There are asteroid belts, and there are collections of bodies in resonant orbits, such as Trojans. Aren't these separate things?

The recent evidence for the 9th planet is an alignment in various parameters of known Kuiper belt objects. Conversely, few or no objects are known in the parts of the belt that would be swept away by the hypothetical planet.

If the Kuiper belt straddles the gravitational neighborhoods of Neptune and this other planet, and its populations reflect their influence, is it really a belt at all? A lopsided belt? Or just a region with a collection of distinct populations, each associated with a dominant body?

  • $\begingroup$ You might be playing up the importance of Neptune a bit, as well as the similarities between the Kuiper belt and other collections of objects. It's 20 AU from its inner radius to its outer radius, and many times more massive than the asteroid belt. $\endgroup$
    – HDE 226868
    Jan 23, 2016 at 17:00
  • $\begingroup$ @HDE226868 I'm not an expert, but Wikipedia presents the "Kuiper Cliff" at 50 AU as the outer edge of Neptune's 2:1 resonance. 30 AU (1:1) to 48 AU (2:1) plus some margin makes 20 AU; is that where you get that figure? $\endgroup$ Jan 24, 2016 at 9:51
  • $\begingroup$ Hmm, it seems I wrongly lumped more distant objects into the Kuiper belt. The objects relating to Planet 9 are beyond 50 AU and not considered to be KBOs. $\endgroup$ Jan 24, 2016 at 9:58
  • $\begingroup$ similar question here astronomy.stackexchange.com/questions/13562/… $\endgroup$ May 7, 2017 at 12:22

1 Answer 1


The recent evidence for the 9th planet is an alignment in various parameters of known Kuiper belt objects.

These are not Kuiper belt objects. The Kuiper belt is taken to refer to the neighborhood of Neptune resonances. The scattered disc is another population, further out, and the objects relating to Planet 9 are further still.

A scatter plot showing orbital parameters of many bodies.

Legend: grey = Kuiper belt, green = integer Neptune resonances, blue = scattered disc. The objects in the recent announcement are all off the chart, although their perihelions are <100 AU.

Source: Wikipedia.

The shape of the grey splotch at the left of this chart shows that the Kuiper belt is indeed a collection of populations dominated by Neptune. This isn't news, and it's still taken to be a belt. It goes all the way around the solar system and the orbital eccentricities are low.

This postulated planet might help explain the "Kuiper cliff", the otherwise unexpected cutoff before the 2:1 resonance. But, that's another question. Failing that, there's simply no relation between the Kuiper belt and Planet 9 except that they're both very far away.


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