The planet that almost became a second sun is Jupiter.

While Jupiter is a gas giant and not a star, it contains a lot of hydrogen and helium, the same elements that make up our sun. If Jupiter had been about 80 times more massive, it would have had enough gravitational pressure and heat to ignite nuclear fusion, turning it into a star.

It's interesting to imagine what our solar system would be like if Jupiter had become a star. It would have been a binary star system, with two suns!