Why is Pluto no longer a planet? Short answer: Because we changed the definition of the word “planet” to better fit its meaning.

Pedro_P

A full view of Pluto's stunning crescent
NASA
▼ When I need to remember the order of the planets I recite the phrase I learned in school: My Very Excellent Mother Just Served Us Nine... and then I stumble just as I get to “Pizzas.”
Because “Pizzas” stands for “Pluto,” and Pluto is no longer a planet. For 76 years people knew it as the smallest, most distant member of the nine-object club, but then, in 2006, the International Astronomical Union (IAU) changed all that. An astronomer from the California Institute of Technology named Mike Brown had discovered a few odd objects out beyond all the known planets. One, Eris, appeared to be larger than Pluto (although we now know they’re almost exactly the same size).
The astronomers who make up the IAU faced a hard choice: label all the new objects and hundreds of future objects as planets, or pick a narrow definition that would save the deeper meaning of the title. They picked the second option. A couple hundred scientists voted to demote Pluto and named it the first of a new group of worlds: the dwarf planets.
Why didn’t Pluto make the cut? When the IAU officially defined the word “planet” for the first time, Pluto simply didn’t fit. To keep its planetary status along with Earth, Saturn, and the rest, it needed to pass three tests:
1.A planet must orbit the sun.
2.A planet must be (mostly) round.
3.A planet must clear its neighborhood of other objects.
Pluto passes the first test with flying colors, making one loop around the sun every 248 years. Things that fail this test include objects that circle other bodies, like how the Moon orbits Earth.
The second test posed no problem either. Smaller asteroids (which do orbit the sun) can have funky shapes. Itokawa, for example, looks like a lumpy potato. But once an object gets big enough, the force of its gravity pulls down any parts that stick out too far, creating a round shape. Pluto is big enough to be round.
The real challenge to the ex-planet was the third test, which gets to the heart of what many astronomers think of when they hear the word “planet.” From Mercury to Neptune, and yes, even Pluto, most planets get their names from Roman gods. As such, we expect them to be masters of their domain. The solar system is full grains of sand, massive gas giants, and many, many objects in between. Having so many objects of different sizes whizzing around the sun gets pretty messy. In the middle of all that chaos, it’s the planets, as Pluto-slayer Mike  Brown writes on his blog, that create order.
Take the asteroid belt, a loose collection of over a million pieces of planetary rubble that live between Mars and Jupiter. The fate of a rocky asteroid is uncertain, because its next orbit could bring it too close to Jupiter, whose gravitational pull could push it in a completely different direction. Or it could crash into another asteroid and burst into pieces. (▪ ▪ ▪)

► Please, read the full note here: Source