Pluto

[gfish]

For some reason the subject of Pluto's status has come up several times recently. (Probably because I've been reading a lot about planetary formation.) I certainly agree that it isn't a planet, but I'll take an even more radical stance: Earth isn't either.

That's right. I don't think Earth should be classified as a planet.

To be more precise, I think it is ridiculous to place Earth (or Mercury or Venus or Mars) in the same class as the gas giants. If you were an intelligence evolved entirely outside the context of planets and you started looking at solar systems, I think you would break things down as follows: stars - planets (gas giants) - random rocky crap (us). Our hypothetical alien intellects (vast and cool and unsympathetic), not obsessed with our particular form of wet and squishy life, would barely even notice the inner solar system. To think that the object we live on must necessarily be grouped with Jupiter and Saturn is pure provincialism. It's nothing more than an updated form of geocentrism.

But at least this means the Kuiper Belt objects would be in the same category as the Earth, so I have to assume those upset about Pluto will be happy.

Comments

[ashley-y.livejournal.com]

The Earth is not a planet, because it is the ground under our feet. It does not wander against the fixed stars. The Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn are all planets, as are various bodies such as Uranus and Neptune that can be seen with magnification devices.

you're welcome

[caladri.livejournal.com]

http://sites.google.com/site/earthdeception/
http://sites.google.com/site/earthdeception/earth-not-a-planet

[neuro42.livejournal.com]

I disagree. The fundamental defining characteristic of a planet is a visible, cyclic, non-sidereal apparent motion. Just look at the etymology. Even the OED lists as 1.a. "Each of the seven major celestial objects visible from the earth which move independently of the fixed stars and were believed to revolve around the earth in concentric spheres centred on the earth (in order of their supposed distance from the earth in the Ptolemaic system: the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn)."

[inaurolillium.livejournal.com]

I have to disagree. I think it's a bit arbitrary to decide that gas giants are planets and rocky spheroids are not. Why not have multiple classifications of "planet," or none, and class the inner-system bodies as rocky spheroids, the gas giants as gas giants, and other bodies as other appropriate types. (I'm leaving Pluto out of consideration entirely.)

[technoshaman.livejournal.com]

Meh. Why do we have to cubbyhole everything? The orbital mechanics are the same for a two-pound chunk of ice as they are for Jupiter. Only the scale is different. We could rate things on average density and temperature, but there's such a continuum... I mean, look at us. Earth isn't all solid. We've got a solid crust, a molten midsection, and a presumably solid core. Who knows what kind of weirdo properties other planets have? And why do folks have to get their shorts in such a knot about it? You'd think it was sexuality or something... (YA thing I don't see why people have to get their shorts in a knot about. It is what it is...)

[anansi133.livejournal.com]

Lately I've notice Earth's moon being called a planet by people you'd think would know better. In that context, a planet is something humans can send probes to. So asteroids and comets could be planets too.

Just wait until we encounter yet another kind of body, that's neither gas giant nor rocky world.

[pielology.livejournal.com]

I'm waiting for the extraterrestrial planetfinders to figure out whether there's an occupied continuum between "rocky, small, thin atmosphere" and "rocky core, humongous, thick atmosphere". It's a pretty clear dichotomy in *this* solar system but that doesn't mean it holds in the rest of the universe.

I'll get back to you with an opinion in a decade or so.

[peteralway.livejournal.com]

Actually, I think that it's as sensible to call a planet "a (celestial) body a person could walk on." The Jovian planets can be booted out of the club because they have no surface, and once asteroids get so small that a conventional walking-bace step sends you off at escape velocity, they are too small to count as planets.

In all seriousness, I think that for nearly 80 years, planets were exemplefied by a list of nine, and since the toppling of the geocentric model every known object on that list was counted as a planet, the list itself is a good source of a functional definition of a planet.

As far as I can tell, Pluto was dethroned in large part because new Kuiper Belt objects larger than Pluto "threatened" to create a long list of planets. It was a reductio ad absurdium argument that "pretty soon we'll have 20 or 30 so-called planets," as if progress in astronomy were a bad thing.

Personally, I think Eris is the tenth planet and if astronomers discover more Kuiper-belt objects bigger than Pluto, then bully for them!

I think it's perfectly reasonable to define planets as objects orbiting a star that are too small for fusion in their cores, and are above an arbitrarily defined size; and these include terrestrial planets, jovian planets, kuiper-belt objects, and some categories that extrasolar planet discoveries are starting to suggest.

Defining terrestrial planets out of the club is to completely discard the common understanding of the word, and would be dictation language beyond reason. Personally I think defining Pluto off the list is pretty darned close to the same thing--the IAU doesn't own the English language.

Yeah, I'm a Plutophile.

[orawnzva.livejournal.com]

We already have a term — "gas giant" — for the category of objects you propose. There are a limited number of reasons to prefer one usage of a term over another. One is closeness to an original meaning; the original meaning of "planet" is intrinsically geocentric as neuro42 points out.

Another is current common usage; this could mean either common usage within the astronomical community, including the idea that a planet is overwhelmingly the largest object in its orbit, since the asteroids are already excluded, or the vernacular meaning whose exact semantics is vague, mainly rooted in the explicit list of planets we learned in school and with the proviso that other objects "like those" would count as planets if discovered.

A third reason to prefer a usage is usefulness, and this must always mean usefulness to some users of the term. Yes, Earth is puny when set beside the mighty Jupiter. Should we then defer to the inhabitants of Jupiter in all questions of terminology— oh, right, as far as we know Jupiter is uninhabited. The question then becomes, which meaning of this term is most useful to us in our current situation (where our situation is taken to include the existence of documents which employ the term in the senses in which it has historically been used up to now)?