Why is a Gas Giant Like Jupiter Considered a Planet?

We asked a constellation (no pun intended) of interrelated questions in order to get to the core principle:

  • Will something that flies into Jupiter make an impact?
  • Does Jupiter have a surface?
  • Why are gas giants considered planets?
  • Why do gas giants have gravity?
  • Why does Jupiter have a magnetic field?

This tour of Jupiter’s layers is very interesting:

For the interior of Jupiter, let’s imagine taking a descent from cloud-tops down to the core based on our best guesses of what lies below.

You start falling through the high, white ammonia clouds starting at 0.5 atmospheres, where the Sun is still visible. It’s very cold here, -150 C (-240 F). Your rate of descent is roughly 2.5x that of Earth, since gravity is much stronger on Jupiter.

You emerge out the bottom of the cloud deck somewhere near 1 atmosphere. It’s still somewhat bright, with sunlight filtering through the ammonia clouds much like an overcast day on Earth. Below, you see the second cloud-deck made of roiling brown ammonium hydrosulphide, starting about 2 atmospheres.

As you fall through the bottom of this second cloud deck, it’s now quite dark, but warming up as the pressure increases. Beneath you are white water clouds forming towering thunderstorms, with the darkness punctuated by bright flashes of lightning starting somewhere around 5 atmospheres. As you pass through this third and final cloud-deck it’s now finally warmed up to room temperature, if only the pressure weren’t starting to crush you.

Emerging out the bottom, the pressure is now intense, and it’s starting to get quite warm, and there’s nothing but the dark abyss of ever-denser hydrogen gas beneath you. You fall through this abyss for a very, very long time.

You eventually start to notice that the atmosphere has become thick enough that you can swim through it. It’s not quite liquid, not quite gas, but a “supercritical fluid” that shares properties of each. Your body would naturally stop falling and settle out somewhere at this level, where your density and the atmosphere’s density are equal. However, you’ve brought your “heavy boots” and continue your descent.

After a very, very long time of falling through ever greater pressure and heat, there’s no longer complete darkness. The atmosphere is now warm enough that it begins to glow – red-hot at first, then yellow-hot, and finally white-hot.

You’re now 30% of the way down, and have just hit the metallic region at 2 million atmospheres of pressure. Still glowing white-hot, hydrogen has become so dense as to become a liquid metal. It roils and convects, generating strong magnetic fields in the process.

Most materials passing through this deep, deep ocean of liquid metallic hydrogen would instantly dissolve, but thankfully you’ve brought your unobtainium spacesuit…which is good, because it’s now 10,000 C (18,000 F). Falling ever deeper through this hot glowing sea of liquid metal, you reflect that a mai tai would really hit the spot right about now.

After a very, very, very long time falling through this liquid metal ocean, you’re now 80% of the way down…when suddenly your boots hit a solid “surface”, insomuch as you can call it a surface. Beneath you is a core weighing in at 25 Earth-masses, made of rock and exotic ices that can only exist under the crushing pressure of 25 million atmospheres.

You check your cell phone to tell you friends about your voyage…but sadly, it melted in the metallic ocean – and besides, they only have 3G down here.

(Do gas giants have a surface?)


Now, these things define a planet:

  • massive enough to be rounded by its own gravity
  • not massive enough to cause thermonuclear fusion
  • has cleared its neighbouring region of planetesimals



So, the atoms are, in fact, close enough to give it density (which I was not previously sure about), which then give it gravity, which makes it a planet.


It is important to note that all of this is theoretical since we are not currently capable of seeing/sampling what exists at the more interesting depths of gas giants (If Jupiter and Saturn are gas giants, could you fly straight through them?).

It is interesting that flying objects can and will visibly punctuate the clouds of Jupiter. Does anyone remember that Shoemaker-Levy comet from the 90’s? This is what it did to Jupiter (the brown spots; after it splintered into a few pieces that spanned over a freaking-million kilometers, all gunning for Jupiter):


(Is it true that Jupiter protects Earth?)


This is the especially-interesting part: Did anyone else hear that part about the center of Jupiter being a churning ball of metallic hydrogen? Yes, the same stuff that air is made out of. Yes, the same stuff that water is made out of. This is obviously not an atom that is a metal in its normal form. In fact, its normal solid form is liquid hydrogen (an ice that is near absolute-zero and is the coldest material that most people will ever observe in their lifetimes and will likely never have direct access to), though it is, technically, a metal:

Though at the top of the alkali metal column in the periodic table, hydrogen is not, under ordinary conditions, an alkali metal. In 1935 physicists Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250000 atm or 3500000 psi), hydrogen atoms would display metallic properties, losing hold over their electrons. Since then, metallic hydrogen has been described as “the holy grail of high-pressure physics”.

The initial prediction about the amount of pressure needed was eventually proven to be too low. Since the first work by Wigner and Huntington, the more modern theoretical calculations were pointing toward higher but nonetheless potentially accessible metallization pressures. Techniques are being developed for creating pressures of up to 500 GPa, higher than the pressure at the center of the Earth, in hopes of creating metallic hydrogen.

(Metallic hydrogen)


Who knew?

Another mildly-interesting (but not greatly-interesting) article about the “Kelvin wave”, and atmospheric waves in general, specifically regarding Jupiter:

NASA Scientists Identify Missing Wave near Jupiter’s Equator


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