Why do larger planets tend to mostly be made up of gases?


Why do larger planets tend to mostly be made up of gases?

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The majority of regular matter in the universe hydrogen. The second place finisher is helium. Both are gases unless placed under extreme conditions. The discs around proto stars during formation primarily consist of these two atoms. The protoplanets that grow the fastest do so because they gather more matter than the other planets do. They do so by gathering the most prevalent material available before other protoplanets do: hydrogen.

Most of the mass of the solar system (or the universe for that matter) is hydrogen and helium. Other elements only account for about 1-2% of the solar system. Small rocky planets like earth form where gasses such as hydrogen and helium have been driven off, leaving behind a relatively small amount of solid to form those planets. Hydrogen and helium were the only elements present in enough quantity in the protoplanetary disk to form planets as massive as Jupiter and Saturn.

Firstly there is a lot more gas around than the rocky stuff, so if you want to go big it really has to be gas like hydrogen and helium. Then to grow big you need to do it away from the main star formation and have a larger orbital path where you can “sweep” up more material for planet formation.

It’s hard for a planet to hold on to light gases. Random thermal motion can accelerate light gases to escape velocity, especially with extra kicks provided by the solar wind. The Earth, for example, can’t hold on to either hydrogen or helium at its size and temperature: they both escape quickly (on planetary timescales) into space due to those effects (escape velocity dominates for hydrogen on Earth, and the solar wind kicks dominate for helium, but either effect would be enough to get rid of both gases over the age of the Earth).

To do it, a planet needs to be both big and, ideally, cold, although a big enough planet can do without the cold.

When the planets formed early in the history of the solar system, the protoplanets that would eventually end up as the gas (Jupiter, Saturn) and ice (Uranus, Neptune) giants were larger than the protoplanets that would eventually end up as the terrestrial planets (Mercury, Venus, Earth, Mars). As a result, they were able to hold on to light gases. Since those gases then added to the mass of the protoplanet, that in turn let them hold on to even *more* light gases, in a feedback loop that eventually captured most of the free gas that wasn’t part of the Sun.

Jupiter and Saturn were big enough to hold on to hydrogen and helium, so they ended up the biggest. Uranus and Neptune couldn’t hold on to those, but they could hold on to light molecules like methane, oxygen, ammonia, and water, and so they captured those gases from the outer Solar System. Since there is a *lot* more hydrogen and helium than other elements in the Universe, Jupiter and Saturn ended up much bigger than Uranus and Neptune.

Only a small amount of the remainder was left over, and that last bit (or the elements heavy enough to be held on to by each planet) ended up forming the early atmospheres of the terrestrial planets. Water is close to the limit for Earth and Mars, but Earth’s magnetic field protects us well enough to hold on to water at Earth’s current temperatures, while Mars’ lack thereof has caused it to lose most of its water to space. Earth is expected to suffer the same fate as Mars in the future, as the Sun continues to warm as it ages (well before the Sun becomes a red giant).

It’s a bit of a chicken-and-egg problem.

If you’re a planet, you need gravity to attract stuff. The bigger you are, the harder you pull. But working the other direction, the lighter something is, the weaker you will pull on it.

The really heavy bits like metals and rocks will tend to find themselves and collect into piles pretty quickly. But the lighter stuff will be harder to rope in, since it’s just really hard to pull on in general since it’s so light.

You’re gonna end up with a lot of sort-of small rocky chunks quite easily. Countless many of them very small asteroid-sized, a few of them planet-sized. Of them, only the ones that happen to get big enough by random chance are going to start collecting the lighter gases floating around. And once that happens, it starts to snowball. More gas is more mass, more mass is more gravity, more gravity is an easier time pulling down more gas.

Planets like Earth aren’t really big enough to have been able to do this. Earth has an atmosphere, yes, but as a proportion of its size, it’s *barely* there. If Earth was an apple, the entire atmosphere would be thinner than the apple’s skin. Most of Earth’s atmosphere was not captured; it either rode in on meteors from the far outer solar system where those gases would have been frozen solid in the cold, or generated from solid or liquid materials through chemical reactions.

Planets like our gas giants, however, probably started out kind of Earth-like in makeup, but by random chance, got bigger. Big enough to start clawing up that really light-weight hydrogen and helium gas that was everywhere that planets like Earth were too small to hold onto. After reaching that critical size and shoveling it up, they snowballed into the giants we know today.

Even the Sun itself likely started out vaguely Earth-like at some point, as a heavy chunk of rocky matter. The Sun formed more or less like any other planet did, just stuff clumping together, heaviest stuff first, then slowly clawing in lighter and lighter stuff. The only thing about the Sun that makes it different is that it was the lucky one to get biggest *first*. The runaway snowballing of the lighter gases made it get really big really fast, faster than any of the others, up to the point where it finally ignited its nuclear furnace and started to shine. That shining whipped up solar winds, which blew away all the extra dust and more or less halted the growth of all other planets, freezing them at more or less their final sizes. From that point on the only thing planets could really do to grow was crash into each other and merge.

tl;dr: the gas giants are just the phenomenally lucky ones.