What's a planet?

There was a very interesting BBC programme last night about whether or not Pluto qualifies as a planet. Apparently no-one - I mean no-one - can agree on the definition of a planet. Must it be spherical or bigger than a certain size (if so, what size) or what? Is it disqualified if it’s a moon? (Many moons are bigger than Pluto).

So, how do you define a planet?

Does your username imply a vested interest in the status quo? :slight_smile:

There is no objective way to define a planet that includes the current 9 but excludes the farther, larger object from Pluto and is not totally arbitrary.

Answering in a IMHO manner, I’d say a planet is:

– A discrete entity,
– That directly orbits a star (so moons are out)
– And is large enough to be spherical due to the effects of gravity (so most or all asteroids would be out.)

Does “discrete entity” include not having any satellites big enough relative to the planet candidate that they have a common center of gravity located outside the volume of the planet candidate?

I think this is about the only sensible definition possible - it does mean that Ceres qualifies - perhaps also Vesta, but I think folks should just get over it.

Hah! I hadn’t thought of that. :slight_smile:

I have no views. I just thought it was wonderful - all those astronomers getting so worked up! The head of (I think) the New York planetarium has removed Pluto from his planetary display, and he’s being pilloried by thousands of kids across the USA, demanding the reinstatement of said planet. But he says it isn’t, so he won’t. (Very amusing guy, BTW).

I just thought it was surprising that such a long established object had no accepted definition.

I hear what you’re saying, but the case you’re likely thinking of would come closer to breaking rule #2 (directly orbiting a star.) Perhaps we can modify it to include a definition:

Objects that would be planets except for their mutual gravitational pull, which puts their common center of gravity outside the volume of the potential planets, shall be classified as twin planets (or triple, quadruple, etc.)

Definition of ‘Planet’ Expected in September

Well, as I hope all of you already know, when Ceres was first discovered everyone was quite positive that it was a planet. It wasn’t until the interest sparked by Ceres’ discovery lead to the identification of several other masses in similar Solar orbits that Ceres planetary status was revoked in favor of just marking off several million kilometers of solar orbit as ‘The Asteroid Belt’.

The upheaval and confusion astronomy is now experiencing is a direct result of having settled on a arbitrary definition to begin with, and insisting that a 100,000 Astronomical Unit-wide star system be fully describable to children by the mnemonic “My Very Excellent Mother Just Served Us Nine Pizzas.”

A situation that was once a triumph of affordable education may soon change to one that education administrators worldwide deplore would be a weighing-down of the curriculum with pointless distinctions, needless facts, and irrelevant explanations about what is almost entirely empty space.

Considered from an objective viewpoint, Pluto is probably best defined as a larger than average Kuiper body. You’d have to come up with a pretty convoluted definition for planet in order to include Pluto on the list and not include bodies like Ceres, Sena, Quaoar, or “Xena”. Basically, you’d have to decide which bodies you wanted on the list first and then work backwards to create a definition that included them.

Politics, as is so often the case, is a factor. Pluto was discovered by Clyde Tombaugh in 1930. Based on the observed date at the time, Pluto appeared to be a pretty unique object and was easily accepted as a new planet. It was a big source of pride in the United States to have had an American astronomer discover a new planet. And there is still a reluctance in the US to retract Pluto’s special status.

Clyde Tombaugh is from the same small town in Illinois as my mother, so I have always had a little extra affection for Pluto. :slight_smile:

Don’t forget the upper limit as well:

– And is not so large that it can support nuclear fusion through gravitational compression.

Otherwise, you’d confuse with the case of small star orbiting big star.

And what do you mean by ‘diectete object’?

And while we’re at it, let’s define ‘star’ as ‘an object that does support nuclear fusion through gravitational compression’. :slight_smile:

This does mean that any number of objects that are shining only by the heat of gravitational compression or impact energies would not be stars–protostars and really big planets maybe–but not stars.

