which abruptly trails off by 50, the sharpness of the edge implying something holding them in/clearing the zone/having done that in the past. An undiscovered massive object is as reasonable an explanation as any other at this point. Works for Saturn's rings, after all. This is what I mean by thinking about it backwards. take a bunch of M&M's scatter them on you're desk then push all the ones in the center out, you get a big high density belt and a steep drop off due to that high density. And I don't think Pluto is large enough to do much "pushing" - the whole reason it was downgraded to sub-planet status was because it had failed to clear its orbit. Neptune, sure, but even that doesn't have much mojo beyond about 48AU. Pluto got de-ranked for a whole suite of things beyond it's local orbit, the largest reason being nobody wanted to admit that if Pluto counts so does about 100 other things and thats inelegant. personal bitterness aside it doesn't need to be very big for a resonant orbit to push things around, which Neptune helps set up (disclaimer this is largely a recollection of Mike's work on the Belt from last time I heard him lecture about it) As Brown himself said, "Could Lykawka's planet be out there and have been missed? Easily." Okay fair enough, but possibly there and missed isn't anywhere close to necessary to explain physical phenomena
- sho 18y ago
  "take a bunch of M&M's scatter them on you're desk then push all the ones in the center out, you get a big high density belt and a steep drop off due to that high density." Huh? The whole issue is that it looks like it's been pushed from both sides. We know what pushed it from the inner, what pushed it/limited it from the outer? Here's a good visualisation of the edge: http://www.cfa.harvard.edu/iau/lists/OuterPlot.html http://www.cfa.harvard.edu/iau/lists/OuterPlot.html Density + mass v. orbital AU / period: http://en.wikipedia.org/wiki/File:TheTransneptunians_73AU.svg http://en.wikipedia.org/wiki/File:TheTransneptunians_73AU.sv... "Pluto got de-ranked for a whole suite of things beyond it's local orbit" Well, I'm just going by what the IAU says, I can't speculate as to the rest of it. I don't have any particular emotional attachment to the notion of Pluto being classed as a planet or not, or whether we have hundreds of the things. I think Pluto is still regarded as a kind of "honourary planet" in practise. Anyway there's plenty of other stuff in resonant orbits, especially at 2:3 and around 4:7, so again Pluto's influence seems likely to be minor. Pluto might be the local actor but it's Neptune providing the periodic "kick". "possibly there and missed isn't anywhere close to necessary to explain physical phenomena" You're the first person I've heard claim there isn't anything to explain. This is a recognised phenomenon and the consensus is we don't know the answer. Hence the need for these theories to explain it.

by sho 18y ago

Avshalom 18y ago

Perhaps, but that's not the question - the question is why the sudden drop-off beyond a certain point? It's not about what is pushing objects out, it's about what is (apparently) pushing them back in. There is a large spike in density centred around 44 - 46 AU which abruptly trails off by 50, the sharpness of the edge implying something holding them in/clearing the zone/having done that in the past. An undiscovered massive object is as reasonable an explanation as any other at this point. Works for Saturn's rings, after all. This is what I mean by thinking about it backwards. take a bunch of M&M's scatter them on you're desk then push all the ones in the center out, you get a big high density belt and a steep drop off due to that high density. And I don't think Pluto is large enough to do much "pushing" - the whole reason it was downgraded to sub-planet status was because it had failed to clear its orbit. Neptune, sure, but even that doesn't have much mojo beyond about 48AU. Pluto got de-ranked for a whole suite of things beyond it's local orbit, the largest reason being nobody wanted to admit that if Pluto counts so does about 100 other things and thats inelegant. personal bitterness aside it doesn't need to be very big for a resonant orbit to push things around, which Neptune helps set up (disclaimer this is largely a recollection of Mike's work on the Belt from last time I heard him lecture about it) As Brown himself said, "Could Lykawka's planet be out there and have been missed? Easily." Okay fair enough, but possibly there and missed isn't anywhere close to necessary to explain physical phenomena

sho 18y ago
"take a bunch of M&M's scatter them on you're desk then push all the ones in the center out, you get a big high density belt and a steep drop off due to that high density." Huh? The whole issue is that it looks like it's been pushed from both sides. We know what pushed it from the inner, what pushed it/limited it from the outer? Here's a good visualisation of the edge: http://www.cfa.harvard.edu/iau/lists/OuterPlot.html http://www.cfa.harvard.edu/iau/lists/OuterPlot.html Density + mass v. orbital AU / period: http://en.wikipedia.org/wiki/File:TheTransneptunians_73AU.svg http://en.wikipedia.org/wiki/File:TheTransneptunians_73AU.sv... "Pluto got de-ranked for a whole suite of things beyond it's local orbit" Well, I'm just going by what the IAU says, I can't speculate as to the rest of it. I don't have any particular emotional attachment to the notion of Pluto being classed as a planet or not, or whether we have hundreds of the things. I think Pluto is still regarded as a kind of "honourary planet" in practise. Anyway there's plenty of other stuff in resonant orbits, especially at 2:3 and around 4:7, so again Pluto's influence seems likely to be minor. Pluto might be the local actor but it's Neptune providing the periodic "kick". "possibly there and missed isn't anywhere close to necessary to explain physical phenomena" You're the first person I've heard claim there isn't anything to explain. This is a recognised phenomenon and the consensus is we don't know the answer. Hence the need for these theories to explain it.