List of gravitationally rounded objects of the Solar System

This is a list of possibly gravitationally rounded objects of the Solar System, which are objects that have a rounded, ellipsoidal shape due to their own gravity (hydrostatic equilibrium). Their sizes range 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.

Star

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]
Astronomical symbol^[q]
Mean distance from the Galactic Center	km light years	~2.5×10^17 ~26,000
Mean radius	km :E^[f]	696,342 109.3
Surface area	km^2 :E^[f]	6.0877×10^12 11,990
Volume	km^3 :E^[f]	1.4122×10^18 1,300,000
Mass	kg :E^[f]	1.9855×10^30 332,978.9
Gravitational parameter	m^3/s^2	1.327×10^20
Density	g/cm^3	1.409
Equatorial gravity	m/s^2	274.0
Escape velocity	km/s	617.7
Rotation period	days^[g]	25.38
Orbital period about Galactic Center[3]	million years	225–250
Mean orbital speed[3]	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[4]	K	1–2×10^6
Photospheric composition		H, He, O, C, Fe, S

Planets

Key
* Terrestrial planet	° Gas giant	‡ Ice giant

The 2006 International Astronomical Union (IAU) defines 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".[5] 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. By the IAU's definition, 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.

		*Mercury[6][7]	*Venus[8][9]	*Earth[10][11]	*Mars[12][13]	°Jupiter[14][15]	°Saturn[16][17]	‡Uranus[18][19]	‡Neptune[20][21]
Astronomical symbol^[q]
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]	2,439.64 0.3825	6,051.59 0.9488	6,378.1 1	3,397.00 0.53260	71,492.68 11.209	60,267.14 9.449	25,557.25 4.007	24,766.36 3.883
Surface area	km^2 :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	km^3 :E^[f]	6.083×10^10 0.056	9.28×10^11 0.857	1.083×10^12 1	1.6318×10^11 0.151	1.431×10^15 1,321.3	8.27×10^14 763.62	6.834×10^13 63.102	6.254×10^13 57.747
Mass	kg :E^[f]	3.302×10^23 0.055	4.8690×10^24 0.815	5.972×10^24 1	6.4191×10^23 0.107	1.8987×10^27 318	5.6851×10^26 95	8.6849×10^25 14.5	1.0244×10^26 17
Gravitational parameter	m^3/s^2	2.203×10^13	3.249×10^14	3.986×10^14	4.283×10^13	1.267×10^17	3.793×10^16	5.794×10^15	6.837×10^15
Density	g/cm^3	5.43	5.24	5.52	3.940	1.33	0.70	1.30	1.76
Equatorial gravity	m/s^2	3.70	8.87	9.78	3.71	23.12	8.96	8.69	11.00
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 1.000702364	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[10]	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^+ P^+	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	79	82	27	14
Rings?		No	No	No	No	Yes	Yes	Yes	Yes
Planetary discriminant^[l]^[o]		9.1×10^4	1.35×10^6	1.7×10^6	1.8×10^5	6.25×10^5	1.9×10^5	2.9×10^4	2.4×10^4

Dwarf planets

Key
^† asteroid belt	^‡ trans-Neptunian

Dwarf planets are bodies 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, though of these only Ceres, which orbits in the asteroid belt between the orbits of Mars and Jupiter, has been confirmed.^[ae] The others all orbit beyond Neptune. Astronomers generally agree that several other trans-Neptunian objects may be large enough to be dwarf planets, given current uncertainties. It seems that dark, low-density objects like Salacia retain internal porosity from their formation, and thus are not planetary bodies.[22] Both Quaoar and Orcus have moons that have allowed their mass and density to be determined, and they are either bright enough to suggest resurfacing and thus planetary geology at least at some point in their past, or are dense enough that they are clearly solid bodies and thus at least potentially dwarf planets.

