List of gravitationally rounded objects of the Solar System

This is a list of most likely gravitationally rounded objects of the Solar System, which are objects that have a rounded, ellipsoidal shape due to their own gravity (but are not necessarily in hydrostatic equilibrium). Apart from the Sun itself, these objects qualify as planets according to common geophysical definitions of that term. The sizes of these objects range over three orders of magnitude in radius, from planetary-mass objects like dwarf planets and some moons to the planets and the Sun. This list does not include small Solar System bodies, but it does include a sample of possible planetary-mass objects whose shapes have yet to be determined. The Sun's orbital characteristics are listed in relation to the Galactic Center, while all other objects are listed in order of their distance from the Sun.

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Star
Main article: Sun

The Sun is a G-type main-sequence star. It contains almost 99.9% of all the mass in the Solar System.[1]

Sun[2][3]
Symbol (image)[q]
Symbol (Unicode)[q] ☉
Discovery year Prehistoric
Mean distance
from the Galactic Center		 km
light years		 ≈ 2.5×1017
≈ 26,000
Mean radius km
:E[f]		 695,508
109.3
Surface area km2
:E[f]		 6.0877×1012
11,990
Volume km3
:E[f]		 1.4122×1018
1,300,000
Mass kg
:E[f]		 1.9855×1030
332,978.9
Gravitational parameter m3/s2 1.327×1020
Density g/cm3 1.409
Equatorial gravity m/s2
g		 274.0
27.94
Escape velocity km/s 617.7
Rotation period days[g] 25.38
Orbital period about Galactic Center[4] million years 225–250
Mean orbital speed[4] km/s ≈ 220
Axial tilt[i] to the ecliptic deg. 7.25
Axial tilt[i] to the galactic plane deg. 67.23
Mean surface temperature K 5,778
Mean coronal temperature[5] K 1–2×106
Photospheric composition H, He, O, C, Fe, S

Planets
Main article: Planet

In 2006, the International Astronomical Union (IAU) defined a planet as a body in orbit around the Sun that was large enough to have achieved hydrostatic equilibrium and to have "cleared the neighbourhood around its orbit".[6] The practical meaning of "cleared the neighborhood" is that a planet is comparatively massive enough for its gravitation to control the orbits of all objects in its vicinity. In practice, the term "hydrostatic equilibrium" is interpreted loosely. Mercury is round but not actually in hydrostatic equilibrium, but it is universally regarded as a planet nonetheless.[7]

According to the IAU's explicit count, there are eight planets in the Solar System; four terrestrial planets (Mercury, Venus, Earth, and Mars) and four giant planets, which can be divided further into two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). When excluding the Sun, the four giant planets account for more than 99% of the mass of the Solar System.

Key
* Terrestrial planet
° Gas giant
× Ice giant
  *Mercury[8][9][10] *Venus[11][12][10] *Earth[13][14][10] *Mars[15][16][10] °Jupiter[17][18][10] °Saturn[19][20][10] ×Uranus[21][22] ×Neptune[23][24][10]
 
