Image credit: compiled from NASA public images
At times, people still express bemusement, or confusion, that Pluto was reclassified as a "dwarf planet", after having been considered a full-fledged planet for the better part of a century. One thing I always point out to them is that we're discovering more and more roughly Pluto-sized bodies. If we call them all planets, the list of planets would rapidly grow unmanageable! (Do you want to memorize the names of 20 planets? How about 80?)
Today the list just got bumped up by one. Meet Makemake, the first body in our solar system to be classified as a dwarf planet since the 2006 IAU decision that established the category of dwarf planets in the first place. When the IAU made its ruling, they put three worlds in that category: Pluto, Ceres, and Eris. Now Makemake makes the fourth.
Follow up:
It's also the third world to be classified as plutoid - a dwarf planet beyond Neptune's orbit. Pluto and Eris (and now Makemake) are plutoids; Ceres, which orbits in the asteroid belt, isn't.
The discoverer of Makemake, Mike Brown, is the same guy who discovered Eris. (That discovery triggered a lot of the most recent discussion of what makes a planet, so in a way, Dr. Brown's discoveries led to the whole dwarf planet issue in the first place.) He has a fascinating discussion of why he proposed the name Makemake for this world. The name was confirmed by the IAU at the same time as the world was classified as dwarf planet and plutoid.
So now the planetary record stands thusly:
- Major planets (terrestrial): Mercury, Venus, Earth, Mars
- Major planets (jovian): Jupiter, Saturn, Uranus, Neptune
- Dwarf planet (asteroid, or terrestrial): Ceres
- Dwarf planets (plutoid): Pluto, Eris, Makemake
- Total score: 8 major planets, 4 dwarf planets
You can keep track with this official IAU list.
Expect this list to expand, however. There are many bodies that stand a good chance of being added to the dwarf planet list. In most cases, it's a matter of confirming that these bodies are actually more or less spherical - that they're large enough for their own gravity to have pulled them into a sphere. (Geek speak for this: hydrostatic equilibrium.) Some future candidates:
- Possible dwarf planets (asteroid, or terrestrial): Vesta, Pallas (unlikely), Hygiea
- Possible dwarf planets (plutoid): 2003 EL61 ("Santa"), Sedna, Quaoar, Orcus, Ixion, and 2002 AW197 ("No cute nickname"), plus over 70 smaller candidates currently being investigated.
That's not counting Charon, which is currently listed as a moon of Pluto. Physically speaking, though, that's questionable, since it would be more accurate to say that Pluto and Charon both orbit around their common center of mass, which lies somewhere between them. I personally think that Pluto and Charon should be considered a "double dwarf planet"...
