Re: Orbital dominance

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
>Brett Paul Dunbar wrote:
>> In message <1156435693.925223.207120@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, David
>> Tate <dtate@xxxxxxx> writes
>> >OK, I'm not anything like an astronomer, but I really can't see what
>> >"orbital dominance" has to do with whether something is a planet or
>> >not. And I don't understand why "binary planet" isn't a natural and
>> >obvious variation, the same way "binary star" is.
>>
>> The aim is to exclude orbital debris, like the Asteroid and Kuiper belt
>> objects. A planet has to be much bigger than everything else in its
>> orbital region (moons, trojans, stuff in an orbital resonance &c.)
>> combined. Hence Neptune is a planet, Pluto which is a tiny iceball in a
>> 3:2 resonance with Neptune is not, as Neptune is nearly 8000 times
>> Pluto's mass.
>
>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.

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. It doesn't much matter anyway as we hadn't evolved at the time and this is an attempt at classifying the existing bodies in the solar system.


>> As this definition only applies to the solar system, which lacks binary
>> planets the question of defining them is moot.
>
>I was assuming that the "only applies to the solar system" part was a
>simple oversight, on the grounds that it is unutterably inane for an
>official body of astronomers to define 'planet' in a way that is
>specific to the solar system. As well define 'star' in terms that are
>specific to the solar system. Does the next conclave get to debate the
>(independent) definition of "extra-solar planetary body"?

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). A formal definition may be adopted when one is actually
needed and we know a bit more about their characteristics.

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 and is the only system around a normal star which has any known bodies small enough to not be planets. The upper limit is clearly defined the lower limit currently irrelevant.

A more formal definition of extra solar planet will be needed at some point in the future but at the moment it isn't


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.

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

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.

>Or, to put it another way: if the definition is to be specific to the
>solar system, then a far more honest and defensible approach would be
>to just issue the list, and forget trying to concoct a "featherless
>biped" sort of definition that happens, contingently, to produce the
>same list (today).

The definition of the class of objects is key

Apparently not, if it's both arbitrary and restricted to just one
stellar system.

My fundament problem here is not that this or that body is (or is not)
a 'planet' any more; it's that the IAU doesn't even seem to know what
the *point* of a definition is.

They are well aware of the purpose of this definition, to establish a lower limit for planet in this solar system. The eight planets are all the sole major bodies in their orbital regions. Even the least massive is more than double the mass of the most massive non-planet.
--
Great Internet Mersenne Prime Search http://www.mersenne.org/prime.htm
Brett Paul Dunbar
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