Why are the solar system’s planets situated where they are?

A: The planets are in their present order because they formed from the same spinning disk of gas and dust that became the sun. Parts of this protoplanetary disc that were closer to the sun were hotter, so only materials with high melting points like metals and rocky minerals could stay solid. These solids clumped into small bodies and eventually into Mercury, Venus, Earth, and Mars.

The disc was cooler farther out, leaving water, ammonia, and methane to freeze. When these lumps became massive enough, they pulled in large quantities of hydrogen and helium before the disk dispersed, creating the giant planets. This is why Jupiter and Saturn are big and gas-rich and why Uranus and Neptune, which formed even further out, ended up as ‘ice giants’ with less hydrogen and helium.

While the disc existed, young planets tugged on the surrounding gas and on each other, thus being able to move planets inwards or outwards. Some evidence suggests the giant planets shifted positions early on. So the current layout reflects both where different materials could form and how planets migrated and reshuffled during the solar system’s infancy.