Revived question: Is Pluto a planet?

On the “welcome” dialog box for AOL, one of the news items shown is a squib about some astronomers making an effort to have the designation of Pluto as a “dwarf planet,” by the International Astronomical Union, rescinded so Pluto will be the ninth planet again. One of the links was to a website with a rather sentimental message board saying things like “Save Pluto.”

Pluto should probably be considered a planet.

Its the same thing as Europe being considered a continent. Its on the “maps” as such, and has a historical record.

By Modern standards, not really…

but let’s “grandfather” it in…

But only for a few decades, not unlike Ceres. It just happens to be the time when most people who are around now went to elemetary school.

Let’s let astronomy be a science not ruled by popular whim, shall we? Pluto isn’t a planet due to a reasonable and self-consistent definition of the term, not due to arbitrary decree and certainly not because anyone wants to make anyone else angry. Must everything degenerate into a popularity contest?

How many of you think everything should degenerate into a popularity contest?

Come on, let’s get those hands in the air. You, there! No, the person sitting by the monitor–yes, you. Raise that hand!

I thought there were objections that the new rules that excluded Pluto were also sort of arbitrary. But I agree that grandfathering Pluto in makes absolutely no sense - it’s either a planet or it isn’t and there doesn’t need to be exceptions. The whole problem was that there wasn’t a clear definition of what a planet was. Creating a definition and then making up exceptions doesn’t exactly help.

When I was your age, Pluto was a planet.Obligatory facebook link (you may have to sign in.)

Is Jupiter a planet?

I mean, how can you call a gas giant the same thing you call a rockball like Earth? Other than the fact that they both orbit the Sun, the two bodies have nothing in common.

Maybe this October there will be an announcement about the world Ceres.

Well they both meet the official definition of orbiting the sun, having enough mass to overcome rigid body forces (It’s mostly round) and has cleared it’s immediate neighborhood.

If we include Pluto as a planet don’t we have to include a bunch of other objects as official planets as well?

Because they both fit the definition of planet. Pluto doesn’t.

See, astronomy is easy!

Sure they do - they’re both massive enough to maintain a roughly spherical shape, but not nearly massive enough to undergo nuclear fusion.

The same way you can call both humans and whales “mammals”, even though the two species are noticeably different in many ways. It’s how the category Mammal is defined — and as broad as that category may seem, it’s presumably defined that way for a useful purpose. The term “planet” as it’s now defined by the IAU is considered useful to astronomers.

For what it’s worth, Earth is a member of the terrestrial planets, whereas Jupiter is one of the gas giants — when that distinction is important to make.

OK, how doesn’t Pluto meet the definition of a planet?

The current definition of a planet was proposed during the 2006 congress of the IAU. It was approved by popular vote.

ETA: I should add that the current definition excludes Pluto as a planet.

Is, say, Mercury more similar to Jupiter than it is to Pluto? And if so, how?

Yes. The definition of planet includes “has cleared the neighbourhood around its orbit”: Mercury and Jupiter both satisfy this, but Pluto does not.

Erstwhile Doper Phil Plait, aka The Bad Astronomer, has a retort to this:

Where do we watch this debate?

:frowning: registration was required and is now closed.