According to the International Astronomical Union, a dwarf planet in our solar system is a body that orbits the Sun (is not a moon), has sufficient mass to be round, yet has not cleared the neighborhood around its orbit. But, astronomers continue to debate the definition, also looking at criteria such as a body’s location in the solar system and whether or not is has moons.
How Many Dwarf Planets Are There?
How many dwarf planets there are depends on who you ask. Most astronomers accept 5 dwarf planets.
- 3 Dwarf Planets: Pluto, Eris, and Ceres most closely meet the criteria for dwarf planets. Some astronomers exclude Ceres because of its location and consider it a large asteroid, but not a dwarf planet. Pluto has at least five moons, while Eris has one moon, and Ceres has none.
- 5 Dwarf Planets: The usual list of dwarf planets is Pluto, Eris, Ceres, Haumea, and Makemake. Haumea has two known moons, while Makemake has one. Haumea is not round. Whether or not either Haumea or Makemake have hydrostatic equilibrium is unclear.
- 9 Dwarf Planets: Quaoar, Sedna, Orcus, and Gonggong likely meet the criteria and are dwarf planets. But, there is a lot we don’t know about these worlds.
- 10 or more: Salacia (one known moon), 2002 MS4, Varuna, Ixion, 2013 FT27, 2003 AZ84, 2004 GV9, and 2002 AW197 are candidates for classification as dwarf planets. Their density is not known well enough at this time.
List of Dwarf Planets
Compiling a list of dwarf planets is harder than you might think. Partly, this is because some bodies incompletely meet the definition. Mostly, it’s because we don’t have enough data on many of these distant worlds.
- Pluto
- Eris
- Ceres
- Haumea
- Makemake
- Quaoar
- Sedna
- Orcus
- Gonggong
- Salacia
- 2002 MS4
- Varuna
- Ixion
- 2013 FT27
- 2003 AZ84
- 2004 GV9
- 2002 AW197
Is Charon a Dwarf Planet?
Some astronomers consider Charon a moon of Pluto, while others think it is a dwarf planet because its mass is comparable to Pluto’s, it is round, and the two worlds are tidally locked. The IAU is considering whether or not Pluto and Charon are a binary system. If Pluto and Charon are a binary system, then Charon orbits the Sun. However, scientists don’t know whether or not Charon is in hydrostatic equilibrium.
Vesta and Triton
Neither Vesta nor Triton are dwarf planets under the current definition. Vesta is roughly spherical, but it appears it is not in hydrostatic equilibrium. Triton is round and even more massive than Pluto or Eris, but it orbits Neptune rather than directly orbiting the Sun.
Where Are Dwarf Planets?
Ceres is the only dwarf planet in the asteroid belt. Pluto, Orcus, Haumea, Salacia, Quaoar, and Makemake are in the Kuiper Belt, which is a region beyond Neptune’s orbit, but in the same plane as solar system. The other dwarf planets are trans-Neptunian objects (TNOs) further out and not necessarily in the same plane.
Dwarf Planet Facts
Here is a table summarizing key dwarf planet facts:
| Name | Location | Moons | Diameter (km) | Orbital Radius (AU) | Orbital Period (years) |
|---|---|---|---|---|---|
| Ceres | Asteroid belt | 0 | 939.4 ± 0.2 | 2.768 | 4.604 |
| Orcus | Kuiper belt | 1 | ~910 | 39.40 | 247.3 |
| Pluto | Kuiper belt | 5 | 2377 ± 3 | 39.48 | 247.9 |
| Charon | Kuiper belt | – | 1212 ± 1 | 39.48 | 247.9 |
| Salacia | Kuiper belt | 1 | 846 ± 21 | 42.18 | 274.0 |
| Haumea | Kuiper belt | 2 | ~1560 | 43.22 | 284.1 |
| Quaoar | Kuiper belt | 1 | 1110 ± 5 | 43.69 | 288.8 |
| Makemake | Kuiper belt | 1 | ~1430 | 45.56 | 307.5 |
| Gonggong | Scattered disc TNO | 1 | 1230 ± 50 | 67.38 | 553.1 |
| Eris | Scattered disc TNO | 1 | 2326 ± 12 | 67.78 | 558.0 |
| Sedna | Detached TNO | 0? | 995 ± 80 | 506.8 | ~11,400 |
References
- Agnor, C. B.; Hamilton, D. P. (2006). “Neptune’s capture of its moon Triton in a binary–planet gravitational encounter”. Nature. 441 (7090): 192–194. Bibcode:2006Natur.441..192A. doi:10.1038/nature04792
- International Astronomical Union (August 24, 2006). “Definition of a Planet in the Solar System: Resolutions 5 and 6.” IAU 2006 General Assembly. International Astronomical Union.
- Margot, Jean-Luc (October 15, 2015). “A Quantitative Criterion for Defining Planets”. The Astronomical Journal. 150 (6): 185. doi:10.1088/0004-6256/150/6/185
- Verbiscer, Anne J.; Helfenstein, Paul; et al. (April 2022). “The Diverse Shapes of Dwarf Planet and Large KBO Phase Curves Observed from New Horizons”. The Planetary Science Journal. 3 (4): 31. doi:10.3847/PSJ/ac63a6. 95