Symbol[q] or
Symbol (Unicode)[q] ☿ ♀ 🜨 ♂ ♃ ♄ ⛢ or ♅ ♆
Discovery year Prehistoric Prehistoric Prehistoric Prehistoric Prehistoric Prehistoric 1781 1846
Mean distance
from the Sun		 km
AU		 57,909,175
0.38709893		 108,208,930
0.72333199		 149,597,890
1.00000011		 227,936,640
1.52366231		 778,412,010
5.20336301		 1,426,725,400
9.53707032		 2,870,972,200
19.19126393		 4,498,252,900
30.06896348
Equatorial radius km
:E[f]		 2,440.53
0.3826		 6,051.8
0.9488		 6,378.1366
1		 3,396.19
0.53247		 71,492
11.209		 60,268
9.449		 25,559
4.007		 24,764
3.883
Surface area km2
:E[f]		 75,000,000
0.1471		 460,000,000
0.9020		 510,000,000
1		 140,000,000
0.2745		 64,000,000,000
125.5		 44,000,000,000
86.27		 8,100,000,000
15.88		 7,700,000,000
15.10
Volume km3
:E[f]		 6.083×1010
0.056		 9.28×1011
0.857		 1.083×1012
1		 1.6318×1011
0.151		 1.431×1015
1,321.3		 8.27×1014
763.62		 6.834×1013
63.102		 6.254×1013
57.747
Mass kg
:E[f]		 3.302×1023
0.055		 4.8690×1024
0.815		 5.972×1024
1		 6.4191×1023
0.107		 1.8987×1027
318		 5.6851×1026
95		 8.6849×1025
14.5		 1.0244×1026
17
Gravitational parameter m3/s2 2.203×1013 3.249×1014 3.986×1014 4.283×1013 1.267×1017 3.793×1016 5.794×1015 6.837×1015
Density g/cm3 5.43 5.24 5.52 3.940 1.33 0.70 1.30 1.76
Equatorial gravity m/s2
g		 3.70
0.377		 8.87
0.904		 9.8
1.00		 3.71
0.378		 24.79
2.528		 10.44
1.065		 8.87
0.904		 11.15
1.137
Escape velocity km/s 4.25 10.36 11.18 5.02 59.54 35.49 21.29 23.71
Rotation period[g] days 58.646225 243.0187 0.99726968 1.02595675 0.41354 0.44401 0.71833 0.67125
Orbital period[g] days
years		 87.969
0.2408467		 224.701
0.61519726		 365.256363
1.0000174		 686.971
1.8808476		 4,332.59
11.862615		 10,759.22
29.447498		 30,688.5
84.016846		 60,182
164.79132
Mean orbital speed km/s 47.8725 35.0214 29.7859 24.1309 13.0697 9.6724 6.8352 5.4778
Eccentricity 0.20563069 0.00677323 0.01671022 0.09341233 0.04839266 0.05415060 0.04716771 0.00858587
Inclination[f] deg. 7.00 3.39 0[13] 1.85 1.31 2.48 0.76 1.77
Axial tilt[i] deg. 0.0 177.3[h] 23.44 25.19 3.12 26.73 97.86[h] 28.32
Mean surface temperature K 440–100 730 287 227 152 [j] 134 [j] 76 [j] 73 [j]
Mean air temperature[k] K 288 165 135 76 73
Atmospheric composition He,  Na+
K+ 		 CO2, N2, SO2 N2, O2, Ar, CO2 CO2, N2
Ar		 H2, He H2, He H2, He
CH4		 H2, He
CH4
Number of known moons[v] 0 0 1 2 95 146 27 14
Rings? No No No No Yes Yes Yes Yes
Planetary discriminant[l][o] 9.1×104 1.35×106 1.7×106 1.8×105 6.25×105 1.9×105 2.9×104 2.4×104

Dwarf planets
Main article: Dwarf planet
See also: List of possible dwarf planets

Dwarf planets are bodies orbiting the Sun that are massive and warm enough to have achieved hydrostatic equilibrium, but have not cleared their neighbourhoods of similar objects. Since 2008, there have been five dwarf planets recognized by the IAU, although only Pluto has actually been confirmed to be in hydrostatic equilibrium[25] (Ceres is close to equilibrium, though some anomalies remain unexplained).[26] Ceres orbits in the asteroid belt, between Mars and Jupiter. The others all orbit beyond Neptune.

