system
Given the following post, respond as if you're commenting on Reddit:
human
Please help me understand the structure of a non-terrestrial planet (gas giant?)

I understand that terrestrial planets (earth, venus, mercury, mars) are rocky-surfaced planets. I am curious, though, about the non-terrestrial planets. I hope I don't come across as an idiot; I just don't really quite understand. Is there an actual physical planetary body as with the others? Or are they totally made of gas? I've done some googling, but I can't really find a concise answer that explains it. 

Thanks!
gpt
Okay, there's a good deal of speculation here presented as fact. Please allow me set the record straight.

From the most widely held theory, we *think* there should be a rocky core at the center of gas giant planets, but we currently have no observational evidence to back this up. This all depends on how you assume these planets form.

The most popular theory of gas giant formation is that dust grains in the disc surrounding the primordial Sun began to stick and form "planetesimals", small rocky bodies the size of asteroids. These began to grow in size by clumping together due to gravity, eventually forming decently-sized rocky planets. If you have a rocky planet that's ~5 Earth-masses and in a part of the disc that has a lot of gas, the planet can begin accreting a lot of gas that cannot escape the planet's relatively strong gravitational pull. This process would eventually form a gas giant with a rocky core.

The alternate theory here is "gas collapse" - it's possible that the gas itself in the primordial disc (and not the dust) is slightly clumpy and will begin to collapse from its own self-gravity with no need for planetesimals as seeds. This would form gas giants without cores.

Although there are some reasons to prefer the planetesimal theory, we really don't have enough evidence to rule out the gas collapse theory at this time. Moreover, we have very little idea about how big that core actually is - I've seen theories ranging anywhere from 5 to 40 Earth-masses for Jupiter's core mass.

We'll know a lot more about this in 2015 when the Juno spacecraft arrives at Jupiter. It will begin a series of very tight orbits around the planet, picking up very tiny gravitational changes that should allow us to probe deep density variations. It should not only be able to confirm the presence of a core, but also how massive it is.


**TL;DR**: Although we think there's probably a rocky core at the center of most gas giants, we simply aren't sure since there's no observational evidence - and we really have no idea how big it is. We'll should have a definitive answer in 2015 when the Juno spacecraft gets to Jupiter.