Why isn't Mercury classified as a dwarf planet? It's not much larger than Pluto.
A planet has to satisfy three conditions:
1: It has to be round. No problem with that.
2: It has to orbit a star. Still no problem. (This is why Titan, one of Saturn’s moons which is larger than Mercury, is not a planet.)
3. It has to “clear the neighbourhood”.
Criterion number 3 has always been vague. Well, NEOs (Near-Earth Objects) are in Earth’s orbital path, and Pluto is occasionally but regularly in Neptune’s. Does that make them not planets? Pluto is decisively not a planet because the Kuiper Belt is a relatively dense “neighbourhood”, home to a not insignificant portion of the Solar System’s asteroids, as well as 4 of the 5 currently accepted dwarf planets. Meanwhile, Mercury, while rather small, is large enough to not have so many “neighbours”.
Another more reasonable reason is probably history. Unlike Pluto, Mercury has been recorded since Ancient Greek times. Due to its closeness to our Earth, it appears bright enough in the night sky for the naked eye to spot. With very simplistic “technology” (or lack thereof), it was one of only a few objects seen moving in the night sky constantly. The Sun, Moon, Mars, Jupiter, Venus, and Mercury. The “planets” of the geocentric model of the Solar System. Mercury has just been a planet for too long. It caused a lot of uproar when Pluto, an object classified for 70 years as a planet. The dissent from the community when an object classified for 2000 years would probably be much larger. The IAU might just not have wanted to take such risks.
TL;DR: The Kuiper Belt, which makes Pluto not have “cleared the neighbourhood”, or possibly historical reasons are why Mercury is not a dwarf planet.
To be classified as a planet , an object should meet 3 different criteria:
- It should orbit a star.
- It should be massive enough to possess a gravitational field strong enough to make it collapse into a spherical shape under the influence of it's own gravity but not massive enough to undergo thermonuclear fusion (which would make it a star).
- It's gravitational influence should clear all surrounding region of other stuff (asteroid belts, dust etc)
Pluto fails number 3 ( look up plutoids and the kuiper belt)
P.S. stuff orbiting the object in question does not violate number 3. Stuff orbiting the parent star does, hence our moon and planet earth or Saturn's rings and planet Saturn.