While we can't definitively say which planet formed *first*, the current leading hypothesis suggests that Jupiter was likely one of the earliest planets to form in our solar system.

Here's why:

* Core Accretion Model: This model is the dominant theory for planet formation. It suggests that planets form by the gradual accumulation of dust and gas around a solid core.

* Giant Gas Planets: Jupiter is a gas giant, meaning it has a large core and a massive atmosphere. To form such a core, it likely needed to gather material very early on in the solar system's history.

* Early Solar Nebula: The early solar nebula, the disk of gas and dust from which the planets formed, was denser and hotter closer to the Sun. This would have allowed Jupiter to accrete material more quickly.

However, it's important to note:

* No Direct Evidence: We can't directly observe the formation of planets, so there's no definitive proof.

* Other Possibilities: While Jupiter is the most likely candidate, other gas giants like Saturn may have formed early as well.

* Ongoing Research: Scientists are constantly studying the formation of planets, and new discoveries could change our understanding.

So, while Jupiter is currently the leading contender for the first planet, it's an ongoing area of scientific research.