Re: "Pluto Now Called a Plutoid"

K_h wrote:
"Brett Paul Dunbar" <brett@xxxxxxxxxxxxxxxxxx> wrote in message news:FV$OyfDNgSdIFwSw@xxxxxxxxxxxxxxxxxxxxxxxxx
In message <PYKdnToYLvdOlenVnZ2dnUVZ_rbinZ2d@xxxxxxxxxxx>, K_h <KHolmes@xxxxxxxxx> writes
* defines a planet as "nearly round" suggesting that only nearly spherical
bodies qualify as planets. But Saturn is over 12% oblate and so it is not
clear if it qualifies as a planet.

It does, the nearly round is hydrostatic equilibrium so it is round modulo any distortions caused by spin.

Well, then, the IAU definition should say that. It says nothing about modulo spin distortions.

The IAU definition _does_ say that. Quoting it directly:

"(1) A planet [1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit."

"Hydrostatic equilibrium" inherently includes oblateness due to rotation. A rotating self-gravitating object that's in hydrostatic equilibrium will be an oblate spheroid, just like Saturn is.

* defines a planet as having "cleared the neighborhood around its orbit"
which would exclude Jupiter type planets in other solar systems. For
example, consider a solar system with a 6 solar-mass star orbited by a 25
Jupiter mass brown dwarf (at 20AU from the star) and one Jupiter mass planet
at each LaGrange point L4 and L5. Neither Jupiter mass planet would have
"cleared the neighborhood around its orbit" and so they would not qualify as
planets.
That is an actual potential ambiguity, at the moment we don't know of any such bodies if we find one then we will have to decide. This is true of most possible definitions. How you might classify a giant Trojan can await actually discovering one.

Definitions should be generic enough to cover the kinds of cases that exist in nature. With trillions of solar systems in the Milky Way galaxy alone, there have to be configurations like this. The defintion of a planet should be generic enough to cover whatever nature can produce.

Exactly how generic was the previous definition by comparison? In case you weren't aware, the previous definition of "planet" was just the following list: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It was arbitrary.

The new definition is still explicitly limited to just the solar system but that's an easy limit to lift. I don't think a few hypothetical and highly unlikely hard cases are enough of a problem to call this an "extremely flawed" definition on that basis, you'll find hard cases for practically any definition if you look hard enough.

Also, the Milky Way doesn't have trillions of stars, so I think you're a bit off on the "trillions of solar systems" thing.

* requires that a Kuiper belt world with the same size and mass as the Earth
is not a planet. A world with 70 percent the mass of the Earth may very
well reside within the Kuiper belt.
The minimum size for orbit clearing depends on distance from the primary; and this is a problem why?

An Earth sized world in the Kuiper belt is a planet under any reasonable definition. If / when and Earth sized world if found orbiting another star, at 30 AU say, it will almost certainly be called a planet.

And, according to the calculations I showed earlier, such an object is approximately as capable of clearing its orbit as Mars is. So yes, it would likely be called a planet, even under the IAU's definition. What's the problem here?

In English the noun with the modifier often denotes a subset, it can also denote a disjoint set. A few examples: In Alpine Skiing the Slalom Giant Slalom and Supergiant Slalom are three distinct events and there is no overlap between the types of courses used. Moles, Golden Moles and Marsupial Moles are three quite distinct, if superficially similar, groups of burrowing mammals again they are entirely disjoint sets and are not closely related.

On something as fundamental as defining planets, the usual rules of English should be obeyed. The IAU defintion does not follow the usual rules and that is bad.

I'd be in favor of dropping the "dwarf planet" category entirely and just let them be asteroids and KBOs and whatnot, but I somehow doubt that would go over better than the current definition did with most of those who are objecting to it. :)

We have a lot more information about the solar system than about any other system, we can actually resolve individual objects that are too small to be planets in the solar system, with one exception this is not true of any exoplanetary system, so for the moment we don't need to worry about the lower limit of planetary size outside the solar system.

Never put off to tomorrow what you can do today. The IAU's bad defintion should be cleared up as soon as possible.

Actually, it makes sense to put off decisions until tomorrow when you haven't got enough information to make good ones today. When extrasolar planets started being discovered the abundance of close-orbiting "hot Jupiters" was a big surprise, turning our accepted theories of planet formation on their heads. If the IAU had tried coming up with some hard-and-fast definition before then that assumed any close-in bodies would be small and rocky like they were in our solar system they could wind up with misclassifications even sillier than Pluto's was.

The IAU's definition as it currently stands is explicitly limited to just our solar system, and except in highly unusual situations we can't yet detect extrasolar objects small enough and far enough out from their primaries to get anywhere near the boundary of planethood anyway, so I don't see what the rush is. We can wait until we know more about planetary systems in general.

* fails to specify what kind of orbit a planet must take. It is unclear if
a world, with the same size and mass as the Earth, traveling on a hyperbolic
orbit around the sun qualifies as a planet.
A hyperbola isn't actually an orbit, as it isn't closed so no that would not have cleared its orbit. Orbits are either elliptical or circular and repeat. Parabolas and hyperbolas do not repeat.

No, they are hyperbolic orbits. Go to http://en.wikipedia.org/wiki/Orbits and do a search for "hyperbolic orbit".

The first time the phrase is mentioned in that article is in the sentence "A comet in a parabolic or hyperbolic orbit about a central star is not gravitationally bound to the star and therefore is not considered part of the star's planetary system."

This is another one of those obscure edge cases that just isn't worth fretting about, IMO. The likelihood of something like this coming up are negligible. A better issue to think about is how to classify free-floating "rogue" planets, which hyperbolic planets are a subcategory of. But since we can't detect those yet there's no big rush here either.

Yes, those numbers were arbitrary and I should have used "Equatorial Diameter" instead of "Diameter" for the planets. The phrase "orbit clearing" is ambiguous and that is one of the problems with the IAU's definition.

At least orbit clearing has a large discontinuity between "very clear" and "not at all clear" in practice, and a solid theoretical basis indicating that this discontinuity will be seen in most other cases too. So even if the exact numeric threshold is undefined one can say "somewhere between Mars and Pluto" and it's unlikely to be ambiguous which side a candidate object falls on.

Planetary diameter for any given mass, on the other hand, can vary widely with composition and temperature and has no "natural" cutoff points.
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