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I am thinking of a scenario in which a dwarf planet, previously declared as a planet, is reclassified as a planet.

What could possibly cause this? Could any possible mineral findings cause this, given certain treaties, or not? Might the classification system itself change to allow this?

Thanks for any help provided

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    $\begingroup$ The reason why a certain former planet was re-classified as a dwarf planet is that astronomers have found that the solar system contains many celestial bodies like it. As a consequence, they had to choose between admitting that the solar system has many planets, or else divide the large-ish bodies in the solar system into planets and dwarf planets. It has nothing to do with composition etc. The criteria are: (1) is it big enough so that its own gravity makes it round-ish? If yes, it's a planet or dwarf planet. (2) It is big enough to clear its orbit of junk? Yes, planet, no, dwarf planet. $\endgroup$
    – AlexP
    Mar 7, 2019 at 3:48
  • $\begingroup$ @Alex p by that criterion, we are discovering that Earth's orbit has not been cleared of debris, which is an argument for reversing the reclassification of said celestial body $\endgroup$
    – pojo-guy
    Mar 7, 2019 at 4:10
  • $\begingroup$ @AlexP, I know about Pluto. Thank you for giving me some more information about why this happened. I am glad to have more information that I had before. I, however, am specifically asking about a situation, fictional or not, found in our solar system or not, that might cause a dwarf planet, like Pluto, to be reclassified as a planet. $\endgroup$
    – GaryS
    Mar 7, 2019 at 4:16
  • $\begingroup$ Welcome to worldbuilding, Gary. As it stands, I suspect that your question is in danger of being closed as primarily opinion based. Ultimately we demonstrated with Pluto that we an reclassify a celestial body simply by changing the rules to suit ourselves. As @AlexP states, we often change the rules based on further discoveries. So the broad answer is yes, because we discover something that makes it more convenient for us to reclassify a dwarf-planet back to full planet status. $\endgroup$
    – Tim B II
    Mar 7, 2019 at 4:48
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    $\begingroup$ @pojo-guy, the definition is "Clearing the neighbourhood around its orbit". What that means is that, in and near its orbit, it is the most gravitationally dominant object and thus there are no bodies of comparable size sharing a similar solar obit. So Earth would count unless there were, for instance, bodies in the Mercury to Venus range zipping around in similar orbits. $\endgroup$
    – Keith Morrison
    Mar 7, 2019 at 4:49

4 Answers 4

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Yes

The International Astronomical Union (IAU) defines planets by three criteria:

  1. "is in orbit around the sun"
  2. "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape"
  3. "has cleared the neighbourhood around its orbit"

The last criterion seems to cause some confusion, but it is mean to imply "[the body] has become gravitationally dominant, and there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence." This is where Pluto fails on multiple counts as it is "constrained in its orbit by the gravity of Neptune and shares its orbital neighbourhood with Kuiper belt objects."

If we assume that a solar system body being studied is easily determined to be "nearly round" then all contention will center around the third criterion. If a planet is declared, and then "demoted" to dwarf planet because it is found not to have cleared its neighborhood, I can think of two ways it would be deemed a planet again in the future:

  1. The neighborhood becomes cleared. This is incredibly unlikely, but consider the case where two massive bodies share an orbit - possibly in stable equilibrium. The loss of one body (say due to large gravitational influence of a passing object) would leave the remaining body in a "cleared" neighborhood and thus it would be reclassified to a planet. I cannot stress how improbable this is, but I can't prove it couldn't happen so I will assume it may be possible. For Pluto, however, this is essentially an impossibility. Any event that somehow cleared the Kuiper belt of objects would almost certainly affect Pluto as it is itself a Kuiper belt object. If some event were to destroy or move Neptune (somehow without affecting Pluto), Pluto would still remain in the uncleared Kuiper belt.

  2. The classification system changes. The system is essentially just semantics that are agreed upon by a single governing body. For instance, people in the field of planetary science tend to refer to moons, asteroids, dwarf planets, and Kuiper belt objects in the solar system without really invoking the planet/dwarf planet distinction. They all know the eight most massive bodies, and they know quite a few smaller ones as well, and their research isn't really based on how these bodies are classified by the IAU. In much the same way, the classification system could change and the solar system would keep on turning.

