How many planets would there be if Pluto were still a planet?

[Chronos]

Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity. The two known and named plutoids are Pluto and Eris. It is expected that more plutoids will be named as science progresses and new discoveries are made.

Wouldn’t Quaoar and Sedna (both discovered before Eris) also qualify as plutoids? Sure, Sedna has a really weird orbit, but I don’t see anything in that definition that rules out weird orbits, and I can’t think of anything at all which would disqualify Quaoar.

[Alka_Seltzer]

@Chronos

Wouldn’t Quaoar and Sedna (both discovered before Eris) also qualify as plutoids? Sure, Sedna has a really weird orbit, but I don’t see anything in that definition that rules out weird orbits, and I can’t think of anything at all which would disqualify Quaoar.

Yes, Sedna qualifies as a plutoid. The terms just means “dwarf planet beyond Neptune”, it’s a bit of a catch-all term.

@Doubticus

As our observational abilities changed, so did the definition of “planet.” Future discoveries are likely to require additional revisions. Such is science.

It will be interesting to see if the dwarf planet definition will survive once the outer solar system is surveyed more throughly, and once we start looking at other stellar systems in detail. Will we find a lot of planatary systems similar to ours, or a wide variety? I expect progress to be very slow on the latter investigation though, there are currently no plans to build the kind of telescopes we’d need to look at other star systems in that kind of detail.