		^†Ceres[23]	^‡Pluto[24][25]	^‡Haumea[26][27][28]	^‡Makemake[29][30]	^‡Eris[31]
Astronomical symbol^[q]
Minor planet number		1	134340	136108	136472	136199
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	1,187[32] 0.186	816 (2100 × 1680 × 1074) 0.13[33][34]	715 0.11[35]	1,163 0.18[36]
Volume	km^3 :E^[f]	4.21×10^8 0.00039^[b]	6.99×10^9 0.0065	1.98×10^9 0.0018	1.7×10^9 0.0016^[b]	6.59×10^9 0.0061^[b]
Surface area	km^2 :E^[f]	2,770,000 0.0054^[a]	17,700,000 0.035	8,140,000 0.016^[z]	6,900,000 0.0135^[a]	17,000,000 0.0333^[a]
Mass	kg :E^[f]	9.39×10^20 0.00016	1.30×10^22 0.0022	4.01 ± 0.04×10^21 0.0007[37]	< 4.4 ×10^21 < 0.0007	1.65×10^22 0.0028
Gravitational parameter	m^3/s^2	6.263 × 10^10	8.710 × 10^11	2.674 × 10^11	< 2.937 × 10^11	1.108 × 10^12
Density	g/cm^3	2.16	1.87	2.02[33]	2.10	2.43
Equatorial gravity	m/s^2	0.27^[d]	0.62	0.63^[d]	< 0.57	0.82^[d]
Escape velocity	km/s^[e]	0.51	1.21	0.91	< 0.91	1.37
Rotation period^[g]	days	0.3781	6.3872	0.1631	0.9511	14.560
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	?	≈78
Mean surface temperature^[w]	K	167[38]	40[39]	<50[40]	30	30
Atmospheric composition		H2O	N2, CH4, CO	?	N2, CH4[41]	N2, CH4[42]
Number of known moons^[v]		0	5	2[43]	1[44]	1[45]
Planetary discriminant^[l]^[o]		0.33	0.077	0.023	0.02	0.10

Of the remaining trans-Neptunian objects, the most likely dwarf planets include:

		Orcus[46]	Salacia[47]	Quaoar[48]	Gonggong[49]	Sedna[50]
Minor-planet number		90482	120347	50000	225088	90377
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[51] 0.0720	423[52] 0.0664	560.5[53] 0.0870	615[54] 0.0982	497.5[55] 0.0780
Surface area^[a]	km^2 :E^[f]	2,641,700 0.005179	2,248,500 0.004408	3,948,000 0.007741	4,932,300 0.009671	3,110,200 0.006098
Volume^[b]	km^3 :E^[f]	403,744,500 0.000373	317,036,800 0.000396	737,591,000 0.000681	1,030,034,600 0.000951	515,784,000 0.000476
Mass^[t]	kg :E^[f]	6.32×10^20[56] 0.0001	4.9×10^20[52] 0.0001	1.41×10^21[57] 0.0003	1.75×10^21[54] 0.0003	?
Density^[t]	g/cm^3	1.5±0.3[56]	1.50±0.12[52]	1.99±0.46[57]	1.74±0.16	?
Equatorial gravity^[d]	m/s^2	0.27	0.18	0.24	0.285	?
Escape velocity^[e]	km/s	0.50	0.39	0.45	0.604	?
Rotation period^[g]	days	?	?	0.3683	0.9333	0.4280[58]
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
Mean surface temperature^[w]	K	~42	~43	~41	~30	~12
Number of known moons		1[59]	1	1[60]	1	0
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

Satellites

Key
^🜨 Satellite of Earth	^♃ Satellite of Jupiter	^♄ Satellite of Saturn	^⛢ Satellite of Uranus	^♆ Satellite of Neptune	^♇ Satellite of Pluto

There are 19 natural satellites in the Solar System that are known to be massive enough to be close to hydrostatic equilibrium. Alan Stern calls these satellite planets, although the term major moon is more common.

Several of these were once in equilibrium but are no longer: these include Earth's moon and all of the moons listed for Saturn apart from Titan and Rhea. The status of the moons of Uranus, Pluto and Eris are uncertain. Other moons that were once in equilibrium but are no longer very round, such as Saturn's Phoebe, are not included. Satellites are listed first in order from the Sun, and second in order from their parent body.