Better make that “does not support nuclear fusion of [sup]1[/sup]H.”

Larger brown dwarfs go through a short phase of deuterium ([sup]2[/sup]H) burning.

Part of the reason there’s no good definition for “planet” is that it’s such a damn boring question. Doesn’t matter if we call Pluto a planet, a Kupier-Belt Object, a Trans-Neptunian Object, or My Aunt Fanny, we’re still going to study it the same way. It’s still a ball of ice and rock. It’s still got a tenuous atmosphere (for now), probably some kind of weather, maybe some interesting geological activity (if it’s anything like Triton.) You could call it “The Solar System’s Least Tasty Sno-Cone,” for all I care.

A song about whether or not Pluto is a planet or not

Just one other point: Mercury was originally thought to be much larger than we now know. Its size was steadily revised downward over the years.

Pluto is definitely a Kuiper belt object. The question is just whether it’s possible for the categories “Kuiper belt object” and “planet” to overlap.

You also have to be careful about the “directly orbits a star” part of the definition. Everyone agrees that, say, Titan is not a planet. But what about our own Luna? Everyone knows that Luna orbits the Earth, not the Sun, but “everyone knows” is a notoriously unreliable source of information: In fact, the Sun exerts a larger gravitational force on Luna than does the Earth (a situation unique to Luna, among all the “moons” in the Solar System).

Depending on exactly how “planet” is defined, I can argue for any number from 8 to 18 “planets” in the Solar System (ironically, in one case, shrinking the definition of “planet” actually increases the total number). The longest list I can come up with is Mercury, Venus, Earth, Luna, Mars, Ceres, Vesta, Io, Europa, Ganymede, Callisto, Saturn, Uranus, Neptune, Pluto, Xena, Quaoar, Sedna: This list comes from the definition that a planet is an object large enough to form into a sphere gravitationally (with enough leeway on “sphere” to include Vesta), but small enough that dR/dm > 0 and hydrogen does not fuse in its core, on which the strongest gravitational force is exerted by a star.

Really, though, I think the real problem with classifying Pluto is that we’ve taken at least two completely different types of objects, or three if we include Pluto, Xena, and the like, and lumped them into a single category. I’d much prefer if we had completely separate names for terrestrial bodies like Earth and Mercury, gas giants like Neptune and Saturn, and icy bodies like Pluto and Sedna. This would still include ambiguous cases like Luna (with its peculiar orbit), Ceres (perhaps below the lower size bound), and Jupiter (arguably a brown dwarf), but it would at least avoid the question of whether Pluto is a “planet”.

This raises an interesting point: the term ‘planet’ was coined not based on detailed analysis of what the planets were, but on how they appeared to naked-eye observations. A planet is, simply put, a wandering star. That’s all they were for thousands of years. During the Renaissance, the fact that planets resolved to discs rather than staying points of light like ‘other stars’ when viewed through a telescope became important; this played against Ceres because it takes a relatively powerful telescope to get a disk out of the little world, and eventually lead to the coining of the term asteroid. (‘star-like’) Composition, Mass, and Orbital Altitude played no role until the 20th century.

I should add that, by the original definition, only moving points of light in the night sky regularly visible to the naked eye are planets; that would be: Mercury, Venus, Mars, Jupiter, and Saturn. Therefore there would be only five planets.

Then we could make up several actually useful names for all the things those planets actually represent!

I’ve always wondered if there was a lot of childish giggling going on when they named Uranus.

It’s a good thing you stuck ‘regularly’ in there or you’d have to include Vesta as a planet. It’s only 340 miles in diameter, but it does sometimes brighten to magnitude 5.2 at opposition.

The classical definition of a planet is a “wanderer” that moves about the “fixed” layer of stars throughout the year. The Moon and the Sun were lumped together with the classical “planets”. By this definition, not only would you also have to include Vesta but also Uranus, as it is also sometimes bright enough to see with the naked eye.