Key
† Asteroid belt
‡ Kuiper belt
§ Scattered disc
× Sednoid
†Ceres[27] ‡Pluto[28][29] ‡Haumea[30][31][32] ‡Makemake[33][34] §Eris[35]
Symbol[q] or
Symbol (Unicode)[q] ⚳ ♇ or ⯓ 🝻 🝼 ⯰
Minor planet number 1 134340 136108 136472 136199
Discovery year 1801 1930 2004 2005 2005
Mean distance
from the Sun		 km
AU		 413,700,000
2.766		 5,906,380,000
39.482		 6,484,000,000
43.335		 6,850,000,000
45.792		 10,210,000,000
67.668
Mean radius km
:E[f]		 473
0.0742		 1,188.3[10]
0.186		 816
(2100 × 1680 × 1074)
0.13[36][37]		 715
0.11[38]		 1,163
0.18[39]
Volume km3
:E[f]		 4.21×108
0.00039[b]		 6.99×109
0.0065		 1.98×109
0.0018		 1.7×109
0.0016[b]		 6.59×109
0.0061[b]
Surface area km2
:E[f]		 2,770,000
0.0054[a]		 17,700,000
0.035		 8,140,000
0.016[y]		 6,900,000
0.0135[a]		 17,000,000
0.0333[a]
Mass kg
:E[f]		 9.39×1020
0.00016		 1.30×1022
0.0022		 4.01 ± 0.04×1021
0.0007[40]		 ≈ 3.1×1021
0.0005		 1.65×1022
0.0028
Gravitational parameter m3/s2 6.263 × 1010 8.710 × 1011 2.674 × 1011 2.069 × 1011 1.108 × 1012
Density g/cm3 2.16 1.87 2.02[36] 2.03 2.43
Equatorial gravity m/s2
g		 0.27[d]
0.028		 0.62
0.063		 0.63[d]
0.064		 0.40
0.041		 0.82[d]
0.084
Escape velocity km/s[e] 0.51 1.21 0.91 0.54 1.37
Rotation period[g] days 0.3781 6.3872 0.1631 0.9511 15.7859
Orbital period[g] years 4.599 247.9 283.8 306.2 559
Mean orbital speed km/s 17.882 4.75 4.48[o] 4.40[o] 3.44[n]
Eccentricity 0.080 0.249 0.195 0.161 0.436
Inclination[f] deg. 10.59 17.14 28.21 28.98 44.04
Axial tilt[i] deg. 4 119.6[h] ≈ 126[h]  ? ≈ 78
Mean surface temperature[w] K 167[41] 40[42] <50[43] 30 30
Atmospheric composition H2O N2, CH4, CO ? N2, CH4[44] N2, CH4[45]
Number of known moons[v] 0 5 2[46] 1[47] 1[48]
Rings? No No Yes  ?  ?
Planetary discriminant[l][o] 0.33 0.077 0.023 0.02 0.10

Astronomers usually refer to solid bodies such as Ceres as dwarf planets, even if they are not strictly in hydrostatic equilibrium. They generally agree that several other trans-Neptunian objects (TNOs) may be large enough to be dwarf planets, given current uncertainties. However, there has been disagreement on the required size. Early speculations were based on the small moons of the giant planets, which attain roundness around a threshold of 200 km radius.[49] However, these moons are at higher temperatures than TNOs and are icier than TNOs are likely to be.

Many TNOs in the 200–500 km radius range are dark and low-density bodies, like 229762 Gǃkúnǁʼhòmdímà (radius 321±14 km) or (55637) 2002 UX25 (radius 332.5±14.5 km). They have densities too low to be solid mixtures of ice and rock, which the larger TNOs are. It was once considered that this was because they were predominantly icy like some moons of Saturn, but TNOs both above and below this size range contain significant rock fractions, and William Grundy et al. pointed out that there is no evolutionary mechanism that would allow large and small TNOs to be rocky while medium ones would not be. They hypothesise instead that medium-sized TNOs are also rocky, and have low densities because they retain internal porosity from their formation, and hence are not planetary bodies (as planetary bodies have sufficient gravitation to collapse out such porosity). Ice–rock mixtures at Kuiper belt temperatures are expected to be strong enough to support significant open spaces in objects up to 350 km radius. At 450 km radius, the interior might eventually start to collapse, but the process might not reach the surface, which would remain cold and uncompressed (as collapsing out porosity would shrink an object, likely differentiated objects now in this size range like Orcus were probably once even larger). Dark surfaces indicate that a body has never been resurfaced (in contrast to Orcus and Charon with bright, relatively clean water ice on their surfaces), and thus that it has at most incompletely differentiated (and might not have differentiated at all).[50] This is roughly in agreement with estimates from an IAU question-and-answer press release from 2006, giving 400 km radius and 0.5×1021 kg mass as cut-offs that normally would be enough for hydrostatic equilibrium, while stating that observation would be needed to determine the status of borderline cases.[51]

If this assessment is correct, then only the largest few TNOs could be dwarf planets.[52] This assessment considered 120347 Salacia (radius 423±10.5 km) and 174567 Varda (radius 370±7 km) to also be dark and low-density bodies; later studies suggest nonetheless that their densities might be higher, potentially high enough to be solid.[53][54]

The only known additional TNOs that have a radius greater than 450 km are Gonggong, Quaoar, Sedna, and probably Orcus. Lowering the cutoff to 400 km increases the certainty for Orcus, and adds Salacia and perhaps also (307261) 2002 MS4, though 2002 MS4's mass is unknown and Salacia's is just below the IAU Q&A's stated mass limit. Astronomers generally agree that the first four are dwarf planets, while disagreeing on smaller bodies. Gonggong, Orcus, and Quaoar have moons that have allowed their mass and density to be determined using Kepler's third law, and they are either bright enough (Orcus) to suggest resurfacing and thus planetary geology at least at some point in their past, or are dense enough (Gonggong and Quaoar) that they are clearly solid bodies and thus at least potentially dwarf planets. Sedna, which is bright but has unknown density, has been included as a strong additional candidate. Salacia, being the only other candidate known to be over 400 km radius, has been included as a sample object at the borderline: it is dark, but might be dense enough to be solid, and is above the IAU Q&A's cutoff radius and just below its cutoff mass.

