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."

Will they finally discount pluto?

[Shrithe]

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.

Re:Who cares?

[RsG]

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.

How about the following?

[cy_a253]

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.

Re:How about the following?

[RsG]

I think that last part that would screw you up. How large, or how spherical, would a random orbiting body have to be to fit the defninition of "minor planet"? You're gonna need an arbitrary cutoff no matter what you do.

Plus, how do brown dwarf stars fit into that definition? A brown dwarf can't fuse hydrogen, and in many regards is similar to a planet; however it can fuse deuterium, so it does undergo fusion during it's initial collapse. Wouldn't a brown dwarf fit your definition of a major planet, since it is not undergoing fusion? Or would it have to be orbiting a star (as is the case in a binary system) first?

What about rogue planets? They don't fit the "orbiting a star" half of the planet definition you give. What does that make them, their own category, or just interstellar rocks? And if they do have their own category, do the aformentioned brown dwarfs fall under it?

"Moon system"

[vlad_petric]

The baricenter of Pluto and Charon is outside Pluto (in space). That's one reason, I guess, for scientist to also call it a "dual planet" system.

OTOH the other two moons are small enough to be called moons.

my homebrew nomenclature

[circletimessquare]

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.

I love western thinking

[Weaselmancer]

Things are either black or white, up or down, good or bad. There are no shades of grey.

Can't we just say that there are different objects in the universe that have similar properties? What's wrong with saying an object is 30% like the planet we're on, but 70% like Jupiter?

Must everything have a category?

It's a real flaw in western thinking. We can't just simply let things be - we have to pin them to cork boards like preserved butterflies. Why not just describe what you find as you found it? Nature doesn't fit things into categories, why should we?

Importance of a definition

[VincenzoRomano]

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 [wikipedia.org]. from dust to galaxy filaments.
I suspect that someone is going to claim the possession over those planets (apply the definition here).

Re:anyone want to place bets on

[Anonymous Coward]

Since everyone is still going to be calling Pluto a planet regardless, and it doesn't really matter - how about a definition which preserves the status quo:

'A planet is any astronomical body whose mass is greater than or equal to the mass of pluto (as at midnight June 10th 2006) and is in orbit around a star and not in orbit around any body which is not a star'

Now, if we could just sort out the real stars from those pesky brown dwarfs.

Comment removed

[account_deleted]

Comment removed based on user account deletion

1 Earth = 1 Planet

[layer3switch]

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

Re:Will they finally discount pluto?

[moranar]

This is very limited thinking. Basing the definition of planet only on our own solar system leaves lots of things to be desired.
E.g., maybe other planetary systems have more planets out of the elliptical plane, if they even have one, and might have even more irregular orbits than that of Pluto.

easy

[Anonymous Coward]

I've read about this debate numerous times, and I've seen size, atmosphere, orbit plane, eccentricity, composition, mass, shape, whether it undergoes fission, what it orbits, and probably other things considered as factors. One factor I've never seen mentioned before is whether or not the body lies in a unique orbit.

I think the following definition should suffice: a planet is a non-fusioning body (or system of two (or more) near-mass bodies in close orbit) in orbit around a star (or system of two...), massive enough to maintain a spherical shape, lying in an orbit that is unique, except for its satellites (or twins).

This definition includes the current nine planets, none of their moons, and none of the "belts." I dont think the composition, presence of an atmosphere, orbital plane or eccentricity, or size should be important, but it should be in a first-order orbit around a star, and it should be able to maintain a spherical shape, and probably most importantly, it should be the only body that follows its orbit, unless it has satellites. As far as I know, there aren't any currently known objects that fit this description besides the nine planets and the three or four bodies that have recently been discovered beyond pluto, and I have no problem calling those planets.

Surface Gravity

[ObsessiveMathsFreak]

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.

Three(ish) conditions

[MrNougat]

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?

Re:I love western thinking

[Jugalator]

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.

Re:anyone want to place bets on

[fbjon]

I wouldn't mind adding a few more, but perhaps we could have a "minor planet" definition for pluto-sized balls?

terrestrial, jovian, cometary

[TTK Ciar]

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