A lot of the laws of physics I’ve studied, like Boyle’s Law and Charles’ Law, describe the behavior of “An ideal confined gas.”
I’ve had to tell several flight students to unlearn what they’ve learned about that in the meteorology chapter, because, for example, in a confined gas, increasing the temperature causes an increase in pressure while the density stays the same. In the Earth’s atmosphere, increasing temperature does nothing to the pressure and decreases the density. Because the Earth’s atmosphere isn’t “confined,” there’s no lid, the air is relatively free to change volume. Heat the entire planet up and the atmosphere will just get a little taller.
But, I think, even if we put a magical vacuum tight shell around the planet 200 miles up, making the volume finite, I think the atmosphere would still act like an unconfined gas, because 1. it’s so vast that it never homogenizes, parcels of different temperatures, pressures and moisture content take days to slosh across the available space, and 2. the Earth’s gravity will cause a pressure gradient; most of the air is at the bottom and if you heat it up, it may not change volume but the pressure at the top will increase.
So I guess there has to be an upper limit to the volume and/or mass of air that can be “confined” and it’s somewhere below planetary scale.
Would that massless box also be a perfect insulator? My understanding of thermodynamics breaks at “massless”. Assuming it’s a solid, sunlight would heat it, and that heat would be conducted to Jupiter, but again an object with a mass of zero breaks the math.
Is your massless box a better thermal insulator than the vacuum of space?
It is also my understanding that Jupiter, unique among our planets, radiates more heat into space from it’s own contraction than it receives from the Sun.