What Is a Planet? Options

[djellison]

My objection has nothing to do with what is and isn't a planet under the definition....

well, actually, it does.

As it stands, the status of Jupiter, Mars and Earth as planets is frankly, shakey.

So - as long as you have no problem with a rule that excludes Pluto....and Earth....then we're all good


Doug

[Pavel]

Well, that could be spun into an April Fool joke. IAU proclaims that Earth is a minor planet now because, as it turns out, it failed to clear its neighborhood. Earth gets its provisional MPC number - now it's 100000 Earth. NASA assures everybody that it's not curtailing its Earth observation programs yet, as long as financing allows, but it will gradually assign more resources to the real planets, such as Mars. In the meantime, protests against the IAC decision are planned worldwide.

[David]

I want to argue strenuously for a definition that will distinguish objects of Neptune-size or larger from the rest of the rubble. Maybe "large enough not to have a clearly defined solid surface"? I realize there's a need to distinguish small bodies like Neptune and Uranus from real planets like Jupiter and Saturn, but I don't believe that the difference is so great that we can't call them planets anymore, even if they weren't discovered until after the American Revolution. If necessary, I'll settle for a compromise and call Neptune and Uranus "dwarf planets".

[JRehling]

I want to argue strenuously for a definition that will distinguish objects of Neptune-size or larger from the rest of the rubble. Maybe "large enough not to have a clearly defined solid surface"? I realize there's a need to distinguish small bodies like Neptune and Uranus from real planets like Jupiter and Saturn, but I don't believe that the difference is so great that we can't call them planets anymore, even if they weren't discovered until after the American Revolution. If necessary, I'll settle for a compromise and call Neptune and Uranus "dwarf planets".

Funny stuff. And yet, as you look down the list of all the sun-orbiting bodies, ranked by size, the biggest gaps (ratio of an object's diameter to that of the next-biggest object) are Earth/Neptune (by FAR the biggest gap: 0.258) and Uranus/Saturn.

There is a gap of about 2:1 between Mercury and Eris, although there's a good chance that future discoveries will wipe that gap out entirely. And that is the gap that the IAU's planet definition has chosen to make hay of, although they chose different variables that correlate with size to try to make the gap seem bigger. Because they are clinging to the assumptions that "planet" is a meaningful category, that it shouldn't have too many members (how scientific!) and that Mercury has to remain in the set.

If it takes more discoveries to drive this illogic out of people, then so be it -- the trend suggests that we may keep discovering yet-bigger objects out there for some time. But some more thought should do the same thing without more discoveries.

[Mongo]

Okay, I would like to step back from this debate and ask: What is a scientifically defensible classification system that includes all objects that could be included under the term 'planet'? In fact, the system I came up with includes all stars, and all sub-planetary bodies as well.

Basically, this is a two-axis system. One axis classifies objects by mass, with 'bins' spanning a factor of two in mass, while the second axis classifies objects by their position in an orbital hierarchy.

MASS

The largest stars appear to be somewhere between 100 and 150 times the mass of the Sun. Occasionally, a larger-mass candidate is announced, but they inevitably turn out to be binary systems, or otherwise below the limit. The Sun is a bit over 330 thousand Earths in mass, so the most massive known stars would have masses of around 50 million Earths.

