General Question

Protagoras's avatar

Why is Pluto not a planet?

Asked by Protagoras (135points) January 1st, 2008

I read that Pluto isn’t considered a planet anymore. What is it then? Who decided this and why?

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20 Answers

boydieshere's avatar

It’s a satellite of whichever planet comes before it. Even while Pluto was considered a planet, it always had an irregular orbit, which has apparently been decided is an irregular satellite obit; instead of just obiting the planet, it orbits the sun as well.

gailcalled's avatar

The International Astronomical Union (IAU) is the Decider. Pluto occasionally wanders inside the orbit of Neptune but is not considered its satellite.

Pluto is now officially called a dwarf planet (aka , rocky boring thing smaller than our moon) altho there are still multitudes of lovers of Pluto the Planet.

For technical details:

Pluto NASA

omfgTALIjustIMDu's avatar

Also, I read somewhere when this was first decided, that other very large space-rocks were found to be the same size or bigger than Pluto, and if Pluto was considered a planet, than these rocks would also need to be taken into account.

LanceVance's avatar

Yeah, the basic answer would be: it’s too small. But Americans complained a lot over this, since Pluto is the only planet to have been discovered by them, and they took this away from them. :)

gailcalled's avatar

Not just Americans expressed their unhappiness over the demotion of Pluto, but huge numbers of both professional and amateur astronomers and the hangers-on——the planetary geologists, the astronphysicists, etc.—-were vocally furious.

gailcalled's avatar

Viz: the discovery of Pluto in 1930 by the young American, Clyde Tombaugh, is a wonderful story. He did it the old-fashioned way w. dozens of photographic plates and hours of eye-popping staring.

Read all about it

Perchik's avatar

Because Pluto is a dog

andrew's avatar

Actually, from what I remember about the debate, the main reason Pluto is a dwarf planet is not because of its size, but because it doesn’t clear its gravitational orbit—hence its irregular orbit with Uranus.

El_Cadejo's avatar

@andrew yea that is the reason, thing that never really made sense to me is if pluto was a planet and it went into Uranus’ orbital pattern, didnt Uranus go into Pluto’s? Why did they decide to ditch Pluto and not Uranus, i think Pluto is much a cooler planet

gailcalled's avatar

Check out the hard science. Pluto was demoted for three reasons (planetary orbits are a science in themselves, and P roams occasionally onto Neptune’s and not Uranus’s turf).

Pluto demoted.


atr408's avatar

Cuz there r moons bigger THM Pluto

gailcalled's avatar

Dems and Reps. in Iowa. And you ain’t seen nothing until you’ve seen an enraged astronomer.

gailcalled's avatar

Sorry; that went off unfinished. The pro-pluto-plannet (PPPS) and the Antis (APPS) are
like the Dems etc…

“Pluto has three known moons, Hydra and Nix, as well as its companion moon, Charon . At about 1,186 km (737 miles), Charon’s diameter is a little more than half of Pluto’s 1413miles.” Source is a NASA site.

The smallest moon is one of Mars – Deimos at 7 miles in diameter. And there are some moons bigger than Pluto, but most of the 139 official planetary satellites known as of 2:!9 EST Jan. 3, 2008 are smaller.

Planetary moons


andrew's avatar

Ug, yes, Neptune, of course. My brain totally froze.

@gail: According to your link, the only difference between a planet and a dwarf planet is that a dwarf planet doesn’t “clear the neighborhood around its orbit”. Seems like one reason to me.

Also, on a side note, Pluto and its satellite, Charon, are tidally locked with each other. This was from a link that gail posted on another question.

@uberbatman: I think the thinking is that pluto is straying much farther from its “natural” orbit than Uranus, so Uranus wins. I think it also may have to do with the fact that Pluto doesn’t affect the orbits of asteroids in its orbit enough, but that’s just speculation. Further research on “clearing your orbital neighborhood” would help.

And wasn’t there also scandal around one of the terms they were going to use before they settled on “dwarf planet”? Didn’t they accidentally use a well-known geological term?

gailcalled's avatar

Nothing like a bunch of planetary astronomers having at each other…the controversy is funny if you have ever hung out with these guys…not exactly the Sat. night wrestling gang. An event is considered scandalous, for example, if you publicize your comet siting second and thus don’t get your name on the comet. If the messages come into the Harvard-Smithsonian clearing house really close together, then, voilĂ„, comet Hale-Bopp. I am sure that Bopp wanted it to be Bopp-Hale.

@Andrew: I had to check what “clearing the neighborhood around its orbit” meant. That was new language to me.

To many of us, Pluto is still the beloved runt of the litter. Its orbit is more elliptical than the other 8 planets, and more tilted to the ecliptic (the path that the earth and the other 7 planets travel around the sun) than the other planets. Not too much is known about Pluto since it is really far away.But at times it is 17 degrees tilted to the ecliptic.

And as I am sure you are aware, even a three-body problem is bloody complicated. More will be known about what is going on out there in 2015, I think, when a probe of some kind ( aha -New Horizons Spacecraft) will arrive. Earth is c. 93 mil/miles from the sun; Plutos is c. 3.6 billion – not exactly around the corner.

It is 0 degrees here now, so I have a frozen brain too.

gailcalled's avatar

@uber; cute but not to scale. Check out this great site: (Pluto is half the size of Earth's moon).

If you want to read the entire story (the astronomers were able to make fun of themselves):
Pluto shrunk

Perchik's avatar

@uber LOVE IT!

flo's avatar

So, when I hear “it is a dwarf planet”, I am getting the message that it is a planet. Isn’t that describing what size of planet it is? Why use the word “planet” in the new name? Shouldn’t there be a new word for it? I haven’t read the answers above

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