‡Orcus[55] ‡Salacia[56] ‡Quaoar[57] §Gonggong[58] ×Sedna[59]
Symbol[q]
Symbol (Unicode)[q] 🝿 🝾 🝽 ⯲
Minor-planet number 90482 120347 50000 225088 90377
Discovery year 2004 2004 2002 2007 2003
Semi-major axis km
AU		 5,896,946,000
39.419		 6,310,600,000
42.18		 6,535,930,000
43.69		 10,072,433,340
67.33		 78,668,000,000
525.86
Mean radius[s] km
:E[f]		 458.5[60]
0.0720		 423[53]
0.0664		 555[61]
0.0871		 615[62]
0.0982		 497.5[63]
0.0780
Surface area[a] km2
:E[f]		 2,641,700
0.005179		 2,248,500
0.004408		 3,870,800
0.007589		 4,932,300
0.009671		 3,110,200
0.006098
Volume[b] km3
:E[f]		 403,744,500
0.000373		 317,036,800
0.000396		 716,089,900
0.000661		 1,030,034,600
0.000951		 515,784,000
0.000476
Mass[t] kg
:E[f]		 5.48×1020[64]
0.0001		 4.9×1020[53]
0.0001		 1.20×1021[65]
0.0002		 1.75×1021[62]
0.0003		  ?
Density[t] g/cm3 1.4±0.2[64] 1.50±0.12[53] ≈ 1.7 1.74±0.16  ?
Equatorial gravity[d] m/s2
g		 0.17
0.017		 0.18
0.018		 0.25
0.025		 0.31
0.029		  ?
Escape velocity[e] km/s 0.41 0.39 0.53 0.62  ?
Rotation period[g] days 9.54?[64]  ? 0.7367[65] 0.9333 0.4280[66]
Orbital period[g] years 247.49 273.98 287.97 552.52 12,059
Mean orbital speed km/s 4.68 4.57 4.52 3.63 1.04
Eccentricity 0.226 0.106 0.038 0.506 0.855
Inclination[f] deg. 20.59 23.92 7.99 30.74 11.93
Axial tilt[i] deg.  ?  ? 13.6[65] or 14.0[67]  ?  ?
Mean surface temperature[w] K ≈ 42 ≈ 43 ≈ 41 ≈ 30 ≈ 12
Number of known moons 1[68] 1 1[69] 1 0
Rings?  ?  ? Yes[65]  ?  ?
Planetary discriminant[l][o] 0.003 <0.1 0.0015 <0.1  ?[x]
Absolute magnitude (H) 2.3 4.1 2.71 1.8 1.5

As for objects in the asteroid belt, none are generally agreed as dwarf planets today among astronomers other than Ceres. The second- through fifth-largest asteroids have been discussed as candidates. Vesta (radius 262.7±0.1 km), the second-largest asteroid, appears to have a differentiated interior and therefore likely was once a dwarf planet, but it is no longer very round today.[70] Pallas (radius 255.5±2 km), the third-largest asteroid, appears never to have completed differentiation and likewise has an irregular shape. Vesta and Pallas are nonetheless sometimes considered small terrestrial planets anyway by sources preferring a geophysical definition, because they do share similarities to the rocky planets of the inner solar system.[71] The fourth-largest asteroid, Hygiea (radius 216.5±4 km), is icy. The question remains open if it is currently in hydrostatic equilibrium: while Hygiea is round today, it was probably previously catastrophically disrupted and today might be just a gravitational aggregate of the pieces.[72] The fifth-largest asteroid, Interamnia (radius 166±3 km), is icy and has a shape consistent with hydrostatic equilibrium for a slightly shorter rotation period than it now has.[73]

Satellites
Main article: Planetary-mass moon
Further information: List of natural satellites

There are at least 19 natural satellites in the Solar System that are known to be massive enough to be close to hydrostatic equilibrium: seven of Saturn, five of Uranus, four of Jupiter, and one each of Earth, Neptune, and Pluto. Alan Stern calls these satellite planets, although the term major moon is more common. The smallest natural satellite that is gravitationally rounded is Saturn I Mimas (radius 198.2±0.4 km). This is smaller than the largest natural satellite that is known not to be gravitationally rounded, Neptune VIII Proteus (radius 210±7 km).

