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xkcd #3063: Planet Definitions (xkcd.com)

submitted 4 days ago by [email protected] to c/xkcd

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Under the 'has cleared its orbital neighborhood' and 'fuses hydrogen into helium' definitions, thanks to human activities Earth technically no longer qualifies as a planet but DOES count as a star.

https://explainxkcd.com/3063/

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[–] CosmicCleric 0 points 4 days ago* (last edited 4 days ago) (2 children)

I personally don’t think they can be counted as skies and oceans etc. anymore when they’re being mixed in with multi-thousand-degree hydrogen/helium plasma.

So Mars never had oceans? Or an atmosphere?

So Saturn's moon Titan doesn't have lakes? Or an atmosphere?

What happens if a body is found in the Oort Cloud that has an internal heat source so that it has a internal ocean like Europa? It's it still not a planet because the space around it is busy?

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[–] [email protected] 2 points 4 days ago (1 children)

One of the idea with "has the space around it cleared" is that the body has the right size and gravitational pull. The theoretical object in the Oort Cloud would relative fast clear the space around himself if it had the size to have a stable and long living internal heat source (that would either need lots of decaying nuclear material or would need to be at least about earth size to have enough stored energy to have a molten core).

So if you put earth into the Oort Cloud it would still be a planet, because we know that earth has the potential to clean it's neighborhood. Not that our definition would be relevant, because Earth in the Oort Cloud would be a lifeless rock very fast, with nobody left to care about definitions.

[–] CosmicCleric 0 points 4 days ago (2 children)

So if you put earth into the Oort Cloud it would still be a planet, because we know that earth has the potential to clean it’s neighborhood.

Would it be a non-planet for the millions of years it would take to clear its orbit?

Does Earth's body/features magically change somehow for the duration of the clearing process, so that it doesn't resemble a planet?

The point is that using external criteria to identify what an internal thing is is not logical, or scientific.

The theoretical object in the Oort Cloud would relative fast clear the space around himself if it had the size to have a stable and long living internal heat source.

You don't know that, especially with the size of the Oort Cloud, and the size of the orbit to clear. And the rules for how much clearance has to be done is very arbitrary.

Also bodies can be small and have a decaying heat source that'll last many millions of years, or renewing heat source via tidal interactions. It's not necessarily a size thing.

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[–] AnyOldName3 1 points 3 days ago

Would it be a non-planet for the millions of years it would take to clear its orbit?

Does Earth’s body/features magically change somehow for the duration of the clearing process, so that it doesn’t resemble a planet?

Clearing the orbit of other material, and in the process, accumulating it and incorporating it into a protoplanet is the process that turns a protoplanet into a planet. While that's happening, it's getting bigger and rounder and is constantly surrounded by impact debris that's in the process of forming moons and rings or is in a decaying orbit. All of these are processes local to the protoplanet that don't happen anymore once it's become a planet.

[–] [email protected] 1 points 4 days ago* (last edited 3 days ago)

Many million years is nothing in geological or astronomical timescales. Any (small) body with a heat source that short is dead cold by know. The solar system is 4.6 billion years old and the formation of planets or larger bodys has ended about 4 billion years ago.

And tidal forces mean that a way larger body is close by, like a gas giant, as far as I remember are all known body's with tidal heating moons not planets.

[–] AnyOldName3 1 points 4 days ago (1 children)

Your original idea only holds if it's still valid to claim Mars still has oceans, even though they're all gone. When things stop existing, it changes their properties.

[–] CosmicCleric 0 points 4 days ago* (last edited 4 days ago) (1 children)

Your original idea only holds if it’s still valid to claim Mars still has oceans, even though they’re all gone. When things stop existing, it changes their properties.

My latest point was to counter your latest point that things like bodies of water or atmosphere should not be considered criteria for identifying a planet or not,

Also, Mars may still have water, under the surface.

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[–] AnyOldName3 1 points 4 days ago (1 children)

That wasn't a point I made. You said the Earth's skies and oceans would be the same after the hypothetical Earth swaps places with an Earth-sized lump of the Sun event, and I pointed out that they'd be destroyed within seconds. That was kind of separate to the original poorly-thought-through suggestion you made about planet location swaps, and was a second poorly-thought-through claim.

[–] CosmicCleric 0 points 4 days ago (1 children)

You said the Earth’s skies and oceans would be the same

No, that's not what I was saying, at all.

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[–] AnyOldName3 1 points 3 days ago

Everything about Earth is still the same, skies, oceans, etc. Only difference is that it’s crowded in by other bodies now.

Is pretty clearly saying the skies and oceans would be the same after the Earth's been swapped with part of the Sun.