An idea for defining planets.

[Daecon]

I've heard that there is a lack of consensus about whether Pluto should be considered a planet or not when compared to other similar bodies, and that outer ring planets and inner ring planets also have nothing in common to both fall under the same definition of "planet".

 

What about a system where planets are categorised based on the thickness of their atmospheres?

 

Either by the depth to/from the core or by percentage of radius that is their atmosphere.

 

For example, you could say that Planet X is a gas giant with a 1000Km thick atmosphere with a small iron core, or you could say that Planet X has a ratio of 90% atmosphere to 10% solid core, etc.

 

Compared to Planet Y which is a rocky planet with 10Km of atmosphere, or a ratio of 5% atmosphere to 95% solid "core".

 

Compared to another body in the solar system with doesn't have an atmosphere at all, like a typical asteroid.

 

Or whatever the actual real numbers would be in real-life situations.

 

If atmosphere-less bodies were described as type-1 planets, those with very small atmospheres were called type-2 planets, and so on up to Jupiter would would be a type-x planet depending on whatever basis you would use for incrementation.

 

Then with planets such as Pluto and Mercury with similar atmospheric thickness so would fall under the same category.

[ajb]

Some moons have an atmosphere. They will then be included in the definition of a "planet"?

 

Comets are also surrounded by a cloud of gas and particles known as the coma. This is also like an atmosphere.

 

So, I think it is a nice idea, and it does make the distinction between gas giants and rocky planets. However, I think it would include other objects not usually called planets.

[Sisyphus]

Yeah, that sounds more like an idea for classifying objects you already agree fall within the category of "planets." After all, an "atmosphere-less body" could be just about anything. Including me.

[Arch2008]

Why not simply classify planets by their mass? That's pretty much what we do with stars and black holes. The Earth’s mass is approximately 6.0 x 10^24 kg, so you could have this as a unit. This way you could tell at a glance which planets would have one MotE (mass of the Earth) and therefore be in our normal gravity range. This would apply to objects with less mass than a brown dwarf star but large enough to be a sphere and that orbit a star (a moon would then orbit a planet). So Venus would be something like a .91 MotE planet.

[ajb]

Why not simply classify planets by their mass? That's pretty much what we do with stars and black holes. The Earth’s mass is approximately 6.0 x 10^24 kg, so you could have this as a unit. This way you could tell at a glance which planets would have one MotE (mass of the Earth) and therefore be in our normal gravity range. This would apply to objects with less mass than a brown dwarf star but large enough to be a sphere and that orbit a star (a moon would then orbit a planet). So Venus would be something like a .91 MotE planet.

 

I am not sure if the original post was about trying to redefine a "planet" or classify what we all accept as being a planet.

 

The ideas of using mass could be useful, but I suspect that there will not be a clear divide between brown dwarfs and gas giants. Also, using mass alone would not differentiate between moons and "planets". You would need to include something like "principally orbiting a star" or similar.

[Arch2008]

Right, I have the differentiation of planets orbit a star and moons orbit a planet, although there are tiny moons that orbit some asteroids too. I think brown dwarf stars are like 20-80 Jupiter masses and they are stars because they sustained hydrogen fusion for a while. It should be possible to calculate a planet size (19 Jupiter masses?) that doesn’t permit fusion. This would be the upper limit for planets. This system would then mesh perfectly with the H-R diagram for stars.

http://abyss.uoregon.edu/~js/ast122/lectures/lec11.html

(The star’s temp and luminosity are related to the star’s mass)

[Mr Skeptic]

Personally, I don't care for a definition of planet to distinguish it from a moon. What does it matter where it is? Reminds me of when bats were considered birds because they could fly. You could turn a planet into a moon, or a moon into a planet by simply moving it without any change to the planet itself.

[Sisyphus]

Personally, I don't care for a definition of planet to distinguish it from a moon. What does it matter where it is? Reminds me of when bats were considered birds because they could fly. You could turn a planet into a moon, or a moon into a planet by simply moving it without any change to the planet itself.

 

Yeah, and you can change a boulder into an asteroid just by flinging it out into space.

[Mr Skeptic]

Yeah, and you can change a boulder into an asteroid just by flinging it out into space.

 

It would be a boulder in space, and recognizable as such. Boulders have significantly different physical and chemical composition than asteroids. I suppose it might qualify as an asteroid though.

[Sisyphus]

It would be a boulder in space, and recognizable as such.

 

i.e., an asteroid

 

Boulders have significantly different physical and chemical composition than asteroids.

 

Not really. There's nothing in the definition of an asteroid about physical or chemical composition, aside from it being a solid, coherent body within a certain size range. And a boulder is just a big rock.

 

I suppose it might qualify as an asteroid though.

 

You would suppose correctly.

 

Not trying to be a jerk, just making the point that using context in a definition is sometimes useful and important. Even if, by some technicality, you were to prove me wrong, I think you still get my point.

[Klaynos]

The difference is about how things are formed... Or at least that is where we're aiming, there are objects sometimes called "free planets" which are not planets at all but form in exactly the same way as stars but are not massive enough to ignite.

[Mr Skeptic]

Not really. There's nothing in the definition of an asteroid about physical or chemical composition, aside from it being a solid, coherent body within a certain size range. And a boulder is just a big rock.

 

Not in the definition itself, no. All I am saying is that I would call a boulder in space a boulder in space, even if I could properly call it an asteroid. Just like I would call a fossilized dinosaur bone in space a fossilized dinosaur bone in space even if I could call it an asteroid. If the earth was smashed to smithereens in a collision with another heavenly body, I'd call the fragments asteroids.

 

Not trying to be a jerk, just making the point that using context in a definition is sometimes useful and important. Even if, by some technicality, you were to prove me wrong, I think you still get my point.

 

I get your point. I'm just saying that the location of something is far less important to me than most of its other attributes, in general. For example, if someone were to make the moon more massive or farther away, so that the center of mass of the earth-moon system was no longer within earth, then by the current definition the earth would no longer be a planet. That seems silly to me.