Re: "Pluto Now Called a Plutoid"

K_h wrote:
"Bryan Derksen" <bryan.derksen@xxxxxxx> wrote in message news:GCvdk.69025$Jx.48462@xxxxxxxxxxxx
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.

No, it does not say that. The phrase "modulo spin distortions" is contained nowhere in the definition.

This is pointless semantic quibbling and nitpickery, not worth any further effort to refute. You're being silly.

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?

New studies suggest that a world, about 70 percent as massive as the Earth, could exist in the Kuiper built without clearing its orbit.

Please check back a few posts earlier in this thread. I showed my calculations, a planet 70 percent as massive as the Earth _would_ clear its region in the Kuiper belt. It would be approximately as capable of clearing objects from its orbit as Mars is, and Mars is considered a planet.

It's at http://groups.google.com/group/rec.arts.sf.science/msg/2cf5c261a6112032 in case it didn't make it to your newsserver.

A world that massive deserves to be called a planet even though it hasn't cleared out its region in the Kuiper belt.

Agreed. Get back to me if they actually _do_ designate it a plutoid and I'll share your annoyance. Until then you're getting bent out of shape about some hypothetical scenario that hasn't occurred and based on the numbers I've dug up is not particularly likely to occur.

Also, check out:
http://www.spacedaily.com/reports/The_Great_Planet_Debate_Dwarf_Planets_Are_Planets_Too_999.html

Alan Stern's been making some peculiar statements on this subject even though he really should know better. He says in that article, for example, that "Earth would not be considered a planet if it orbited the Sun beyond Neptune, because its gravitational influence would be insufficient to clear out the Kuiper Belt." By _his own method_ of determining orbit clearing capacity, however, I've shown that it'd be about as capable of clearing its orbit as Mars is (see my previous post linked to above). So if Mars is considered a planet based on its orbit-clearing prowess then Earth in the Kuiper belt would be too.

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. :)

A bad idea to call "dwarf planets" asteroids. A world like Pluto has an atmosphere, polar caps, three moons, and interesting geological activity at its surface.

So come up with some additional subdivisions for asteroids and KBOs and whatnot. I'm not too concerned what you do there, I'm merely pointing out that I consider the division between dwarf planet and non-dwarf planet to be a fairly arbitrary one (in contrast to the division between planet and non-planet, which has a theoretical basis and is unlikely to produce many ambiguous cases as a result).

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.

It is not obscure at all, there are probably billions of "free-floating" planets in the local group alone.

And we're unlikely to detect any of them any time soon, and if we do we're even less likely to find any of them in trajectories worth calling "hyperbolic orbits." Hence not worth fretting about. Eris and her friends, on the other hand, are there right now and so needed to be dealt with.

The defintions I provided qualify them as planets but, by the IAU's definition, they are not planets at all since they are not "in orbit" around a star. If the definition of a planet depends on its orbit being closed then the IAU definition should have used the phrase "in a closed orbit" instead of "in orbit". Obviously the IAU's defintion contains substantial weaknesses.

This is more semantic quibbling. The exact wording the IAU used is "is in orbit around the Sun," which implies a closed orbit to everyone except the most obsessive and lawyerly-minded - a hyperbolic orbit isn't _around_ an object, it _passes by_ it. I think you've decided that this definition is bad for some other reason and are just casting about for every little nook you can hammer an objection into.

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.

It is too soon to claim that there is a "solid theoretical basis" indicating that a large discontinuity will be seen in most cases.

What objections do you have to the theory behind it? It's a fairly straightforward positive feedback loop - once an object gets big enough relative to the other objects sharing its orbit to start ejecting or accreting them, it rapidly gets even bigger.

The aforementioned issue with a 70 percent mass world not clearing out its Kuiper belt region underscores this point.

My earlier calculations showing that a 0.7-Earth mass _would_ clear an orbit in the Kuiper belt aside, have you heard of the Kuiper cliff? It was also mentioned earlier in this thread, you can read about it here: <http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22>. It may be a case of orbit-clearing by a yet-undiscovered planet.
.

Relevant Pages

  • Re: "Pluto Now Called a Plutoid"
    ... If Stern is correct then your argument on this point is invalid. ... the side that says "it can clear its orbit" is from ... as the Earth orbiting within the Kuiper belt. ... needs to be part of the definition of a "planet" under the IAU's incomplete ...
    (rec.arts.sf.science)
  • Re: Harmonia (Mars/Venus system)
    ... Mars and Venus are both thoroughly terraformed first... ... They'll orbit each other every ... But is there any /obvious/ reason a double planet at 1.1 au ...
    (rec.arts.sf.science)
  • Re: Harmonia (Mars/Venus system)
    ... Mars and Venus are both thoroughly terraformed first... ... They'll orbit each other every ... But is there any /obvious/ reason a double planet at 1.1 au ...
    (rec.arts.sf.science)
  • Harmonia (Mars/Venus system)
    ... Mars and Venus are both thoroughly terraformed first... ... They'll orbit each other every ... But is there any /obvious/ reason a double planet at 1.1 au ...
    (rec.arts.sf.science)
  • Re: "Pluto Now Called a Plutoid"
    ... Sun, has sufficient mass for its self-gravity to overcome rigid body ... approximately as capable of clearing its orbit as Mars is. ... would likely be called a planet, ... cleared out its region in the Kuiper belt. ...
    (rec.arts.sf.science)
Loading