It's impossible to say for sure which planet formed first in our solar system. We don't have a perfect timeline for planet formation, and there are a lot of unknowns.

Here's what we do know:

* Planets formed from a protoplanetary disk: This disk of gas and dust surrounded the young Sun.

* The process was messy: It wasn't a neat, orderly process. Planets likely collided and merged, and their formation timelines might have overlapped.

* Planetary formation models: Scientists use computer models to simulate planet formation, but these models still have limitations.

Some scientists believe Jupiter might have formed earliest, but it's not confirmed. Others suggest it might have been Mercury or even a rocky planet that later merged with another to form Venus.

The mystery of the first planet remains open!