Re: "Pluto Now Called a Plutoid"


"Bryan Derksen" <bryan.derksen@xxxxxxx> wrote in message
news:vNQdk.63575$kx.53870@xxxxxxxxxxxx
K_h wrote:
"Bryan Derksen" <bryan.derksen@xxxxxxx> wrote in message
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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.


No, it is not "pointless semantic quibbling and nitpickery" and I am not
being silly. The IAU's definition is very bad for many reasons, including
those that I outlined in my first reply to this thread.


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 agree with Stern that it would not clear out the Kuiper Belt. So, under
the IAU's definition it would not be a planet. That is very 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. :)

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).


As astronomers learn more and more about other solar systems, more and more
unexpected discoveries arise. It is premature to cleaim that it is
"unlikely to produce many ambiguous cases". The definitions I provided are
clear and comprehensive and do not require additional subdivisions for
asteroids and KBO's. In fact, the term "Plutoid" is not needed.


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 example definitions I provided clearly qualify these billions of
free-floating planets as planets. A good definition of a planet should
cover these now, rather than wait for lots of them to be discovered and then
have to modify the definition of a planet again.


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.

No. To define a planet as "in orbit around the Sun" fails to define planets
around other starts as well as planets that are in galactic or intergalactic
space. It is bad to have one definition of a planet for our solar system
and another for everywhere else. Yes, hyperbolic orbits around the sun will
be quite rare, for worlds the size we typically call planets, but that was
not the point. The point is that there is no excuse for such bad wording by
the IAU. If it meant a closed orbit around the sun then it should have
specified that kind of orbit in its definition so that it is clear that the
definition is not applying to open orbits around the sun.


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.

See my original post, I listed a case where Jupiter size and mass worlds
would not qualify as planets under the IAU's definition. Cases like this
will not eject or accrete the other worlds and it is too soon to know how
common or rare systems like these are.


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.

"May be the case" is the operative term. It may be that a 0.7 Earth mass
world has not cleared out its orbit, far out in the Kuiper Belt, as Stern
has pointed out. If that turns out to be the case then it will be yet
another example of why the IAU's definition is so flawed. The point is that
the definitions should be fixed so that it is clear what a planet is no
matter what its changing environmental conditions are. The example
definitions I provided do just that.

K


.

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: OT Poll: Does our solar system have 8 or 9 or 10 known planets?
    ... defintion of planet, I would suggest any body that has sufficient mass ... to force itself to be spherical and orbits the sun as its primary orbit ... the Moon was orbiting Earth. ... primary orbit is around the Sun. ...
    (talk.origins)
  • Re: Two suns
    ... To shift the position of the orbit, ... zero, after all, because that would mean the sun would stand still. ... would orbit faster than Jupiter does (by a factor of the square root ... I assume you don't mean a planet year of 400 days, ...
    (rec.arts.sf.composition)
  • Re: Gravity and the Sun
    ... > a net electrical charge upon the Earth and Sun, ... between the Sun and planet, and ds being the radius from the Sun's ... > The energy of the orbit will increase, ... > total mechanical energy of the orbit, ...
    (sci.physics)
  • Re: OT Poll: Does our solar system have 8 or 9 or 10 known planets?
    ... to force itself to be spherical and orbits the sun as its primary orbit ... the Moon was orbiting Earth. ... primary orbit is around the Sun. ... Classifying the Earth-Moon system as a double planet is a bit ...
    (talk.origins)