It was only in 1969 (nice) that fungi officially became its own separate kingdom.
The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body that:
- is in orbit around the Sun,
- has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- has “cleared the neighbourhood” around its orbit.
And in that same article:
It has been argued that the definition is problematic because it depends on the location of the body: if a Mars-sized body were discovered in the inner Oort cloud, it would not have enough mass to clear out a neighbourhood that size and meet criterion 3. The requirement for hydrostatic equilibrium (criterion 2) is also universally treated loosely as simply a requirement for roundedness; Mercury is not actually in hydrostatic equilibrium, but is explicitly included by the IAU definition as a planet
That’s not even addressing the issue of rogue planets which were ejected from their star system. Many estimates say they outnumber the stars. Obviously when a planet is ejected it doesn’t just disintegrate but by that poor definition it’s no longer a ““planet””, so it’s clearly a problematic definition.
