How Would You Define a Planet?

It doesn't come easy asks: "The argument over the definition of a planet continues. So far, two definitions are favored but without much consensus so far: base the definition of a planet simply on an object's size. Pluto would be near the lower limit and the newly discovered Kuiper Belt objects could also qualify, giving us 10 or 11 planets so far; or define the single dominant body in its immediate neighborhood as the only qualifying object for planetary status. If no one body dominated (such as the millions of individual asteroids in the asteroid belt) then none would qualify for planetary status. In this case Pluto would be disqualified (Neptune would be the dominant body in Pluto's region of space), and the newly discovered Kuiper Belt objects would also fail to qualify. The International Astronomical Union (IAU) working group charged with pinning down the definition of a planet may vote on the proposals within the next two weeks (or they may decide to start all over again with something new). Maybe Slashdot readers can give them some help. How would you define a planet?"

Disqualifying Pluto

[calibanDNS]

The problem with disqualifying Pluto because of Neptune being the more dominant body falls apart when you consider the eccentric orbit of Pluto and just how far that takes it from Neptune's "region of space".

What exactly is the definition of a region of space?

How much larger must an object be than its neighbors in order to be considered the dominant object of its neighbors? Twice as large? Four times?

Re:Shape and orbit

[slavemowgli]

Round shape pretty much depends on size only, yes; the bigger an object is, the smoother it'll always be (which is why the highest mountain on Earth is less than 9 km above sea level, while on Mars, which is smaller than Earth, it's more than 27 km). However, pretty much everything that's bigger than an asteroid will have a more or less round shape overall, so that's a non-criterion.

A better idea that I've heard being discussed would be to abandon the term "planet" altogether and instead label objects according to their characteristics - so Mercury, Venus, Earth and Mars would be "earth-likes", Jupitur, Saturn, Neptun and Uranus would be "gas giants", and Pluto, Sedna etc. would be - for example - "ice dwarves". "Large Kuiper belt objects" (LKBOs?) would also be a good term, of course, but "ice dwarves" could probably be applied to objects in other solar systems more easily, as it doesn't rely on the existence of a Kuiper belt to define the objects being talked about.

howabout...

[circletimessquare]

anything with an appreciable atmosphere that is NOT a gas giant = planet ...REGARDLESS of what it orbits

therefore, mercury would NOT be a planet (more like a moon of the sun)

and titan, even though it orbits saturn, WOULD be a planet

i think that makes most the most sense: what an object orbits shouldn't matter, it's composition should be the largest consideration

some other nomenclature can address what it orbits ("a moon of the sun" or "a planet of saturn")

it should be considered either

• a moon (like mercury or pluto)
  
  
• a planet (like mars or titan)
  
  
• an asteroid (like deimos and phobos... called moons of mars, they are clearly NOT moons, but captured asteroids of the sun) if it is not spherical
  
  
• and then we have your comets

REGARDLESS of what it orbits

Re:Why bother?

[Cabriel]

I agree with this, at least in part. Usage of "Planet" has become basically meaningless. Sure, it is useful for differentiating bodies that orbit the Sun, but the debate around the definition is evidence that it is too constrictive. I would rather call them "Solar Orbital Bodies" (although, in afterthought, it might be wise to use a term that has an abbreviation less offensive than "SOB").

We could define "Small Orbital Bodies" and "Large Orbital Bodies" while still utilizing other terms, such as "brown dwarf" and "gas giant" which are both based on other qualities of the orbital bodies. Even "Comet" and "Meteor" could remain in usage because, as I am under the impression of, those are defined by the make-up of the body.

While using the terms "Small" and "Large" to define size, it would be simple to arbitrate points within the ranges because every size fits into a given range.

Understanding that I'm not an astronomer, my assumption is that, for some reason, I will be proven wrong. I might appreciate if someone could tell me what I am wrong about. ^_^

The definative answer:

[robbak]

This is what I think to be the key quote from Wiki (yes, mostly because I agree with it):

"Scientists have not yet realized that the term planet no longer belongs to them. But, quite clearly, it does not... The word "planet" has been around much longer than modern science." It may very well be that children will continue to learn of the nine planets in school while scientists work in a solar system of eight, or hundreds, or even abandon the term "planet" altogether. For now, "planet", like "continent," is a word caught between the scientific and cultural worlds, without a clear meaning.

Re:Shape and orbit

[zsau]

Mars has a sea-level? I don't mean to sound overcorrect or anything, but isn't the comparison you draw inaccurate? If you got rid of all the water from Earth and tried making a comparable measurement you might find that the tallest mountain on Earth was more than 9 km off the replacement for sealevel, I would've thought. I doubt it'd increase three times, though; but what I'm more saying is how comparable are the two figures? (I really don't know; it might be that the sea is only a hundred metres deep on average and that make the 9 km 9.1 km, which really is still 9 km...)

Re:caveat

[madstork2000]

Then you have to define "belt" - if you look atthe relative distances of things the object in the kuiper "belt" are more spread out that say the objects (read planets) in the inner solar system of roughly similar size.

Personally I think it is silly to disqualify something because it has "neighbors" or orbits in a "belt".

My $.02:
Any object that revolves around a star, and is not a star, has enough mass to be roughly spherical (say +/- 1% of a perfect sphere) due to its gravity is a PLANET.

Objects that are roughly spherical that revolve around planets are MOONS (regardless of size.) If two objects revolve around each other and their center of gravity lies outside the radius of either partner then it is a binary planet.

Objects that revolve around a star that are not roughly spherical are MINOR OBJECTS. This leads us to a bit of a problem because under my definition there would be no distinction between our friends the comets and asteroids.

I imagine there could be a further classification, based on the shape of the orbit - so we can continue to have "comets" and "asteroids". However I do not like using the orbit shape in any definition.

Objects that revolve around a planet taht are not roughly spherical are SATELLITES.

By the way the reason WHY i do not like using the shape of the orbit or something like the vicinity of other objects in the definition is simple. Those characteristics can be changed. Orbits gradually change over time, especially early in the life cycle of the solar system.

Granted objects are "captured" by planets and stars, and "ejected" in the same way. However, given enough time (and any external influences) don't orbits tend to become circular? So just because comets have highly elliptical orbits now does not mean they will be elliptical forever. The orbital shape is a TRANSITION characteristic. It is not inherent to the object.

Oh well I have rambled on way to long about this....I am ponderig the much larger question - "Why do I care?"

Re:anything with a roman god name

[ThJ]

In Norwegian, this planet's official name is Jorden (pron. YOOR-enn), from "jord" (earth, mold). The -en suffix is the determinator, making the word translate as "The Earth". It's interesting to note that "Earth" derives from Old English "eorthe" (pron. yorth), which is starting to sound an awful lot like "yoord", the literal pronounciation of "jord" if you don't mute the D. This is probably a lot of speculation on my part, but still. ;)