What Defines a Planet?

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Discussion Overview

The discussion centers around the definition of a planet, particularly questioning the classification of gas giants like Jupiter, Saturn, Uranus, and Neptune. Participants explore various criteria that could define a planet, including physical characteristics and orbital dynamics.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant proposes a personal definition of a planet that includes being solid-cored, spherical, in a circular orbit around a sun, not orbiting another planet, and not being a failed star or a gas ball.
  • Another participant expresses indifference to the number of planets, suggesting that the discussion may be awkward for the scientific community.
  • A different participant argues that Jupiter likely has a solid core and questions the arbitrariness of the proposed criteria for defining a planet.
  • It is noted that gas giants are already categorized as such, distinguishing them from terrestrial planets.
  • Some participants express disagreement with specific criteria, such as the requirement of not orbiting another planet and the nature of orbital shapes.
  • Another participant mentions the existence of giant exoplanets and discusses the survival challenges on planets like Venus and Mars, suggesting that definitions should be simple.

Areas of Agreement / Disagreement

Participants exhibit multiple competing views regarding the definition of a planet, with no consensus reached on the criteria proposed.

Contextual Notes

Some criteria proposed are seen as arbitrary, and there are unresolved questions regarding the physical characteristics of gas giants and their classification.

Aquafire
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When is a Planet a planet...?

and when is it just a ball of gas..?

My personal belief is that Jupiter, Saturn, Neptune and Uranus...should NOT be considered planets.

How can you call a spheroid ball of rotating gases a planet ?

Surely, if you can't step on it, land on it or float on it...its not a planet..

So for what its worth my definition of a Planet would any object that is..

A. Solid cored.

B. Spherical

C. In an circular or as close to circular orbit around a Sun.

D. Not in orbit around another planet.

E. Is not a failed or collapsed star or proto-star..

F. Is not a ball of gas with a liquid core...So...maybe..(since we're tossing out Pluto)...we should also give Jupiter, Saturn, Uranus and Neptune the flick...

I stress, this is a personal definition.

In anycase, i'd like to read other peoples POV's... on this..

Cheers

Aquafire
 
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I don't really care wheter there are five, or eight, or fiftyseven planets. It's just a matter of definition.
It think this discussion is kind of awkward for the scientific community.
 
First, Jupiter does likely have a solid core and if it doesn't, it's only because it is so hot it is liquid.

But that's besides the point: you most certainly can float on Jupiter, just like you can float either on the Earth's ocean or in its atmosphere depending on your buoyancy.

Also, why? Why do you choose these criteria? They seem arbitrary to me. (not that other defnitions wouldn't be...)
 
Something else I forgot - gas giants are already labeled as "gas giants" - as opposed to "terrestrial" planets.
 
Those are my thoughts as well, although I disagree with letter D, and I'm unsure about letter C.
 
As Russ pointed out, Jupiter is believed to have a solid core, probably metallic hydrogen.

Surely, if you can't step on it, land on it or float on it...its not a planet..
Is not a valid criterion. But as Russ also indicated, there are giant planets with a lot of gas or liquid, which are composed mainly hydrogen, methane, ammonia, . . . and there are 'rocky', or terrestrial planets.

JUPITER AND SATURN EVOLUTION BY GAS ACCRETION ONTO A SOLID CORE pdf - use 'save target as'

Then there is the matter of the giant exoplanets which are periodically discovered which are Jupiter-sized or larger, or Neptune-sized.

Giant Planets Orbiting Faraway Stars - http://astro.berkeley.edu/~gmarcy/0398marcy.html

Also, if a person tried to land on Venus, he/she would be 'cooked' pretty quickly. As for Mars, one has to take one's oxygen to breath and one would need an artificial environment in order to survive.

Definitions are perhaps arbitrary and should be as simple as possible.
 
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We know the sun is orbiting the galaxy's center. Is it also spinning?
 
heliocentricprose said:
We know the sun is orbiting the galaxy's center. Is it also spinning?


Yes .
 

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