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Hank: At some point, we've all learned one of those mnemonics to help us remember the order of the planets. Something like: “My Very Excellent Mother Just Served Us Nachos...” she's a really good mom.

But have you ever wondered why the planets have the order they do? After all, our Solar System has eight planets: four rocky spheres then four balls of gas. It turns out that this arrangement is less a question of chance and more a matter of physics. Different kinds of planets form in different conditions!

So let’s go back to the beginnings of our Solar System, more than four and a half billion years ago. Back then, we didn’t even have the Sun – just a big ball of gases, mostly hydrogen and helium, that astronomers call a protostar, because it didn’t have fusion reactions to generate heat at its core. For a few million years, this protostar was surrounded by material that would eventually become all the planets and other space rocks – known as a protoplanetary disk.

That material can be split into four basic groups: metals, rocks, ices, and light gases. And those light gases made up around 98% of the disk. Each group melts or condenses at different temperatures. The metals, for example, stay solid except in really high heat, while the ices only condense from gases when it’s really cold. Now, even though the protostar wasn’t generating heat yet, it was still tremendously hot. So, nearby, only the most heat-resistant stuff was solid – mainly the metal and rock chunks.

This is where Earth, with its rocky layers and metallic core, and the other three inner planets could form. The atmospheres of inner planets came later, seeping out from the rocks they’re made of, because they were all too small to trap any of the gases that were floating around. Now, the farther you went from the protostar, the colder it was.

Eventually it was cold enough for gases like water vapor, methane, or ammonia to freeze and form ices. Here on Earth, water freezes at zero degrees Celsius. But in the vacuum of space, where there’s much lower pressure, chemicals need to be much colder to condense.

Astronomers call this distance where ices could form the frost line, but since every gas freezes at a different temperature, it’s really more of a zone. And most estimates put the frost line somewhere between the present-day orbits of Mars and Jupiter. Looking past the frost line, there’s lots of ices, plus more solid metals and rocks.

With all this extra material, the outer planets were able to grow larger and larger. Eventually, scientists think their cores became massive enough that their gravity could pull in a bunch of the hydrogen and helium gases that were floating around in the protoplanetary disk. They were able to hang onto all these gases, and thus the gas giants were born.

Out past Neptune, though, there are no more planets. The protoplanetary disk was less dense, making it nearly impossible for the icy, rocky, metallic cores to form – much less grow large enough to pull in the light gases. That’s why we have the Kuiper belt, which includes Pluto and other dwarf planets, and has way too many icy objects for even the most talented mnemonic makers!

Eventually, after all these planets started forming and our Solar System began to take shape, the protostar had enough heat and pressure to “turn on.” In other words, its internal fusion reactions started, and it began to shine like the Sun we see today. Our Sun’s high-energy solar wind particles could have scattered the rest of the protoplanetary disk, sending some of the leftover dust and gases into interstellar space. Eight remarkable planets remained, more-or-less lined up in a plane.

Plus all those big leftover chunks. And now we humans are here, trying to understand how different kinds of planets came to be, and puzzling out the history of our Solar System and the universe. Thanks for watching this episode of SciShow Space, which was brought to you by our patrons on Patreon, just a bunch of people who give us money so we can make free science information in the internet. If you want to help support content like this, you can go to Patreon.com/SciShow, and don't forget to go to YouTube.com/SciShowSpace to subscribe.