My question is this : could the Charon-Pluto system becomes in the coming few millions years a full fledged planet, as in the IAU classification ?

I ask this because there are two facts that bother me :

  • The system orbit isn't on the ecliptic
  • The system hasn't cleared its neighborhood

So, is it possible that that system isn't stabilized yet, and that 1) its orbit is very slowly changing (for lack of a better word) to the ecliptic, and 2) that the system will slowly collapse, and bring all its moons with it, to clear its neighborhood ?

In other words, is Pluto a "proto-planet" (as I would personally name it), or definitely (and for the next millions years) a dwarf planet ?

Edit : I disagree on the fact that this question is a duplicate of How long will it take Pluto to grow to planet size? as this one ask not for the size of Pluto, but rather the other characteristics, the orbit and the clearing the neighborhood.
Moreover, the question was asked in a broader canvas, to know if it was possible that some elements of the solar system (Pluto, but not only) could be instable enough to reach the level of planet sometime in the futur.

  • $\begingroup$ This question comes from a series of comments on space.SE What is the relationship between New Horizons launch and Pluto's loss of Planet status? $\endgroup$ Commented Jun 24, 2015 at 14:34
  • $\begingroup$ Generalizing the question a bit, one could ask if any of the known trans-Neptunian objects might ever become a "planet". That gets around the "what would we call the result of an Eris-Pluto merger" problem. Generalizing even further, it might be worthwhile to ask whether there is some yet undiscovered body in the outer solar system that does meet the definition of a planet. That's not a good question for this site. The only possible answer is "We don't know (but we've ruled out a number of possibilities)." $\endgroup$ Commented Jun 24, 2015 at 14:58
  • $\begingroup$ @called2voyage are you saying it is not possible to define what amount of mass in the Kuiper belt would be required to form a single body that meets the re requirements of the IAU definition of planet? $\endgroup$ Commented Jun 24, 2015 at 15:42
  • $\begingroup$ @JamesJenkins I'm saying that it is not the mass that is important but whether or not the orbit is clear. $\endgroup$
    – called2voyage
    Commented Jun 24, 2015 at 15:43
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    $\begingroup$ @JamesJenkins Neptune is gravitationally dominant, see this Wiki article. Yes, mass plays a role, but there is no absolute mass limit for a planet; that's what I'm trying to say. $\endgroup$
    – called2voyage
    Commented Jun 24, 2015 at 16:21

2 Answers 2


Pluto will never be a planet.

There are a number of technical papers that give more precise meaning to the concept of "clearing the neighborhood". It's not just now, it's can the object in question clear the neighborhood of its path while the Sun is still a star.

In the case of Pluto, Ceres, Eris, and a host of other not-quite-planet objects, that will not happen. Ever. Or at least so long as the Sun shines. After that, does it matter?

  • $\begingroup$ This is the most reasonable answer. Of course, there is the possibility of extremely unlikely events like a large stream of matter from beyond the Kuiper Belt colliding with Pluto yet it maintaining the same orbit, but including such possibilities in an answer would be like including the possibility of pigs with prosthetic wings in an answer to "Can pigs fly?" $\endgroup$
    – called2voyage
    Commented Jun 24, 2015 at 14:46
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    $\begingroup$ So the rate of accretion is lower than the rate of proton/neutron decay? $\endgroup$ Commented Jun 24, 2015 at 15:14
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    $\begingroup$ @DavidHammen I avoided using the word "free" for that very reason. I just doubt we can say with much certainty what the mass of Pluto will be 10^60 years from now. $\endgroup$ Commented Jun 24, 2015 at 15:23
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    $\begingroup$ @WayfaringStranger - Your 10^60 years figure is irrelevant. What is relevant is on the order of 10^10 years. After that, the Sun will no longer be a star, so who cares what happens after that? (Besides, what happens to the planets after the Sun has spewed 50% or more of its mass during its red giant phase is more than a bit problematic.) $\endgroup$ Commented Jun 24, 2015 at 15:32
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    $\begingroup$ "There are a number of technical papers" Feel free to reference some to give this answer a bit more body and authority. $\endgroup$
    – Mast
    Commented Nov 16, 2016 at 12:12

The question says a few interesting things:

  • The system orbit isn't on the ecliptic
  • The system hasn't cleared its neighbourhood

These are not going to change in the next few million years - or ever.

Orcus is an interesting counter-example. It is in a similar orbit to Pluto - similar aphelion, perihelion and eccentricity, similar orbital period (to within a year or so) and an inclination of around 20 degrees, a little more than Pluto.

In short, in orbital terms Orcus is as much a planet as Pluto is - except of course it's only around half the diameter and has far less mass.

Pluto is never going to displace Orcus from its orbit. In fact they were both pushed into their orbits by Neptune and the Kozai mechanism, which causes its inclination. They will never be able to clear their orbits because Neptune dominates the area.

To answer the final part of the question - Pluto is officially regarded a dwarf planet by the current definition. Partly because of its orbit as your question states. Whether it's also a protoplanet or not may be debatable. I'd say it's too big to be considered a protoplanet because it's too big. (The larger asteroids like Ceres, Vesta etc. are probably closer to that size range.)

  • $\begingroup$ Thanks for your answer, which is very clear :) Except for the last sentence : Pluto won't be a protoplanet because it's bigger than Ceres and Vesta ? $\endgroup$ Commented Nov 15, 2016 at 21:46
  • $\begingroup$ @Cqoicebordel thanks - I've edited a bit as I wasn't clear about the size considerations. (Pluto is probably too big to be considered a protoplanet, though I'm not aware of any "rules" on what size range a protoplanet would be.) $\endgroup$
    – Andy
    Commented Nov 16, 2016 at 8:52
  • $\begingroup$ I'm sorry, but your clarification isn't helping me a lot : intuitively, I would say bigger means more mass. More mass means more chances of clearing its neighborhood. So, following that logic, Pluto has more chance to become a planet than Ceres, Vesta etc. which would make it a protoplanet. So, what am I missing here ? Is there an "official" protoplanet classification I didn't know that you are talking about ? Or is there another celestial mechanism (you didn't mention above) that would prevent Pluto gaining mass because it's too big already ? In any cases, thanks for your answers :) $\endgroup$ Commented Nov 16, 2016 at 15:33
  • $\begingroup$ @Cqoicebordel that's OK. In the first part, I'm saying Pluto's orbit is unlikely to change much, as Neptune is effectively controlling its orbit. In the second half, I understand "protoplanets" to be the small building blocks of a few hundred kilometres across, that later formed planets. Some of the asteroids are said to be protoplanets, but Pluto is much larger than them and should belong to a different class; though I don't believe there is a formal definition of a protoplanet size. $\endgroup$
    – Andy
    Commented Nov 16, 2016 at 15:53
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    $\begingroup$ Ok, now I understand. I was using the term proto planet as "could evolve to become a planet". That's why I didn't understand you. Thanks for the clarification ! :) $\endgroup$ Commented Nov 16, 2016 at 16:02

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