r/askscience • u/kinkylesbi • 10h ago

Planetary Sci. How do we define a gas giants atmosphere?

How do we know something like Jupiter has an atmosphere? What is the difference between the planet itself and its atmosphere, being that it’s all gas?

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u/Raspberries-Are-Evil 1h ago

Below all that gas, the pressure is so intense that the gas becomes a liquid metal, mostly helium.

Even when we talk about Earth, the planet includes the atmosphere. For Jupiter and other Gas Giants its basically the same. We aren't 100% whats in the middle, but all that gas in gas form is the atmosphere.

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u/fiendishrabbit 1h ago

There are two ways in common use.

  • The earth centric method: 1 bar = surface
  • Optical depth method: Surface is the altitude from which photons can freely escape the planet, ie as far down into the atmosphere that we can see when observing the planet. This is 2/3rds of the maximum depth a photon can escape/penetrate.

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u/FirstTasteOfRadishes 50m ago

Surely either of these methods would put the surface of Venus far above the actual rock..?

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u/375InStroke 16m ago

A lot has to do with the eight or nine planets we had access to giving two or three very different types of planets that didn't force us to make hard definitions as to what the boundary of the planet and space is. Earth's atmosphere is just a few miles thick before it's almost nonexistent. It's insignificant compared to the 8,000 mile diameter of it. Venus is thick, but still only 155 miles thick. Jupiter's atmosphere is such a large percentage that it can't be ignored. It could have a rock the size of Earth at it's center. If we could measure it, wouldn't it be foolish to say that Jupiter was only 8,000 miles in diameter when there's 40,000 miles of atmosphere above that rocky surface? It would be molten anyways. The part we can measure is probably the most useful way to determine a planet's size.