Official and Nearly Certain Dwarf Planets
As of 2011, five objects are recognized as dwarf planets by the IAU. Mike Brown considers these and four others to be "nearly certain". For Ceres and Pluto, this is known through direct observation. Brown thinks the other seven are massive enough to be in hydrostatic equilibrium even if they are dense (primarily rocky) and at the lower end of their estimated diameters. Eris is more massive than Pluto; Haumea and Makemake were assigned names as dwarf planets based on their absolute magnitudes. In relative distance from the Sun, the five "official" dwarf planets are:
- Ceres – discovered on January 1, 1801, 45 years before Neptune. Considered a planet for half a century before reclassification as an asteroid. Accepted as a dwarf planet by the IAU on September 13, 2006.
- Pluto – discovered on February 18, 1930. Classified as a planet for 76 years. Reclassified as a dwarf planet by the IAU on August 24, 2006.
- Haumea – discovered on December 28, 2004. Accepted by the IAU as a dwarf planet on September 17, 2008.
- Makemake – discovered on March 31, 2005. Accepted by the IAU as a dwarf planet on July 11, 2008.
- Eris – discovered on January 5, 2005. Called the "tenth planet" in media reports. Accepted by the IAU as a dwarf planet on September 13, 2006.
The other four "nearly certain" objects per Brown, Tancredi, et al. are:
- Orcus – discovered on February 17, 2004.
- Quaoar – discovered on June 5, 2002.
- 2007 OR10 – discovered on July 17, 2007.
- Sedna – discovered on November 14, 2003.
No space probes have visited any of these. This will change if NASA's Dawn and New Horizons missions reach Ceres and Pluto, respectively. The plans are for Dawn to orbit Ceres, and New Horizons to fly by Pluto, both in 2015. Dawn entered orbit around the potential dwarf planet Vesta on 16 July 2011. On 5 September 2012, Dawn left Vesta in order to orbit Ceres, which it is scheduled to reach in February 2015.
| Orbital attributes | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Name | Region of Solar System |
Orbital radius (AU) |
Orbital period (years) |
Mean orbital speed (km/s) |
Inclination to ecliptic |
Orbital eccentricity |
Planetary discriminant |
||||
| Ceres | Asteroid belt | 2.77 | 4.60 | 17.882 | 10.59° | 0.079 | 0.33 | ||||
| Pluto | Kuiper belt (plutino) | 39.48 | 248.09 | 4.666 | 17.14° | 0.249 | 0.077 | ||||
| Haumea | Kuiper belt (12:7) | 43.13 | 283.28 | 28.22° | 0.195 | 0.020 | |||||
| Makemake | Kuiper belt (cubewano) | 45.79 | 309.9 | 4.419 | 28.96° | 0.159 | 0.02 | ||||
| Eris | Scattered disc | 67.67 | 557 | 3.436 | 44.19° | 0.442 | 0.10 | ||||
| Physical attributes | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Name | Equatorial diameter relative to the Moon |
Equatorial diameter (km) |
Mass relative to the Moon |
Mass (×1021 kg) |
Density (g/cm3) |
Surface gravity (m/s2) |
Escape velocity (km/s) |
Axial inclination |
Rotation period (days) |
Moons | Surface temp. (K) |
Atmosphere | |
| Ceres | 28% | 974.6±3.2 | 1.3% | 0.94 | 2.08 | 0.27 | 0.51 | ≈ 3° | 0.38 | 0 | 167 | none | |
| Pluto | 66% | 2306±20 | 17.8% | 13.05 | 2.0 | 0.58 | 1.2 | 119.59° | −6.39 | 5 | 44 | transient | |
| Haumea | ≈ 37% | 1300±? | 5.5% | 4.01 ± 0.04 | 2.6–3.3 (?) | 0.44 | 0.84 | 0.16 | 2 | 32 ± 3 | ? | ||
| Makemake | 41% | 1420±60 | ≈ 4% ? | ≈ 3 ? | 1.7 ± 0.3 | ? | 0.32 | 0 | ≈ 30 | none | |||
| Eris | 67% | 2326±12 | 22.7% | 16.7 | 2.5 | ≈ 0.8 | 1.3 | ≈ 1 (0.75–1.4) | 1 | ≈ 42 | transient? | ||
| Orbital attributes | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Name | Region of Solar System |
Orbital radius (AU) |
Orbital period (years) |
Mean orbital speed (km/s) |
Inclination to ecliptic |
Orbital eccentricity |
Planetary discriminant |
||||
| Orcus | Kuiper belt (plutino) | 39.17 | 245.18 | 20.57° | 0.227 | 0.003 | |||||
| Quaoar | Kuiper belt (cubewano) | 43.405 | 285.97 | 8.00° | 0.039 | 0.007–0.010 | |||||
| 2007 OR10 | Scattered disc (10:3?) | 67.21 | 550.98 | 30.70° | 0.500 | ? | |||||
| Sedna | Detached | 518.57 | ≈11,400 | 11.93° | 0.853 | ? | |||||
| Physical attributes | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Name | Equatorial diameter relative to the Moon |
Equatorial diameter (km) |
Mass relative to the Moon |
Mass (×1021 kg) |
Density (g/cm3) |
Surface gravity (m/s2) |
Escape velocity (km/s) |
Axial inclination |
Rotation period (days) |
Moons | Surface temp. (K) |
Atmosphere | |
| Orcus | ≈ 22% | 760–810 | 0.9% | 0.63 | ° | 0.55 | 1 | ||||||
| Quaoar | ≈ 26% | 890±70 | 1.8–2.6% | 1.6 ± 0.3 | ° | 0.74 | 1 | ||||||
| 2007 OR10 | ≈ 37% | 1280±210 | ? | ? | ? | 0 | |||||||
| Sedna | ≈ 30% | 995 ± 80 | ≈1.4% | ≈1 | ? | 0.42 | 0 | ≈ 12 | |||||
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