Astromike23 t1_iye1tyo wrote

> if the added mass was solid iron or silicon, so that it would sink to the core, I understand that the solid core would grow. But would the outer gassy layers also grow?

That depends sensitively on "how much" and "what kind of stuff"...but generally electron degeneracy is electron degeneracy. You can't pack too much stuff too tightly or it goes degenerate, and starts expanding outwards into momentum space while shrinking in position space.

The resulting outer gassy layers are tricky, but there's a closely analogous phenomenon in stars: as an old main sequence star fuses up all the hydrogen at its core, it leaves behind a ball of helium "ash" that collapses under its own weight, supported only by electron degeneracy pressure. The central density in our Sun is already very high, about 160x greater than water, but this helium ash ball is closer to 16,000x denser than water. If you could stand on this ball, it would have enormous surface gravity - also meaning fresh hydrogen in a shell surrounding the degenerate helium ball is compressed, and fusion proceeds much more rapidly, expanding the outer layers into a red giant.

In the case of Jupiter, it's still a long way from the pressures / temperatures needed for any fusion, so the gassy layers would just get compressed by the increased density of a more massive, more degenerate core...and that's about it.


Astromike23 t1_iy92xpc wrote

> I'm sticking to my statement that with more mass, Jupiter would shrink.

You're almost right.

As you add mass to Jupiter, its diameter increases up until a mass of 3 x Jupiter, after which point the diameter starts to shrink because of the sheer amount of degenerate matter at the core.

Degenerate matter is weird stuff, a macro-scale substance only made possible by some obscure rules quantum physics. Prime among these rules is the Pauli Exclusion Principle, which states that, "no two electrons can exist in the same quantum state at the same time." Thing is, a quantum state is more than just position - it also includes momentum. You can have two electrons occupy the same position at the same time, so long as they're moving at different speeds through each other.

The above mechanism produces a very non-intuitive quality: the more material you add to an electron degenerate body, the smaller it gets in size, as electrons are forced to move faster and faster in speed. Counterintuitively, if you had an electron degenerate bookshelf, you'd have more room the more books you added.

Not only is this the case for really massive planets, but also brown dwarfs: the more massive it is, the smaller the radius.

Source: did my PhD researching Jupiter.