Several of these were once in equilibrium but are no longer: these include Earth's moon[74] and all of the moons listed for Saturn apart from Titan and Rhea.[75] The status of Callisto, Titan, and Rhea is uncertain, as is that of the moons of Uranus, Pluto[25] and Eris.[50] The other large moons (Io, Europa, Ganymede, and Triton) are generally believed to still be in equilibrium today. Other moons that were once in equilibrium but are no longer very round, such as Saturn IX Phoebe (radius 106.5±0.7 km), are not included. In addition to not being in equilibrium, Mimas and Tethys have very low densities and it has been suggested that they may have non-negligible internal porosity,[76][77] in which case they would not be satellite planets.

The moons of the trans-Neptunian objects (other than Charon) have not been included, because they appear to follow the normal situation for TNOs rather than the moons of Saturn and Uranus, and become solid at a larger size (900–1000 km diameter, rather than 400 km as for the moons of Saturn and Uranus). Eris I Dysnomia and Orcus I Vanth, though larger than Mimas, are dark bodies in the size range that should allow for internal porosity, and in the case of Dysnomia a low density is known.[50]

Satellites are listed first in order from the Sun, and second in order from their parent body. For the round moons, this mostly matches the Roman numeral designations, with the exceptions of Iapetus and the Uranian system. This is because the Roman numeral designations originally reflected distance from the parent planet and were updated for each new discovery until 1851, but by 1892, the numbering system for the then-known satellites had become "frozen" and from then on followed order of discovery. Thus Miranda (discovered 1948) is Uranus V despite being the innermost of Uranus' five round satellites. The missing Saturn VII is Hyperion, which is not large enough to be round (mean radius 135±4 km).

