3 ms·

Astronomers call this boundary the Kuiper cliff, because the density of space rocks drops off so steeply. What caused it? The only answer seems to be a 10th pla

Astronomers call this boundary the Kuiper cliff, because the density of space rocks drops off so steeply. What caused it? The only answer seems to be a 10th planet. We're not talking about Quaoar or Sedna: this is a massive object, as big as Earth or Mars, that has swept the area clean of debris. They've got this kind of backwards. They describe this like something basically pushed parts of the Kuiper belt inward to form the steep gradient. We're pretty sure Neptune and Pluto pushed things out to form the Kuiper belt. 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. Maybe we can't work out what dark matter is because it doesn't actually exist. That's certainly the way Rubin would like it to turn out. "If I could have my pick, I would like to learn that Newton's laws must be modified in order to correctly describe gravitational interactions at large distances," she says. "That's more appealing than a universe filled with a new kind of sub-nuclear particle." Actually modified Newtonian Dynamics or MOND equations are WAAAAY harder to swallow than dark matter. Dark Matter: The majority of particles don't interact significantly with photons MOND: Particles shooting through space either spontaneously replicate or behave non geometrically. Gravity falling off at 1/R^2 is a consequence of geometry, the surface area of a sphere. MOND proposes 1/R^2 + R/N (for some very large but arbitrary number N) which makes NO physical sense. Meanwhile gravitational lensing observations continue to provide evidence for "invisible" matter. If the observations are correct, the only vaguely reasonable explanation is that a constant of physics called the fine structure constant, or alpha, had a different value at the time the light passed through the clouds. But that's heresy. Alpha is an extremely important constant that determines how light interacts with matter - and it shouldn't be able to change. Its value depends on, among other things, the charge on the electron, the speed of light and Planck's constant. Could one of these really have changed? Actually any sophomore or junior in college is exposed to the concept of evolution in the fine structure constant, while it isn't taught as definitive, it's hardly heresy. AND it goes on, none of these (at least the physics based ones) really have much debate in the scientific world.

aristus 18y ago
Were they debatable 4 years ago, when this was written?
- Avshalom 18y ago
  Kuiper Belt stuff, yes...ish Dark Matter, eh we had a lot less direct evidence of it but MOND still made no sense. The rest? not particularly.
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
philwelch 18y ago
"Actually modified Newtonian Dynamics or MOND equations are WAAAAY harder to swallow than dark matter." Maybe, but "dark matter" smells so much like "magical substance we can't observe directly." It's like the controversy over Mercury's orbit—Mercury's orbit was "wrong", so astrophysicists posited another planet, Vulcan, between Mercury and the Sun, that affected Mercury's orbit but (due to the brightness of the sun) was not observable. Then, after Einstein made his revisions to Newtonian dynamics, they recalculated Mercury's orbit according to Einstein's formulas, and it was right. The thing is, Einstein's physical equations were, for all intents and purposes, the same as Newton's for pretty much everything we had a chance to measure, until we started measuring things like the orbit of Mercury. It wouldn't be that odd if even Einstein's physics are, on the galactic scale, only approximate in their own right--close enough for GPS to work but not close enough to explain galaxies. Or maybe there is dark matter. I'm not qualified to give a good judgment.

aristus 18y ago

Were they debatable 4 years ago, when this was written?

Avshalom 18y ago
Kuiper Belt stuff, yes...ish Dark Matter, eh we had a lot less direct evidence of it but MOND still made no sense. The rest? not particularly.

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

philwelch 18y ago

"Actually modified Newtonian Dynamics or MOND equations are WAAAAY harder to swallow than dark matter." Maybe, but "dark matter" smells so much like "magical substance we can't observe directly." It's like the controversy over Mercury's orbit—Mercury's orbit was "wrong", so astrophysicists posited another planet, Vulcan, between Mercury and the Sun, that affected Mercury's orbit but (due to the brightness of the sun) was not observable. Then, after Einstein made his revisions to Newtonian dynamics, they recalculated Mercury's orbit according to Einstein's formulas, and it was right. The thing is, Einstein's physical equations were, for all intents and purposes, the same as Newton's for pretty much everything we had a chance to measure, until we started measuring things like the orbit of Mercury. It wouldn't be that odd if even Einstein's physics are, on the galactic scale, only approximate in their own right--close enough for GPS to work but not close enough to explain galaxies. Or maybe there is dark matter. I'm not qualified to give a good judgment.