The discovery of a new object in our solar system larger than Pluto will once again re-open the debate, "What is a planet?" This question is especially tricky because the definition of the word "planet" is unclear on two ends, both the Juptier-sized objects being found around other stars, and large, icy bodies being found in our own Solar System. A big part of the problem, as articulated by Mike Brown, the co-discoverer of the new "planet," is that the word "planet" is not owned by scientists. Rather, the term "planet" also has cultural definitions that scientists cannot and should not control.
Let's start with the nine (ten? more?) planets in our own Solar System. Nobody would claim that the Moon is a planet, but the Moon is larger than Pluto! So size is not the sole criterion. So you might say that a planet has to orbit the sun and not another body, but since nobody would call asteroids and comets "planets," this definition is not enough either.
Finally, looking at the nine planets (Mercury through Pluto), Pluto is clearly the odd man out. Pluto is far smaller than any other planet; its composition is very different from any other planet; its orbit is not nearly circular, and its orbit rises out of the plane that all the other planets lie in. But Pluto's orbit and chemical makeup are very similar to the dozens of smaller "Kuiper Belt Objects" that orbit the sun beyond Neptune.
Because of this, many astronomers want to "de-list" Pluto as a planet. But when astronomers try to do this, there is a huge cry from the public. In your eyes, Pluto is a planet. My suspicion is that, in time, astronomers will learn to live with the dichotomy between what astronomers call a planet and what the public calls a planet. Also, only time will tell whether the public will accept this new object and any more objects larger than Pluto as "planets." Stay tuned!
Tomorrow we'll discuss the other side of the definition of "planet," the large objects around other stars.