Jupiter, biggest of the planets. If it had a bit more mass it might shine slightly as a brown dwarf star? In the story 2001 a Space oddessy it’s converted to a second star in the solar system. That would make the solar system a binary star system

How would it affect Earth? More light, more heat? It depends on its solar output. It might make plants grow more, but it might add to current global warming. Birds are being affected by the light pollution from streetlamps, imagine having the extra light of maybe a full moon when Jupiter was close to us in its orbit. Someone can probably work out the ramifications. I think in the original story in the book was set at Saturn and the aliens that turned it into a star wanted to heat the moon Europa as it was supposed to have organisms living in an ocean below it’s frozen crust.

Have a read of the books (it’s a series) or watch the film. I think Arthur C Clarke was a very interesting writer.