And then there were 8
According to the new definition, to be called a planet an object has to exhibit 3 traits: it must orbit the Sun*; its own gravity must pull it into a nearly round shape; and it must have cleared away other objects in its neighbourhood—this is where Pluto comes up short.
So, we now find ourselves with just 8 planets in the Solar System:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The IAU has created 2 other categories to classify celestial bodies that are not planets.
The 1st of these is the “dwarf planet” category to which Pluto has been relegated.
For example, the asteroid Ceres and the distant object 2003 UB313 (nicknamed Xena) fall into this category.
And the list probably won’t end there, since the IAU has a dozen potential dwarf planets on its watchlist.
Other bodies orbiting the Sun that are neither planets nor dwarf planets will be called “small solar-system bodies”. Earth’s Moon conserves its status as a natural satellite.
Pluto no longer fits
Its status as a planet has long stirred controversy due to its small size and an elliptical orbit highly tilted to the ecliptic plane.
What’s more, at about 2,300 km across Pluto is in fact much smaller than our Moon!
* This definition does not apply to planets orbiting other stars.