No, it is not possible for a planet to be as big as a star. Stars are significantly larger and more massive than planets. Stars are formed through a process called stellar nucleosynthesis, where nuclear fusion reactions occur in their cores, converting hydrogen into helium and releasing vast amounts of energy. This energy production is what defines a star.
In contrast, planets are formed through the accretion of material within a protoplanetary disk surrounding a young star. While planets can vary in size and mass, they are significantly smaller than stars. The largest known planets, such as gas giants like Jupiter, are still orders of magnitude smaller than even the smallest stars.
If a planet were somehow able to grow to the size of a star, it would likely undergo significant changes. The increased mass and gravitational forces would compress the planet's material, leading to increased pressure and temperature in its core. Eventually, it might reach a point where the conditions for nuclear fusion are met, and it would transition from a planet to a star. However, this is highly unlikely to occur naturally, given the different formation processes and characteristics of stars and planets.