EDIT (to answer more precisely about when and how this could happen):

To clarify, the insistence that 1. is an unlikely scenario is based on the fact that a situation such as I describe above is unlikely to occur at present in the established solar system. The only scenario that comes to mind in the current solar system is the case where a planetary moon is ejected and finds its way into a stable orbit (again, the likelihood is low but it is possible under the right conditions). As our moon moves slowly away from the Earth, it is theorized that it may one day escape the Earth's gravitational influence entirely (of course, the opposite is theorized as well). If it were to escape and find its way to a stable orbit, it would become the largest dwarf planet in the solar system.

However, in the case of a newly forming solar system, an observer would most likely observe planets being demoted to dwarf planet and then later reestablished as a planet. In such a case, an existing planet could find a large body of comparable mass kicked into its orbit. For as long as the orbit was shared, both objects would be dwarf planets. The same planet could also find itself a new member of a co-orbiting system where another large body has become quite close (think of the situation if the moon was considerably larger). Then as before, it would be demoted to dwarf planet. These situations are inherently unstable: in the former case the new object would be kicked into another orbit or collide with the original planet, and in the latter case the two objects would almost certainly collide (maybe forming one big planet or a planet-moon sytem). The instability in these situations is what makes a planet->dwarf planet->planet change in status unlikely at present. All/most of the unstable conditions of the early solar system have since settled into equilibrium.

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    $\begingroup$ By definition, dwarf planets have to meet the first two criteria. If they don't, they aren't dwarf planets. For example, almost the entire asteroid belt is in orbit around the Sun, but only Ceres is large enough to be hydrostatically round and thus a dwarf planet. And there are any number of round objects that do not directly orbit the Sun. E.g. Earth's Moon. $\endgroup$
    – Brythan
    Mar 7, 2019 at 17:23
  • $\begingroup$ I think you're underestimating how unlikely the "neighbor becomes cleared" event could be. The question isnt specifically asking about Pluto. If a hypothetical dwarf planet had 3 or 4 asteroids in the neighborhood that prevented it from being called "cleared" that years later eventually collided with the body, then it could be reclassified. Maybe someone knocked the remaining objects on a collision course in order to help the process along; or maybe they were mined out. Doesnt really matter how, as long as it happens. $\endgroup$
    – Stephan
    Mar 7, 2019 at 19:27
  • $\begingroup$ @Stephan right it's not specifically about Pluto, which is why I said it is highly unlikely in general and then addressed Pluto as it was my running example. My insistence that this chain of events is unlikely was based on the fact that a system such as you describe is unlikely to form to begin with. Also remember that the IAU doesn't require NO other objects in the orbit (Jupiter is definitely a planet despite the Trojans), just that the object is gravitationally dominant. $\endgroup$
    – ben
    Mar 7, 2019 at 20:59
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    $\begingroup$ @ben maybe it would help to elaborate on why it would be improbable in the answer. It could help the asker to come up with scenarios that negate those reasons and make the askers world more believable. $\endgroup$
    – Stephan
    Mar 8, 2019 at 14:48
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    $\begingroup$ @Stephan fair enough, added an edit to that end. $\endgroup$
    – ben
    Mar 8, 2019 at 17:44
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Yes, because this is not just scientific, it is a popular issue

Some politician may want to have a campaign promise to Make Pluto a Planet Again (MPPA or MPaPA), and subsequently win American presidency on this platform. Scientists who think Pluto is not a planet would find themselves in a hot seat, with President calling them names on Twitter daily. If they persist in their foolish behavior, federal government will pull their grants and award them to Pluto supporters. Even though International Astronomical Union is not a national organization, USA is having an oversized influence in this body. American president can also push the leaders of other countries to give Pluto a much deserved recognition. Finally, an Executive Order or National Emergency declaration can make Pluto's return a virtual certainty (but I am sure drastic measures like that won't be necessary).

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  • $\begingroup$ I would vote for it. $\endgroup$
    – workerjoe
    Mar 8, 2019 at 1:37
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A dwarf planet might be reclassified if it gained significant mass. If there was a debris cascade in the Kupier Belt that made several dwarfs pile onto Pluto, that would both help clear its orbit and make it more massive to round itself out.

I hereby dub “a hypothetical pile of dwarfs on Pluto” to be called “a Kupier scrum.”

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You could have a Mercury sized body that has not "cleared the neighborhood around its orbit", because there are two Pluto/Eris sized bodies located in the general vicinity. These two bodies could collide creating an asteroid field. The Mercury sized planet could clear these much smaller bodies, and fulfill the "cleared neighborhood" criteria.

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