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Definition of Planet to be Announced in September
MasaMuneCyrus writes "After over seven years of debating, the International Astronomical Union announced that it expects to announce the official definition of a planet in September. After many-a-deadlock, they handed the task of deciding exactly what a planet is to a new committee, which includes historians and educators. 'They wanted a different perspective from that of planetary scientists,' said Edward Bowell, an astronomer at Lowell Observatory who is also vice president of the IAU's Division III-Planetary Systems Sciences group. If all goes according to plan, the wording will be proposed in their 12-day General Assembly meeting in Prague."
I hope Pluto finally gets excluded from planet definition. It's too small (only twice the size as it's "moon", Charon, and a little less than a fifth as massive as Luna), it's out of the plane of the elliptic (a trait shared with objects like comets, but not any planets), and it's not even orbiting in a stable configuration with regard to Neptune (for part of it's orbit, Pluto is in fact eighth, and Neptune ninth).
Then there's the fact that it only really got counted as a planet in the first place because astronomers at the time of it's discovery were hung up on the idea of discovering a ninth planet. They thought they found a disturbance in Neptune's orbit, which they attributed to a ninth planet, but ended up being caused by the fact that they were working from bad data about Neptune's mass. Pluto's much too small to have any effect on Neptune's orbit.
This might finally put the final nail in the coffin of the idea of nine major planets in our solar system. We can only hope.
I've always thought that looking for intelligent life on other planets is premature, since we haven't conclusively found intelligent life on this one yet :-P
But seriously, what difference does life make? Any planet could support life if you put it in a habitat of some sort. Even gas giants could support life if said habitats floated (and yes, that hs been proposed - human breathable air is a lifting gas when the outside pressure is high enough). If you mean indigenous life, that's another story - we're very likely alone here in our solar system, so seeking to define a livable planet when we have exactly one example is a bit premature.
If you want to find examples of life outside our solar system, good luck. The best we can do currently is look for either signs of intelligence (which is SETI's business), or else look for a planet that shows signs of an oxygen atmospherem, since that would imply biological processes. We're already doing this IIRC.
And even then we'd be unable to show that a rocky body of the right temperature didn't have life - anaeorbic (sp?) life gets along just fine and dandy without toxic oxygen fouling up their enviroment. That doesn't even get into the possibility of life forms existing with completely different chemical composition, which we can't even make an educated guess about.
We couldn't even show that there isn't intelligent life somewhere, since there is no guarantee that they'd use the same methods of communicating as we do - all we can do is hope they're trying to contant us.
Erotic is when you use a feather. Exotic is when you use the whole chicken.
Planet: object orbiting a star, massive enough to be spherical under its own weight, but not enough to undergo nuclear fusion.
Major planets: the eight (Mecury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).
Minor planets: the moon, all the spherical satellites of the major planets, Pluto, all the spherical asteroids in the asteroid belt and all similar spherical kuiper belt objects.
OTOH the other two moons are small enough to be called moons.
The Raven
for what it's worth (not much):
planet = mostly rock/metal sphere with a significant atmosphere (what "significant" means becomes a point of contention then of course)
asteroid = solid rock/metal object that is not spherical
moon = rock/metal sphere without an atmosphere
a gas giant should be considered as something different than a planet (mostly gas, obviously spherical)... a star is simply a gas giant that has achieved thermonuclear fusion... and in between you have your brown dwarfs and other objects occuring at the end of a star's life time/ before it's lifetime/ malformed and never quite stars, etc.
and comets should also come to mean any agglomeration of ice and rock and dust that is loosely packed, not just those we see streaming towards the sun on a regular basis... as we explore the oort cloud, we'll find plenty of these "dormant" comets
and most importantly: all of these objects should be defined independently of what they orbit
so mercury isn't a planet, it's a moon of the sun
likewise, pluto is a moon of the sun
and ceres and vesta are moons of the sun (small perfectly spherical "asteroids")
titan isn't a moon, it's a planet of saturn (it has a significant atmosphere)
the most important thing i think, no matter what nomenclature is agreed upon, is that as we discover weirder extrasolar objects out there, the "what it orbits" part of an object's identity should come to mean something totally different than "what it's made of"
and size should never have meaning
then of course, we have to come to grips with direction of orbit, orbits outside the elliptical, orbits with bizarre shapes, binary/ tertiary objects, binary/ tertiary/ quartanery star systems, etc.
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Is it that important to have a precise definition of what is a planet and what is not?
Unless you are going to precisely define every single astronomical object. from dust to galaxy filaments.
I suspect that someone is going to claim the possession over those planets (apply the definition here).
Maybe Computers will never be as intelligent as Humans.
For sure they won't ever become so stupid. [VR-1988]
Pluto is a planet only because of tradition, and I for one will be somewhat disappointed if Pluto is NOT demoted, because scientific chategorization based on emotion, "what will the kids think" and public opinion is the reason we're dealing with this creationism bullshit.
The only logical definition of planethood that would include Pluto is a broad one, such as any object massive enough to be roughly spherical. If that's what they decide, then fine - but then we have to add at least a couple of asteroids to the tally too.
This space available.
Why not categorize and measure like G(ravity), N(ewton), K(elvin)?
If an object is one half earth's mass, just call it 1/2 Planet. If the object orbits around a planet, just call it satellite or moon or subplanet. After all, planet means "to wander". What doesn't wander around the universe?
*Middle English, from Old French planete, from Late Latin planta, from Greek plants, variant of plans, plant, from plansthai, to wander.
ref. dictionary.com
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The definition should be based on surface gravity, or average surface gravity, that way the definition of a planet would be usefult. The watershed could be say, an average surface gravity that would make it feasable to build a facility of some kind where a long term human prensence can be sustained without major risk to health.
This would also be useful as objects could be classed with a relevance that would be important to any future explorer. Even non elliptical objects could still be given a metric to judge their habitability.
"Sir, object is a Class G planetoid! Our noses will be crushed by our feet if we set foot on it."
"I see"
"However sir, the Halo Ring, despite not being isomorphic to a sphere, does qualify as a planet due to a reasonable average surface gravity."
"Cue music. We're going in.
But even dyson spheres could qualify as planets.
May the Maths Be with you!
A planet must:
Revolve around a star
within a certain maximum aphelion
having a maximum elliptic
Be large enough in volume
Not be artificial in nature (this provides that any intelligence in this universe creating an object that would fit the prior criteria would not be allowed to call it a planet)
Define maximum aphelion and maximum elliptic and minimum volume. What else is there?
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Shouldn't the analysis on extrasolar data be based on the properties of celestial bodies, not if they're called planets or not? It's just a gut feeling, but defining "planet" and then looking specifically for those for life could maybe even hurt the discovery of such celestial bodies with life (which I believe is the extrasolar body analysis that's most interesting to humanity), in case the definition was such to not cover all possible bodies.
Not going specifically for what's defined as "planets" feels like freeing yourself of the boundaries a group of scientists thought up in a conference room, and that feels quite important when we know so little about extrasolar life.
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As Jesapoo points out, it's not about size, but as important as orbit eccentricity is material composition. Planets are historically categorized into two buckets based on their composition -- "terrestrial", which are mostly rock (mercury, venus, earth, mars), and "jovian", which are mostly gas (jupiter, saturn, uranus, and neptune). And then there are comets, which are mostly dirty ice and frozen gas with some rocks.
Pluto is cometary in composition, which has led some to classify it as a comet rather than as a planet. Frankly I can see the argument. Perhaps the best way out is to define "planet" such that some comets can be planets?
-- TTK