* Mass: Stars are born from massive clouds of gas and dust. To ignite nuclear fusion (the process that powers stars), a celestial body needs a certain minimum mass. Jupiter and Saturn, while large, are significantly less massive than our Sun. In fact, Jupiter would need to be about 80 times more massive to become a star.
* Composition: While both Jupiter and Saturn are primarily composed of hydrogen and helium, the composition of the core is also crucial. Stars have a dense, hot core where nuclear fusion takes place. Jupiter and Saturn lack the necessary core density and temperature for fusion to occur.
* Nuclear Fusion: The core of a star needs to reach extreme temperatures and pressures for nuclear fusion to ignite. Jupiter and Saturn simply don't have the internal conditions required for this process.
What could they have become?
While Jupiter and Saturn couldn't become stars, they could have become brown dwarfs. These are objects that are too massive to be planets but not massive enough to be stars. They have a mass between 13 and 80 times that of Jupiter.
In short: While Jupiter and Saturn are gas giants, they lack the necessary mass, core composition, and internal conditions to become stars.
