Why isn't Pluto a planet anymaore?

Answer 1

In a sense, it never really WAS a planet. If Eris had been discovered as quickly after Pluto as Pallas was after Ceres, Pluto would never have been considered to be a planet in the first place; astronomers would have simply said "Huh, looks like there's a second asteroid belt out there." At the time Pluto was first discovered it was thought to be a planet. We now know more about it and there are a number of problematic issues: Firstly, it's significantly smaller than was first thought. At one time it was believed to be approximately Earth-sized; it's actually smaller than the Moon. Secondly, it's got a weird orbit. It's got a relatively high eccentricity and a relatively high inclination compared to the planets. It's also in an orbital resonance with Neptune; some of the other planets have near resonances with each other, but none of them have an exact resonance like the Pluto-Neptune one (Pluto completes exactly three orbits for every two of Neptune's). Thirdly, it's not alone out there; there are plenty of other objects of roughly similar size in similar orbits which are clearly NOT associated with Pluto. This is not true of any of the planets (the current definition of "planet" specifically includes this as a requirement). Fourthly, it's not even the BIGGEST of the objects out there; Eris is larger than Pluto. Taken all together, Pluto looks less like it belongs in a group with Jupiter, Uranus, Earth, and Mars, and more like it belongs in a group with Orcus, Sedna, Haumea and Ceres.

Answer 2

Pluto is now to small to be a planet anymore.