Re: Orbital dominance

In message <1156548280.747074.157510@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, David Tate <dtate@xxxxxxx> writes
Brett Paul Dunbar wrote:
In message <1156474021.146442.208090@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
David Tate <dtate@xxxxxxx> writes
>Brett Paul Dunbar wrote:
>> In message <1156462728.693928.223770@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
>> David Tate <dtate@xxxxxxx> writes
>> >So no binary planets, no roughly-same-size-at-all-trojan-points
>> >planets, etc? Why?
>>
>> That is not a stable configuration,
>
>Stability wasn't part of the adopted definition, that I recall.

And binary planets are potentially stable.


As none are known to exist, and are very unlikely to exist in this solar system, that doesn't matter.

>> the trojans are only stable because
>> their mass is negligible compared to their primary. Theia, the body that
>> caused the big splash, has been theorised to have formed in one of the
>> trojan grew too big to be stable there and hit Earth.
>
>So neither Theia nor Earth was a planet at the time of the collision?
>Doesn't that seem like a sub-optimal definition of 'planet'?

At the time they were protoplanets rather than planets, as that was
during the formation process.

The definition doesn't say anything about protoplanets, either.

It doesn't much matter anyway as we hadn't evolved at the time

So? What does that have to do with whether a given body at a given
time is, or is not, a planet? (The proposed definition doesn't specify
"humanity already evolved, not yet extinct" either.)

and this is an attempt at classifying the existing bodies in the solar system.

No -- it's an attempt at defining 'planet'. The problem seems to be
that nobody at the IAU somehow got confused, and gave up "attempting to
define 'planet'" in favor of the utterly dissimilar activity
"attempting to classify the existing bodies in the solar system".

It is defining planet for the purpose of classifying currently existing objects in this solar system.


>>
>> We don't really need a formal definition for outside the solar system as
>> we can't actually detect anything small enough to not be a planet (with
>> the exception of bodies in the solar system of the millisecond pulsar
>> PSR B1257+12).

Let me pause here for a moment to express astonishment. "We don't need
to define 'red' or 'blue', because at the moment the lights are off, so
we wouldn't be able to see the difference anyway". I'm speechless.


It's more "We'll leave the issue of defining extra solar planets more precisely until we have more information and we actually know of some objects that might actually not be planets." At the moment all of the extra solar system bodies known, outside that pulsar's solar system, are massive enough that they are definitely planets.



[...]

>I seem to be having a hard time expressing my objection to this way of
>thinking, so let me try again.
>
>If the definition is not meant to be extensible to not-yet-discovered
>objects, then IT IS NOT A DEFINITION. It's just a list, and an
>arbitrary list at that.
>
>If the definition is not meant to apply to extra-solar objects, despite
>that fact that the word has been used to apply to certain extra-solar
>objects (and hypothetical not-yet-observed extra-solar objects) for
>decades (if not centuries), then it behooves the IAU to define "solar
>planet" or some equivalent term that explicitly restricts its scope to
>only planets in the solar system. And then to explain why the solar
>system is special, and needs a special term for *its*
>only-planets-for-this-system.

The specific reason is that this by far the best known system

So you would think the definition would at least work *here*, yes, but
that's not a reason to deliberate make it broken elsewhere.


The earlier drafts did include extra solar planets. It was decided to leave the issue of other solar systems until we had more information to work with. As we are currently in a period of rapid discovery it was decided that any definition that attempted to cover other systems would become obsolete very quickly. Hence the deliberate and intentional limitation to the solar system.


and is the only system around a normal star

All stars are normal, if you're an astronomer, which presumably these
IAU folk are.


I mean as opposed to a millisecond pulsar.

which has any known bodies small enough to not be planets.

Um, 'planet' wasn't defined yet, so that can't have been the reasoning.
Well, not *valid* reasoning anyway.

The upper limit is clearly defined the lower limit currently irrelevant.

Once again: a definition that is not extensible to new observations IS
NOT A DEFINITION. It's an arbitrary labelling of known objects --
which is pretty much the *opposite* of a definition. A definition's
purpose is to provide a decision rule for how to label future
observations. Saying that it doesn't need to, because they haven't
been observed yet, is to completely miss the point of coming up with a
definition in the first place.

Actually it is extensible to new observations, if a massive body were discovered out past the kuiper belt it might be classified as a planet (not thought likely, but our theories on planetary formation have been wrong before)


>It's not as though the word 'planet' didn't already have a
>mostly-well-understood meaning, which astronomers used in their
>technical papers, that was NOT restricted to this system.

I note you didn't reply to this bit. Why is the current fiasco any
different from redefining 'star' to mean "the Sun", and deferring any
attempt to define non-Sun formerly-called-stars?

>> At the moment, the de facto definition is anything orbiting a star
>>which is too small to be a brown dwarf.
>
>OK; why isn't that good enough for Sol, too?
>

Because we can see the left over debris in this system, as it is very
close.

Yeeeeeeees? And so?

We don't need to explicitly exclude said rubble in other systems
as we can't see it anyway.

Astronomers are solipsists? All of them? Weird.


We can't see it, so we don't actually know anything about it. Creating a classificatory scheme in the total absence of information is, at best, pointless.

By the time we need to make such a distinction we should have a lot more
knowledge about the smaller stuff so may be able to come up with better
definition. We don't know how typical of solar systems in general ours
is. A definition based on guesswork and the principle of mediocrity may
turn out to be wildly inappropriate if it turns out our system is
atypical. On the whole it seems better to leave it until we have better
information and some actual need for a definition.

If they really thought that, then WHY MAKE A DEFINITION NOW? What
urgent problem did it solve? What urgent purpose does a definition of
solar-system-only-planet serve?


It is of use for defining objects in the solar system, which is useful for solar system astronomy.

They are well aware of the purpose of this definition, to establish a
lower limit for planet in this solar system.

If I could figure out what that's supposed to mean (when 'planet' is
still undefined), or why any astronomer would care, I would be a lot
closer to figuring out what you're trying to say.

The aim was to establish which objects were planets and which ones belonged to the various classes of smaller objects.
--
Great Internet Mersenne Prime Search http://www.mersenne.org/prime.htm
Brett Paul Dunbar
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