Key
🜨 Satellite of Earth
♃ Satellite of Jupiter
♄ Satellite of Saturn
⛢ Satellite of Uranus
♆ Satellite of Neptune
♇ Satellite of Pluto
🜨Moon[78] ♃Io[79] ♃Europa[80] ♃Ganymede[81] ♃Callisto[82] ♄Mimas[p] ♄Enceladus[p] ♄Tethys[p] ♄Dione[p] ♄Rhea[p]
Roman numeral designation Earth I Jupiter I Jupiter II Jupiter III Jupiter IV Saturn I Saturn II Saturn III Saturn IV Saturn V
Symbol[q] ☾ JI JII JIII JIV SI SII SIII SIV SV
Symbol (Unicode)[q] ☾
Discovery year Prehistoric 1610 1610 1610 1610 1789 1789 1684 1684 1672
Mean distance
from primary		 km 384,399 421,600 670,900 1,070,400 1,882,700 185,520 237,948 294,619 377,396 527,108
Mean radius km
:E[f]		 1,737.1
0.272		 1,815
0.285		 1,569
0.246		 2,634.1
0.413		 2,410.3
0.378		 198.30
0.031		 252.1
0.04		 533
0.084		 561.7
0.088		 764.3
0.12
Surface area[a] 1×106 km2 37.93 41.910 30.9 87.0 73 0.49 0.799 3.57 3.965 7.337
Volume[b] 1×109 km3 22 25.3 15.9 76 59 0.033 0.067 0.63 0.8 1.9
Mass 1×1022 kg 7.3477 8.94 4.80 14.819 10.758 0.00375 0.0108 0.06174 0.1095 0.2306
Density[c] g/cm3 3.3464 3.528 3.01 1.936 1.83 1.15 1.61 0.98 1.48 1.23
Equatorial gravity[d] m/s2
g		 1.622
0.1654		 1.796
0.1831		 1.314
0.1340		 1.428
0.1456		 1.235
0.1259		 0.0636
0.00649		 0.111
0.0113		 0.145
0.0148		 0.231
0.0236		 0.264
0.0269
Escape velocity[e] km/s 2.38 2.56 2.025 2.741 2.440 0.159 0.239 0.393 0.510 0.635
Rotation period days[g] 27.321582
(sync)[m]		 1.7691378
(sync)		 3.551181
(sync)		 7.154553
(sync)		 16.68902
(sync)		 0.942422
(sync)		 1.370218
(sync)		 1.887802
(sync)		 2.736915
(sync)		 4.518212
(sync)
Orbital period about primary days[g] 27.32158 1.769138 3.551181 7.154553 16.68902 0.942422 1.370218 1.887802 2.736915 4.518212
Mean orbital speed[o] km/s 1.022 17.34 13.740 10.880 8.204 14.32 12.63 11.35 10.03 8.48
Eccentricity 0.0549 0.0041 0.009 0.0013 0.0074 0.0202 0.0047 0.02 0.002 0.001
Inclination to primary's equator deg. 18.29–28.58 0.04 0.47 1.85 0.2 1.51 0.02 1.51 0.019 0.345
Axial tilt[i][u] deg. 6.68 0.000405
± 0.00076[83]		 0.0965
± 0.0069[83]		 0.155
± 0.065[83]		 ≈ 0–2[83][aa] ≈ 0 ≈ 0 ≈ 0 ≈ 0 ≈ 0
Mean surface temperature[w] K 220 130 102 110[84] 134 64 75 64 87 76
Atmospheric composition Ar, He
Na, K, H		 SO2[85] O2[86] O2[87] O2, CO2[88] H2O, N2
CO2, CH4[89]
♄Titan[p] ♄Iapetus[p] ⛢Miranda[r] ⛢Ariel[r] ⛢Umbriel[r] ⛢Titania[r] ⛢Oberon[r] ♆Triton[90] ♇Charon[28]
Roman numeral designation Saturn VI Saturn VIII Uranus V Uranus I Uranus II Uranus III Uranus IV Neptune I Pluto I
Symbol SVI SVIII UV UI UII UIII UIV NI PI
Discovery year 1655 1671 1948 1851 1851 1787 1787 1846 1978
Mean distance
from primary		 km 1,221,870 3,560,820 129,390 190,900 266,000 436,300 583,519 354,759 17,536
Mean radius km
:E[f]		 2,576
0.404		 735.60
0.115		 235.8
0.037		 578.9
0.091		 584.7
0.092		 788.9
0.124		 761.4
0.119		 1,353.4
0.212		 603.5
0.095
Surface area[a] 1×106 km2 83.0 6.7 0.70 4.211 4.296 7.82 7.285 23.018 4.580
Volume[b] 1×109 km3 71.6 1.67 0.055 0.81 0.84 2.06 1.85 10 0.92
Mass 1×1022 kg 13.452 0.18053 0.00659 0.135 0.12 0.35 0.3014 2.14 0.152
Density[c] g/cm3 1.88 1.08 1.20 1.67 1.40 1.72 1.63 2.061 1.65
Equatorial gravity[d] m/s2
g		 1.35
0.138		 0.22
0.022		 0.08
0.008		 0.27
0.028		 0.23
0.023		 0.39
0.040		 0.35
0.036		 0.78
0.080		 0.28
0.029
Escape velocity[e] km/s 2.64 0.57 0.19 0.56 0.52 0.77 0.73 1.46 0.58
Rotation period days[g] 15.945
(sync)[m]		 79.322
(sync)		 1.414
(sync)		 2.52
(sync)		 4.144
(sync)		 8.706
(sync)		 13.46
(sync)		 5.877
(sync)		 6.387
(sync)
Orbital period about primary days 15.945 79.322 1.4135 2.520 4.144 8.706 13.46 5.877 6.387
Mean orbital speed[o] km/s 5.57 3.265 6.657 5.50898 4.66797 3.644 3.152 4.39 0.2
Eccentricity 0.0288 0.0286 0.0013 0.0012 0.005 0.0011 0.0014 0.00002 0.0022
Inclination to primary's equator deg. 0.33 14.72 4.22 0.31 0.36 0.14 0.10 157[h] 0.001
Axial tilt[i][u] deg. ≈ 0.3[91] ≈ 0 ≈ 0 ≈ 0 ≈ 0 ≈ 0 ≈ 0 ≈ 0.7[92] ≈ 0
Mean surface temperature[w] K 93.7[93] 130 59 58 61 60 61 38[94] 53
Atmospheric composition N2, CH4[95] N2, CH4[96]

See also

Notes

Unless otherwise cited:[z]

