It was a burning question of mine for a while now:

I understand that dwarf planets like Pluto and Ceres aren’t considered planets of the solar system, but why are they called ‘dwarf PLANETS’ if they aren’t planets.

And no one really says, “the sun isn’t a star, it’s a Dwarf Star”. Nor is it declassified as one because of it.

So, why are dwarf planets not planets, but dwarf stars are stars?

  • @brucethemoose
    16•4 hours ago

    Its semantics, and a subject of ongoing debate.

    Per wikipedia, I really like this proposal:

    Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star.[210] The formula produces a value called π that is greater than 1 for planets.[c] The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0.1, or less. Objects with π values of 1 or more are expected to be approximately spherical, so that objects that fulfill the orbital-zone clearance requirement around Sun-like stars will also fulfill the roundness requirement[211] – though this may not be the case around very low-mass stars.

    It basically means a planet should be big enough to consolidate all the stuff in its orbital area, not be part of an asteroid field. That makes sense to me.

    https://en.wikipedia.org/wiki/Dwarf_star

    “Dwarf” stars are even more confusing, as it basically a synonym for “normal,” as opposed to “giant” stars (which are relatively puffy and big for their mass/temperature), or more exotic stars. But the term is also used for special cases, like the relatively exotic white dwarfs (remnants of exploded stars with very strange properties, extreme density, and not “burning” like a star traditionally does), or “barely a star” brown dwarfs.

    TL:DR: If an astronomer asks you to name something, you should say ‘absolutely not.’

    • Rhaedas
      1•2 hours ago

      I don’t like the idea of actually requiring a clearing out of the orbit. Is this not including Trojan areas, because there will always be stuff there for any planet.

      The phrase “clearing the neignborhood” doesn’t mean the orbit is clear, it means the planet in question has gravitational dominance over anything in its orbit, so larger bodies are either captures as moons or removed via gravitational slingshot. This allows wanderers and other captured bodies.

      The same astronomer (Margot) has remarked that gravitational dominance was clearer language, and it’s interesting that I’ve found in many reputable sources like NASA where they’ve dumbed down this third rule to just clearing the orbit, which is NOT correct as I mentioned above.

      As for anyone who ever pulls the “I think Pluto is a planet”…it is a planet, as a subclass with restrictions.

    • @SpaceNoodle
      1•3 hours ago

      Why would I turn down such an opportunity? I’m great at naming things.