Question: why do planet plutto referred to as a dwarf

Jacquie Oliwa answered on 13 Jul 2017:

Hi there,
In 2006, the International Astronomical Union (IAU), which has the final say on matters of astronomical nomenclature, voted on a formal definition of what makes a planet. According to their decision a planet must satisfy the following three criteria:
i) It must be an object which independently orbits the Sun (this means moons can’t be considered planets, since they orbit planets)
ii) It must have enough mass that its own gravity pulls it into a roughly spheroidal (circular) shape. So asteroids are not planets since they are smaller and irregularly shaped
iii) It must be large enough to “dominate” its orbit (i.e. its mass must be much larger than anything else which crosses its orbit). A dwarf planet’s path around the sun is full of other objects like asteroids and comets. They propose that if a body’s mass is greater than the total mass of small stuff which orbits in the same region, it is a planet.

Because Pluto is not large enough to “dominate” its orbit, it is not a planet. (Neptune is about 8000 times more massive than Pluto, so Neptune is a planet and Pluto is a dwarf planet.) From its discovery in 1930, and up until 1978 when Pluto’s moon Charon was discovered, Pluto was thought to be larger than Mercury and possibly even Mars (in reality, it’s much smaller in mass than either of them). Pluto’s mass is now known to be 25 times smaller than Mercury’s and only 9 times larger than that of Ceres, the largest body in the asteroid belt.

I hope this explains why Pluto was reclassified as a dwarf planet?
Best,
Jacquie