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How big does a planet have to be to have an atmosphere? (The planet will be in Earth's place if you need a position)

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Ray  Dart Profile
Ray Dart answered

I don't know definitively, but here are a few observations.

The mass of a planet is probably more important than the size of it (although both will be significant).

What gases make up the atmosphere? Earth's gravity is insufficient to hold on to hydrogen and helium, the two most abundant elements in the universe. On the other hand, carbon dioxide and methane are no problem.

The atmosphere on earth is remarkably thin. If you reduced the size of the earth to a 30cm diameter sphere, then two coats of reasonably thick paint would cover 98% of the space taken up by the atmosphere. So -- there isn't much atmosphere anyway.

Mars - away from our orbit  (although, in astronomic terms, not by much) lost most of its atmosphere in the past, and part of the reason is that its gravity is only about a third of the Earth's - but it still has some atmosphere.

Not an answer, sorry, but food for thought?

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Ray  Dart
Ray Dart commented
As I said in my first paragraph. :)
As to the "gas giants", where do you separate "planet" from "atmosphere"? In fact, in the case of Jupiter, the largest,it is thought that the "surface" of Jupiter just fades from gas to vacuum over several thousand miles......
Walt O'Reagun
Walt O'Reagun commented
It will be interesting to see, when we get the technology to actually send probes into those planets.

It would alter the theory on how planets are formed, if there is no solid mass at the center.
Ray  Dart
Ray Dart commented
yep, the "Coalescence theory" would be dead in the water.....

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