		^🜨Moon[61]	^♃Io[62]	^♃Europa[63]	^♃Ganymede[64]	^♃Callisto[65]	^♄Mimas^[p]	^♄Enceladus^[p]	^♄Tethys^[p]	^♄Dione^[p]	^♄Rhea^[p]
Astronomical symbol^[q]
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]	1×10^6 km^2	37.93	41.910	30.9	87.0	73	0.49	0.799	3.57	3.965	7.337
Volume^[b]	1×10^9 km^3	22	25.3	15.9	76	59	0.033	0.067	0.63	0.8	1.9
Mass	1×10^22 kg	7.3477	8.94	4.80	14.819	10.758	0.00375	0.0108	0.06174	0.1095	0.2306
Density^[c]	g/cm^3	3.3464	3.528	3.01	1.936	1.83	1.15	1.61	0.98	1.48	1.23
Equatorial gravity^[d]	m/s^2	1.622	1.796	1.314	1.428	1.235	0.0636	0.111	0.145	0.231	0.264
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	0	0–0.33[66]	0	0	0	0	0	0
Mean surface temperature^[w]	K	220	130	102	110[67]	134	64	75	64	87	76
Atmospheric composition		Ar, He Na, K, H	SO2[68]	O2[69]	O2[70]	O2, CO2[71]		H2O, N2 CO2, CH4[72]
Rings?		No	No	No	No	No	No	No	No	No	Yes?

		^♄Titan^[p]	^♄Iapetus^[p]	^⛢Miranda^[r]	^⛢Ariel^[r]	^⛢Umbriel^[r]	^⛢Titania^[r]	^⛢Oberon^[r]	^♆Triton[73]	^♇Charon[24]
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]	1×10^6 km^2	83.0	6.7	0.70	4.211	4.296	7.82	7.285	23.018	4.580
Volume^[b]	1×10^9 km^3	71.6	1.67	0.055	0.81	0.84	2.06	1.85	10	0.92
Mass	1×10^22 kg	13.452	0.18053	0.00659	0.135	0.12	0.35	0.3014	2.14	0.152
Density^[c]	g/cm^3	1.88	1.08	1.20	1.67	1.40	1.72	1.63	2.061	1.65
Equatorial gravity^[d]	m/s^2	1.35	0.22	0.08	0.27	0.23	0.39	0.35	0.78	0.28
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	0	0	0	0	0	0	0	?
Mean surface temperature^[w]	K	93.7[74]	130	59	58	61	60	61	38[75]	53
Atmospheric composition		N2, CH4[76]							N2, CH4[77]

See also

• List of Solar System objects by size
• Lists of astronomical objects
• List of former planets
• Planetary object

Notes

Unless otherwise cited:^[ac]

15. ^{^} The planetary discriminant for the planets is taken from material published by Stephen Soter.[78] 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.[79]
16. ^{^} Saturn satellite info taken from NASA Saturnian Satellite Fact Sheet.[80]
17. ^{^} Astronomical symbols for all listed objects except Ceres taken from NASA Solar System Exploration.[81] Symbol for Ceres was taken from material published by James L. Hilton.[82] The Moon is the only natural satellite with an astronomical symbol, and only the dwarf planets Pluto and Ceres are among the few minor planets assigned symbols, which are now known to be round.
18. ^{^} Uranus satellite info taken from NASA Uranian Satellite Fact Sheet.[83]
19. ^{^} Radii for plutoid candidates taken from material published by John A. Stansberry et al.[36]
21. ^{^} 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."[84]
22. ^{^} Natural satellite numbers taken from material published by Scott S. Sheppard.[85]

Manual calculations (unless otherwise cited)