Class : Mass range (in Earths) : Examples

Class 1 : >32,768,000
Class 2 : 16,384,000 - 32,768,000
Class 3 : 8,192,000 - 16,384,000
Class 4 : 4,096,000 - 8,192,000
Class 5 : 2,048,000 - 4,096,000
Class 6 : 1,024,000 - 2,048,000
Class 7 : 512,000 - 1,024,000
Class 8 : 256,000 - 512,000 : Sun (332,830)
Class 9 : 128,000 - 256,000
Class 10 : 64,000 - 128,000
Class 11 : 32,000 - 64,000
Class 12 : 16,000 - 32,000 : lower mass limit for hydrogen burning (~24,000)
Class 13 : 8,000 - 16,000
Class 14 : 4,000 - 8,000 : HD41004B b (>5,850 - largest known 'planet' by radial velocity - might be a brown dwarf)
Class 15 : 2,000 - 4,000
Class 16 : 1,000 - 2,000
Class 17 : 500 - 1,000
Class 18 : 250 - 500 : Jupiter (317.9)
Class 19 : 125 - 250
Class 20 : 62.5 - 125 : Saturn (95.18)
Class 21 : 31.25 - 62.5
Class 22 : 15.625 - 31.25 : Neptune (17.14)
Class 23 : 7.812 - 15.625 : Uranus (14.54)
Class 24 : 3.906 - 7.812 : Gliese 876 d (>7.3117 - smallest known planet by radial velocity)
Class 25 : 1.953 - 3.906
Class 26 : 0.976 - 1.953 : Earth (1)
Class 27 : 0.488 - 0.976 : Venus (0.815)
Class 28 : 0.244 - 0.488
Class 29 : 0.122 - 0.244
Class 30 : 0.061 - 0.122 : Mars (0.1075)
Class 31 : 0.0305 - 0.061 : Mercury (0.0553)
Class 32 : 0.0152 - 0.0305 : Ganymede (0.025) Titan (0.0225) Callisto (0.018) PSR 1257+12 b (0.0222 - smallest known pulsar planet)
Class 33 : 0.0076 - 0.0152 : Io (0.015) Luna (0.012) Europa (0.008)
Class 34 : 0.0038 - 0.0076
Class 35 : 0.0019 - 0.0038 : Triton (0.00359) Eris (0.00268) Pluto (0.00219)
Class 36 : 0.000905 - 0.0019
Class 37 : 0.000452 - 0.000905
Class 38 : 0.000226 - 0.000452
Class 39 : 0.000113 - 0.000226
Class 40 : 0.000056 - 0.000113
Class 41 : 0.000028 - 0.000056
Class 42 : 0.000014 - 0.000028
Class 43 : 0.000007 - 0.000014
.
.
.

In theory, this could be extended to individual 'dust grains'

ORBITAL HIERARCHY

a : free intergalactic wanderer
b : bound to a galaxy, but no smaller object (not counting open clusters, which are dynamically unstable over billions of years)
c : bound to a globular cluster, but no smaller object
d : bound to a single object which is itself class a, b or c
e : bound to a class d object
f : bound to a class e object - i.e. a moon of a planet orbiting the less massive star of a binary star system

This axis could be sub-divided further, with designations for Trojan-type orbits, 'double planets', etc.

EXAMPLE DESIGNATIONS

Sun : Class 8b
Jupiter : Class 18d
Earth : Class 26d
Ganymede : Class 32e
Pluto : Class 35d

If it were up to me, I might junk the existing system of nomenclature and go with something like this. Does this sound like a reasonable system? (not that it has any chance of being adopted)

Bill

[nprev]

Argh...

I'm about ready to junk the term "planet" in favor of "stuff orbiting around a star" (STOAAS)! After all, the entire Solar System is just a minor fraction of the mass of the whole when the Sun is included, and most of it exists in discrete packages along a continuum with local variables based on mass and the amount of solar radiation received.

Planet is a subjective term...let's just call 'em as we feel 'em. Unscientific as hell, of course, but realistic...

[dvandorn]

I don't think you'll ever get rid of the term "planet", if for no other reason than a century and more of science fiction has entrenched the term solidly within the human psyche.

When humans imagine going out into space and exploring the Cosmos, they think about living on chunks of rock with roughly the same size, shape and general characteristics as the Earth. We imagine exploring, but not taking up residence on, other chunks that are less hospitable.

It doesn't matter if you're talking about David Brin or Arthur C. Clarke or Gene Roddenberry -- when you tell stories about traveling to distant worlds, you have to have a generic name for the bodies you visit, and that name, for better or for worse, is "planet".

So, "planet" will continue to describe those chunks of rock that humans imagine visiting. Otherwise, billions of words of science fiction will instantly become "quaintly out-of-date," and that just ain't gonna happen...

-the other Doug