Expert: Courtney Seligman - 1/14/2010

Question
is it possible that the sun could have been a planet?  

Answer
Historically, it was considered a planet, when the term planet meant an object that moved around in the sky, relative to the background stars (using that definition, the Moon was also a planet). However, in the modern sense, no, the Sun could never have been a planet.

Exactly how small an object has to be, to be called a planet, is open to some debate. Objects with more than 80 Jupiter masses generate enough heat, by the compression of their gases, to cause thermonuclear reactions in their center. Such objects are called stars. Objects with less mass than that do not generate enough heat to cause thermonuclear reactions, and are referred to as brown dwarfs, or "failed" stars, if they are closer to 80 Jupiter masses, than not. Objects with masses a few times greater than Jupiter are called super-Jupiters, or large planets. But in either case, we are talking about objects much less massive than the Sun, which has a thousand times the mass of Jupiter, and is at least 30 times more massive than the most massive planet would be.