The answer to this question is a little tricky because the planets formed gradually over a long time, and the order isn't completely clear-cut. However, we can say with a high degree of confidence that the inner rocky planets (Mercury, Venus, Earth, Mars) formed before the outer gas giants (Jupiter, Saturn, Uranus, Neptune).

Here's why:

* Early Solar Nebula: The solar system formed from a spinning disk of gas and dust called the solar nebula.

* Inner Disk: The inner part of the disk was hotter and denser, primarily made up of rocky materials like iron and silicates. These materials condensed into small particles called planetesimals, which eventually accreted to form the inner rocky planets.

* Outer Disk: The outer part of the disk was colder and less dense, containing more ice and gas. These materials condensed into larger planetesimals, which grew rapidly due to their gravitational pull, eventually forming the gas giants.

While the exact order of the inner planets' formation is still debated, the overall timeline points to them forming first, followed by the gas giants. It's important to remember that the planets didn't form in isolation, and their gravitational interactions influenced each other's growth and evolution.