Astronomy of the day

[beccastareyes]

What determines how big a planet gets?

There's an interesting question for you. Most solids are relatively incompressible -- meaning, if you take a rock and try to squish it, this does not work that well. When they do compress (something like a pillow), it is because you are getting ride of the voidspace (pockets of mostly empty space that usually end up filled with air on Earth). So, you might think that if you pile eight times the stuff onto a planet, it would get eight times bigger in volume (or twice as big in radius).

You might think that, but you'd be wrong. When you get millions of tons of rock pressing down on every inch of the core, it compresses to a noticeable degree. If I were to blow up the Earth (it was obstructing my view of Venus), the debris would take up more volume than the planet itself. Planets and stars are essentially balanced between the gravity that is trying to pull everything in and forces that want to resist that. On the Earth, that is mostly the interactions between electrons that make things solid. After all, most atoms are 99% empty space. It only is the electron clouds of those atoms that keeps me from being able to do things like wave my hand through my desk, my desk from vaporizing, or the Earth from collapsing into a chunk of neutronium.

But, solid planets are still pretty hard to compress -- the forces between atoms are pretty strong. Something like Jupiter, which is mostly made up of gases has a lot weaker forces. Consequently, Jupiter would be a lot easier to compress. On the surface of the Earth, Jupiter's atmosphere has a density of about 0.000072 grams per cubic centimeter (for reference, water is about 1 in these units, and rock is about 3). However, Jupiter's average density is 1.33 grams per cubic centimeter. Saturn, which is made of roughly the same stuff is only 0.7 grams per cubic centimeter -- because Saturn is only one third of the mass of Jupiter, it is less packed down. Both of them have maybe 1/10 of their mass as rock and ice and other solid things -- the rest is hydrogen and helium.

(And, yes, Saturn would float if you had a bathtub big enough to fit it in.)

So, what happens? On the outside, Jupiter and Saturn look pretty normal for big balls of gas. As we get deeper into the atmosphere, and more gas is pile don, the hydrogen and helium starts condensing into a liquid. The next thing that happens -- deep into the planet, at pressures that make it like we were at the Earth's core, is that the hydrogen starts acting like a liquid metal. Gas giants are weird and it can be a pain in the neck to figure out what they are like inside.

Anyway, so gas giants get bigger when you make a bigger gas giant, but it's not as much as you'd expect -- Jupiter is three times the mass of Saturn, but it's not three times the volume. It turns out eventually, you hit a limit to how big, size-wise a gas giant gets -- piling on more mass just compresses things more, so the planet actually get smaller in size. The limit is about at a bit bigger than Jupiter. It turns out that any object from Jupiter to a tiny star (from one to 80 Jupiter masses) will be about at Jupiter's radius. Neat, huh?

(This limit isn't there for young planets that are still cooling. There the heat from their formations makes them a lot puffier. Similarly, we've found planets that are very close to their parent stars that also look puffy. For that matter, if a planet had more rock and ice, it would be smaller than a planet made of hydrogen and helium.)

Anyway, the reason I mention is this is that a small 'failed star' (or brown dwarf) was found eclipsing the star it orbits. It's about 20 times Jupiter's mass, but still about the size of Jupiter. As a result, the average density is greater than lead (and most metals on Earth). The upper layers are probably pretty normal atmosphere, but at the core, matter is compressed so much that the electron clouds of the atoms are touching and repelling one another (a state called degenerate matter).

It's the type of thing that seems counter-intuitive -- this thing is probably made of the two lightest elements in the Universe, yet is denser than most metals on Earth. But, it's because sometimes intuition isn't suited for changed circumstances -- most metals aren't under that much pressure here on the surface of the Earth.