If I ask you to name and describe the planets in the solar system, what would you say?
You'd probably say that Mercury, Venus, Earth, and Mars are small and rocky inner planets, and Jupiter, Saturn, Uranus, and Neptune are the gas and ice outer giants.
But do you know why these planets are composed and ordered like this?
The order of the planets actually has to do with the formation of our solar system. Our solar system formed around 4.5 billion years ago. But we’re talking about before the sun even existed!
Protostar
It all started with a large cloud of interstellar gas and dust, called a protostar, like a “primitive star”. The reason it’s a protostar is that it doesn’t have enough energy and fusion reactions to generate heat and light to be a star. Some of the gases in the cloud are hydrogen and helium, which is what our Sun is made of!
So, how does a cloud of, well, stuff, become a radiant star?
The protostar collapses on itself under its own gravity, forming a nebula. As it continues to collapse, its gravity pulls in material from the cloud.
And now we start cooking! The pressure and rising heat generated through the collapses allow nuclear fusion to occur. In a nutshell, during stellar nuclear fusion, hydrogen nuclei fuse together and form helium atoms.
Nuclear fusion releases heat and light, and thus, our Sun was born.
Protoplanetary Disk
Thanks for the FYI, but what does this even have to do with the planets?
I’m getting there.
Welcome to the protoplanetary disk. This is the remaining 1% of the material left from the cloud. That’s right. The Sun is made up of 99% of all of the material in the cloud.
And our planets are going to form from that teeny 1%. I’m not kidding. Seriously.
The nuclear fusion in the nebula caused solar winds around the Sun, scattering the remaining material, creating the disk. The material is mainly metal, rock, light gases, and ice.
Near the sun, obviously, the disk was really hot. Only heat-resistant materials like metal and rock could retain their form in this area. And farther into the disk, it was really cold, where everything froze and became ice.
The area between present-day Mars and Jupiter is called the frost line. Past that, there is a lot of rocks, ice, and other frozen material.
The Great Collisions
So, near the sun, rocky and metallic matter started clumping together, forming the small rocky, inner planets with their metallic cores. These planets were too small to trap any gases, but because of collisions at a certain speed, they became large enough for their gravity to shape them into a sphere.That’s why we get those small, rocky, inner planets at the beginning.
Remember those solar winds?
They blew those gases far away, where it was cold enough for those gases to freeze into ice. And because past that frost line there was so much material, the outer planets grew larger and larger with rocky cores and ices. Eventually their cores were so big that they were able to trap gases such as hydrogen and helium using their gravity, thus becoming gas and ice giants. And that’s why we have our giant gas and ice planets way out in the solar system.
Now you might be wondering: If there was so much material in the disk, why were there only 8 planets in the solar system?
Past Neptune, the disk is too thin and does not allow planets to form. The rest of the material floats around as space junk in the Kuiper belt. And Jupiter is so large that its gravity has a lot of influence in the area around it, gathering material for itself, and other planets could not take their shape, thus creating the asteroid belt.
The rest of the material formed comets, asteroids, moons, meteoroids.
And there you have it! Our very own solar system.