1. ^{^} Surface area A derived from the radius using ${\displaystyle {\begin{smallmatrix}A=4\pi r^{2}\end{smallmatrix}}}$, assuming sphericity.
2. ^{^} Volume V derived from the radius using ${\displaystyle {\begin{smallmatrix}V={\frac {4}{3}}\pi r^{3}\end{smallmatrix}}}$, 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: G*m/r² .
5. ^{^} Escape velocity derived from the mass m, the gravitational constant G and the radius r: sqrt((2*G*m)/r) .
14. ^{^} Orbital speed is calculated using the mean orbital radius and the orbital period, assuming a circular orbit.
20. ^{^} Assuming a density of 2.0
23. ^{^} Calculated using the formula ${\displaystyle {\begin{smallmatrix}T\ =\ {\frac {T_{\textrm {eff}}(1-qp_{\nu })^{1/4}}{\sqrt {2}}}{\sqrt {52/r}},\end{smallmatrix}}}$ where T_eff =54.8 K at 52 AU, ${\displaystyle p_{\nu }}$ is the geometric albedo, q=0.8 is the phase integral, and ${\displaystyle r}$ 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,[36] where emissivity and beaming parameter were assumed equal unity, and ${\displaystyle \pi }$ was replaced with 4 accounting for the difference between circle and sphere. All parameters mentioned above were taken from the same paper.
27. ^{^} Calculated using the formula ${\displaystyle {\begin{smallmatrix}D={\frac {1329}{\sqrt {p}}}10^{-0.2H}\end{smallmatrix}}}$, where H is the absolute magnitude, p is the geometric albedo and D is the diameter in km, and assuming an albedo of 0.15, as per Dan Bruton.[86]
30. ^{^} Mass derived from the density multiplied by the volume.

Individual calculations

25. ^{^} Derived from density
26. ^{^} Surface area was calculated using the formula for a scalene ellipsoid:

    ${\displaystyle {\begin{smallmatrix}2\pi \left(c^{2}+b{\sqrt {a^{2}-c^{2}}}E(\alpha ,m)+{\frac {bc^{2}}{\sqrt {a^{2}-c^{2}}}}F(\alpha ,m)\right),\,\!\end{smallmatrix}}}$ where ${\displaystyle {\begin{smallmatrix}\alpha =\arccos \left({\frac {c}{a}}\right)\,\,\!\end{smallmatrix}}}$ is the modular angle, or angular eccentricity; ${\displaystyle {\begin{smallmatrix}m={\frac {b^{2}-c^{2}}{b^{2}\sin(\alpha )^{2}}}\,\!\end{smallmatrix}}}$ and ${\displaystyle {\begin{smallmatrix}F(\alpha ,m)\,\!\end{smallmatrix}}}$, ${\displaystyle {\begin{smallmatrix}E(\alpha ,m)\,\!\end{smallmatrix}}}$ 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

6. ^{^} Relative to Earth
7. ^{^} Sidereal
8. ^{^} Retrograde
9. ^{^} The inclination of the body's equator from its orbit.
10. ^{^} At pressure of 1 bar
11. ^{^} At sea level
12. ^{^} The ratio between the mass of the object and those in its immediate neighborhood. Used to distinguish between a planet and a dwarf planet.
13. ^{^} This object's rotation is synchronous with its orbital period, meaning that it only ever shows one face to its primary.
24. ^{^} 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.
28. ^{^} Proteus average diameter: 210 km;[73] Mimas average diameter: 199 km[80]
29. ^{^} "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.
30. ^{^} Gravitational measurements by the Dawn orbiter have demonstrated that Ceres is in hydrostatic equilibrium.[87] None of the other putative dwarf planets have been observed this closely, though Pluto and Eris are universally assumed to be in equilibrium as well.

References

1. Woolfson, Michael Mark (2000). "The Origin and Evolution of the Solar System". Astronomy & Geophysics. 41 (1): 1.12–1.19. Bibcode:2000A&G....41a..12W. doi:10.1046/j.1468-4004.2000.00012.x.
2. NASA Solar System exploration Sun factsheet Archived 2008-01-02 at the Wayback Machine and NASA Sun factsheet Archived 2010-07-15 at the Wayback Machine NASA Retrieved on 2008-11-17 (unless otherwise cited)
3. Leong, Stacy (2002). Elert, Glenn (ed.). "Period of the Sun's Orbit around the Galaxy (Cosmic Year)". The Physics Factbook (self-published). Retrieved 2008-06-26.
4. Aschwanden, Markus J. (2007). "The Sun". In McFadden, Lucy Ann; Weissman, Paul R.; Johnsson, Torrence V. (eds.). Encyclopedia of the Solar System. Academic Press. p. 80.
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10. "NASA Earth factsheet". NASA. Retrieved 17 November 2008.
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