It depends what you mean by "planet."

In a previous post, I mentioned that astronomers had used an innovative method to, for the first time, observe light from an extra-solar planet. (An extra solar planet is a planet that orbits a star other than our own sun.) This planet orbits the star GQ Lup. There's no question that the object was observed, but there now seems to be a question of whether it's a planet or just a dim star. Why do we think it's a planet? It's 20 times further from it's GQ Lup than Jupiter is from the our sun. It has a mass of 2 or 3 Jupiters, and anything less massive than 13 Jupiters is small enough to be a planet. But here's the problem: the "planet" is bright; GQ Lup is only 156 times brighter than it. And, it's hot; the "planet" is estimated to be 3,000 degrees F. So for now, astronomers are using the term "companion", rather than "planet". Last year, when Sedna was found orbiting our sun out past Pluto, it sparked a debate as to whether Sedna or Pluto were real planets. The whole problem comes back to the basic question that has yet to be answered: what is a planet? Link.