Pluto no longer a planet

Just heard on the radio a few minutes ago that the announcement has been made regarding the official definition of a planet and which celestial bodies qualify as planets. There was some talk of reclassifying Ceres, Charon, and UB313 as planets, bringing the total to tweleve, but in the end they chose the more sensible option of declassifying Pluto due to its small size and wobbly orbit. Pluto is now considered a "dwarf planet". No word yet on whether Ceres, Charon, or UB313 will also be classified as dwarf planets.


Grr... beat me by 4 minutes!

The BBC has a graphic up in which those latter bodies are shown as dwarf planets. I'm not sure if that's official yet though. Hopefully puts an end to all the debating over Pluto and how it should be classified.

So, what's the difference between a dwarf planet & a large meteoroid? Atmosphere? Size? Orbit-style? Distance to the sun? Proximity to other objects? Just curious

I suspect that a dwarf planet will have to meet the following criteria: gravity strong enough that it has a spherical shape, and orbits a star. Non-spherical would be asteroids. And then there's the Kuiper Belt and Oort Cloud objects that we don't know much about yet; some of them are close to spherical...

Atmosphere determines the type of planet.

Size is what determines dwarf planet versus planet.

By orbit-style do you mean circular versus elliptical? IIRC even our own planet's orbit isn't perfectly spherical, so elliptical seems to be ok for planets too. Pluto was disqualified for having a wobbly orbit, which I think is something else but not sure.

From what I've heard, distance to the sun is not a factor. People were quite upset that distance would be used to discriminate in the definition. AFAIK the only thing distance determines right now is which zone an object exists in: hot zone, terrestrial zone, cold zone, Kuiper Belt, Oort Cloud.

Proximity to other objects is tricky. I read somewhere that Charon doesn't quite orbit Pluto, but that both of them orbit a point somewhere in between. There are some other companion objects to Pluto besides Charon that are being proposed as satellites. Not sure if that is going to be decided at this time though.

hmm, interesting! so if Charon orbits Pluto but Pluto isn't a real planet that would make Charon not a real moon I assume. Perhaps a dwarf moon? Except it's actually quite a large moon from what I've heard. I always thought Pluto had a weird orbit though.

I wonder if Pluto was named after the Disney character or the Disney character named after Pluto-- or perhaps they're not even related!