List of gravitationally rounded objects of the Solar System

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

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.

		*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	28	16
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 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.

		†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. 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.^[50] Many TNOs in the 200–500 km radius range are dark and low-density bodies, which suggests that they retain internal porosity from their formation, and hence are not planetary bodies (as planetary bodies have sufficient gravitation to collapse out such porosity).^[51]

In 2023, Emery et al. wrote that near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 suggests that Sedna, Gonggong, and Quaoar underwent internal melting, differentiation, and chemical evolution, like the larger dwarf planets Pluto, Eris, Haumea, and Makemake, but unlike "all smaller KBOs". This is because light hydrocarbons are present on their surfaces (e.g. ethane, acetylene, and ethylene), which implies that methane is continuously being resupplied, and that methane would likely come from internal geochemistry. On the other hand, the surfaces of Sedna, Gonggong, and Quaoar have low abundances of CO and CO₂, similar to Pluto, Eris, and Makemake, but in contrast to smaller bodies. This suggests that the threshold for dwarf planethood in the trans-Neptunian region is around 500 km radius.^[52]

In 2024, Kiss et al. found that Quaoar has an ellipsoidal shape incompatible with hydrostatic equilibrium for its current spin. They hypothesised that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium, but that its shape became "frozen in" and did not change as it spun down due to tidal forces from its moon Weywot.^[53] If so, this would resemble the situation of Saturn's moon Iapetus, which is too oblate for its current spin.^[54][55] Iapetus is generally still considered a planetary-mass moon nonetheless,^[56] though not always.^[57]

The table below gives Sedna, Gonggong, and Quaoar as additional consensus dwarf planets; slightly smaller Orcus and Salacia, which are larger than 400 km radius, have been included as borderline cases for comparison.

		‡Orcus[58]	‡Salacia[59]	‡Quaoar[60]	§Gonggong[61]	×Sedna[62]
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[63] 0.0720	423[64] 0.0664	555[65] 0.0871	615[66] 0.0982	497.5[67] 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[68] 0.0001	4.9×1020[64] 0.0001	1.20×1021[69] 0.0002	1.75×1021[66] 0.0003	?
Density[t]	g/cm3	1.4±0.2[68]	1.50±0.12[64]	≈ 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?[68]	?	0.7367[69]	0.9333	0.4280[70]
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[69] or 14.0[71]	?	?
Mean surface temperature[w]	K	≈ 42	≈ 43	≈ 41	≈ 30	≈ 12
Number of known moons		1[72]	1	1[73]	1	0
Rings?		?	?	Yes[69]	?	?
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.^[74] 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.^[56] 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.^[75] 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.^[76]

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^[77] and all of the moons listed for Saturn apart from Titan and Rhea.^[55] The status of Callisto, Titan, and Rhea is uncertain, as is that of the moons of Uranus, Pluto^[25] and Eris.^[51] 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,^[78][79] 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.^[51]

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).

		🜨Moon[80]	♃Io[81]	♃Europa[82]	♃Ganymede[83]	♃Callisto[84]	♄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[85]	0.0965 ± 0.0069[85]	0.155 ± 0.065[85]	≈ 0–2[85][aa]	≈ 0	≈ 0	≈ 0	≈ 0	≈ 0
Mean surface temperature[w]	K	220	130	102	110[86]	134	64	75	64	87	76
Atmospheric composition		Ar, He Na, K, H	SO2[87]	O2[88]	O2[89]	O2, CO2[90]		H2O, N2 CO2, CH4[91]

		♄Titan[p]	♄Iapetus[p]	⛢Miranda[r]	⛢Ariel[r]	⛢Umbriel[r]	⛢Titania[r]	⛢Oberon[r]	♆Triton[92]	♇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[93]	≈ 0	≈ 0	≈ 0	≈ 0	≈ 0	≈ 0	≈ 0.7[94]	≈ 0
Mean surface temperature[w]	K	93.7[95]	130	59	58	61	60	61	38[96]	53
Atmospheric composition		N2, CH4[97]							N2, CH4[98]