The Solar System has shrunk as Pluto is demoted from “Planet” to “Dwarf Planet”.
One thing I didn’t know is that the asteroid Ceres was considered a planet for 50 years until the rest of the asteroid belt was discovered. But as of today Ceres is considered a dwarf planet, along with Pluto, Charon (Pluto’s largest moon), and the misfortunately christened “2003 UB313” (but nicknamed Xena). They had been considered for planetary status last week but astronomers decided that a dozen planets would not only make the solar system too cluttered now but probably open the door to 50 or so “planets” being discovered in the near future. I mean, it’s a slippery slope, innit?
The most important effect of today’s decision to me, however, is that it makes the solar system more symmetrical, with four terrestrial planets inside of the asteroid belt and four gaseous planets outside of it, which are themselves enclosed by the rest of these bits and bobs of inferior status.
Graphic courtesy of the BBC.
Here’s the text of the International Astronomical Union resolution:
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets.” The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
The IAU therefore resolves that “planets” and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
(1) 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 neighborhood around its orbit.
(2) 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 neighborhood around its orbit, and (d) is not a satellite.
(3) All other objects, except satellites, orbiting the sun shall be referred to collectively as “Small Solar-System Bodies.” The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
The IAU further resolves:
Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.