The cat is out of the bag: Pluto is no longer a planet. The International Astronomical Union (IAU) made that decision during its 26th General Assembly. Celestial bodies in our Solar System can now be defined into three categories:
- A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
- A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
- All other objects except satellites orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.
This means our Solar System now has 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first members of the dwarf planet class are Pluto, Ceres and 2003 UB313. Pluto is considered to be the prototype of a new class of Trans-Neptunian Objects (TNO). The third category currently includes most asteroids, most TNOs, comets, …
I think this decision is much better than the initial proposal where they’d promote Ceres and Pluto’s satellite Charon to the class of planets. Recent findings indicated that Pluto is in fact a Kuiper Belt Object and that the region contained more Pluto-like objects. This added more fuel to the discussion about Pluto’s status as a planet which has now ended with the IAU’s decision.