There are some pretty strange planets out there. Jupiter has 63 moons, Neptune is blue and Venus spins retrograde. And those are just the planets in our own solar system--the further we explore, the stranger the worlds we discover. That’s certainly the case with XO-3b, a recently discovered exoplanet (a planet beyond our solar system ) that’s 13 times the mass of Jupiter.

Unusual Discovery

A team of astronomers discovered XO-3b during the XO Project, a collaboration between professional and amateur astronomers. The planet is considered unusual compared to its more than 200 fellow exoplanets. For one, it orbits its parent star in only four days, making it the largest and most massive planet to orbit its parent star so tightly. And, because it’s so close to its star, it should follow a circular orbit, owing to the gravitational pull of the star. But XO-3b follows an elliptical orbit. And, XO-3b is a “transiting star,” meaning it passes in front of its star during every orbit. Only a few dozen transiting stars are known.

Planet? Star? Brown Dwarf?

Beyond its novelty, though, XO-3b is also adding to an already existing controversy: where is the boundary between brown dwarfs and massive planets? Brown dwarfs are more massive than planets, but don’t meet the hydrogen fusion requirement that would classify them as stars. XO-3b’s mass lies right on the boundary between the two.

So what’s the difference?

Objects with a mass between 15 and 75 times the mass of Jupiter are usually considered brown dwarfs. At such a large size, an object could not maintain the fusion of hydrogen as a star does, which is why many astronomers call them “failed stars.”

Stars form from clouds of gas which contract, and reach such a high temperature in their center that the hydrogen in the cloud fuses with helium, releasing energy and producing light. Planets form from small dust particles left behind after the formation of a star. After colliding, the particles stick together to form an object, but they don’t have a high enough temperature for fusion to occur or to release energy; therefore, they do not produce light as a star does.