  1. ^ The planetary discriminant for the planets is taken from material published by Stephen Soter.[97] Planetary discriminants for Ceres, Pluto and Eris taken from Soter, 2006. Planetary discriminants of all other bodies calculated from the Kuiper belt mass estimate given by Lorenzo Iorio.[98]
  2. ^ Saturn satellite info taken from NASA Saturnian Satellite Fact Sheet.[99]
  3. ^ With the exception of the Sun and Earth symbols, astronomical symbols are mostly used by astrologers today; although occasional use of the other planet symbols (and Pluto) in astronomical contexts still exists,[100] it is officially discouraged.[101] Astronomical symbols for the Sun, the planets (first symbol for Uranus), and the Moon, as well as the first symbol for Pluto were taken from NASA Solar System Exploration.[102] The other symbols are even rarer in modern astronomy.
    • The symbol for Ceres was taken from material published by James L. Hilton; it was used astronomically when Ceres was thought to be a major planet, and continues to be used today in astrology.[103]
    • The second symbol for Uranus was also taken from there; it is more common in astrology than the first symbol.[103]
    • The other dwarf-planet symbols were invented by Denis Moskowitz, a software engineer in Massachusetts. His symbols for Haumea, Makemake, and Eris appear in a NASA JPL infographic, as does the second symbol for Pluto;[104] they are otherwise mostly astrological symbols. His symbols for Quaoar, Sedna, Orcus, and Gonggong were taken from Unicode;[105] so far they have only been used in astrology. He has also invented a symbol for Salacia (pictured), but it has not received widespread adoption amongst astronomers or astrologers.[105]
    The Moon is the only natural satellite with a standard abstract symbol; abstract symbols have been proposed for the others, but have not received significant astronomical or astrological use or mention. The others are often referred to with the initial letter of their parent planet and their Roman numeral.
  4. ^ Uranus satellite info taken from NASA Uranian Satellite Fact Sheet.[106]
  5. ^ Radii for plutoid candidates taken from material published by John A. Stansberry et al.[39]
  6. ^ Axial tilts for most satellites assumed to be zero in accordance with the Explanatory Supplement to the Astronomical Almanac: "In the absence of other information, the axis of rotation is assumed to be normal to the mean orbital plane."[107]
  7. ^ Natural satellite numbers taken from material published by Scott S. Sheppard.[108]

Manual calculations (unless otherwise cited)

  1. ^ Surface area A derived from the radius using , assuming sphericity.
  2. ^ Volume V derived from the radius using , assuming sphericity.
  3. ^ Density derived from the mass divided by the volume.
  4. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: Gm/r2.
  5. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: √(2Gm)/r.
  6. ^ Orbital speed is calculated using the mean orbital radius and the orbital period, assuming a circular orbit.
  7. ^ Assuming a density of 2.0
  8. ^ Calculated using the formula where Teff = 54.8 K at 52 AU, is the geometric albedo, q = 0.8 is the phase integral, and is the distance from the Sun in AU. This formula is a simplified version of that in section 2.2 of Stansberry et al., 2007,[39] where emissivity and beaming parameter were assumed to equal unity, and was replaced with 4, accounting for the difference between circle and sphere. All parameters mentioned above were taken from the same paper.

Individual calculations

  1. ^ Surface area was calculated using the formula for a scalene ellipsoid:
    where is the modular angle, or angular eccentricity; and , are the incomplete elliptic integrals of the first and second kind, respectively. The values 980 km, 759 km, and 498 km were used for a, b, and c respectively.

Other notes

  1. ^ Relative to Earth
  2. ^ Sidereal
  3. ^ Retrograde
  4. ^ The inclination of the body's equator from its orbit.
  5. ^ At pressure of 1 bar
  6. ^ At sea level
  7. ^ The ratio between the mass of the object and those in its immediate neighborhood. Used to distinguish between a planet and a dwarf planet.
  8. ^ This object's rotation is synchronous with its orbital period, meaning that it only ever shows one face to its primary.
  9. ^ Objects' planetary discriminants based on their similar orbits to Eris. Sedna's population is currently too little-known for a planetary discriminant to be determined.
  10. ^ "Unless otherwise cited" means that the information contained in the citation is applicable to an entire line or column of a chart, unless another citation specifically notes otherwise. For example, Titan's mean surface temperature is cited to the reference in its cell; it is not calculated like the temperatures of most of the other satellites here, because it has an atmosphere that makes the formula inapplicable.
  11. ^ Callisto's axial tilt varies between 0 and about 2 degrees on timescales of thousands of years.[83]

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