List of gravitationally rounded objects of the Solar System

This is a list of objects in the Solar System that have a rounded shape because of their own gravity. These objects are called gravitationally rounded objects (GRO). They include planets, dwarf planets, and many moons. Their sizes are very different, from small objects up to the giant planets and the Sun.

The list does not include very small objects, but it does include some possible planetary-mass objects that scientists are still studying. The Sun’s place is described in relation to the Galactic Center. All other objects are listed based on how far they are from the Sun. This helps us learn how gravity shapes objects in space and how these bodies fit into our solar system.

Star

Main article: Sun

The Sun is a bright star that gives us light and warmth. It is very big and holds almost all the mass in our Solar System. The Sun is a G-type main-sequence star, a common type of star that shines steadily.

		Sun
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	million years	225–250
Mean orbital speed	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	K	1–2×106
Photospheric composition		H, He, O, C, Fe, S

Planets

Main article: Planet

In 2006, the International Astronomical Union said a planet is a body that goes around the Sun and is big enough to be round because of its own gravity. This also means the planet is big enough to control things close to it.

There are eight planets in our Solar System. Four are called terrestrial planets — Mercury, Venus, Earth, and Mars. The other four are giant planets, with Jupiter and Saturn being gas giants, and Uranus and Neptune being ice giants. Together, these giant planets have most of the mass in our Solar System.

		*Mercury	*Venus	*Earth	*Mars	°Jupiter	°Saturn	×Uranus	×Neptune
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	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	115	292	29	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

Main article: Dwarf planet

See also: List of possible dwarf planets

Dwarf planets are objects that go around the Sun. They are big enough to form a round shape, but not big enough to push away other objects from their path. The International Astronomical Union (IAU) has officially recognized five dwarf planets. Pluto is the most famous one. Ceres, which goes between Mars and Jupiter, is another well-known dwarf planet.

Astronomers think there might be more dwarf planets beyond Neptune. Some of these are called trans-Neptunian objects. A few, like Sedna, Gonggong, and Quaoar, might be dwarf planets. However, not all experts agree on how big an object needs to be to be a dwarf planet.

		†Ceres	‡Pluto	‡Haumea	‡Makemake	§Eris
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 0.186	816 (2100 × 1680 × 1074) 0.13	715 0.11	1,163 0.18
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 ≈ 0.0002	1.30×1022 0.0022	4.01 ± 0.04×1021 0.0007	≈ 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	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	40		30	30
Atmospheric composition		H2O	N2, CH4, CO	?	N2, CH4	N2, CH4
Number of known moons[v]		0	5	2	1	1
Rings?		No	No	Yes	?	?
Planetary discriminant[l][o]		0.33	0.077	0.023	0.02	0.10

		‡Orcus	‡Salacia	‡Quaoar	§Gonggong	×Sedna
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 0.0720	423 0.0664	555 0.0871	615 0.0982	497.5 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 0.0001	4.9×1020 0.0001	1.20×1021 0.0002	1.75×1021 0.0003	?
Density[t]	g/cm3	1.4±0.2	1.50±0.12	≈ 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?	?	0.7367	0.9333	0.4280
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 or 14.0	?	?
Mean surface temperature[w]	K	≈ 42	≈ 43	≈ 41	≈ 30	≈ 12
Number of known moons		1	1	1 (2?)	1	0
Rings?		?	?	Yes	?	?
Planetary discriminant[l][o]		0.003		0.0015		?[x]
Absolute magnitude (H)		2.3	4.1	2.71	1.8	1.5

Satellites

Main article: Planetary-mass moon

Further information: List of natural satellites

There are 19 natural satellites in the Solar System that are big enough to have a rounded shape because of their own gravity. These include seven moons of Saturn, five of Uranus, four of Jupiter, and one each from Earth, Neptune, and Pluto. The smallest of these rounded moons is Mimas, a moon of Saturn, with a radius of about 198 kilometers.

Some of these moons used to be rounded but are not anymore, such as Earth’s Moon and most of Saturn’s moons except Titan and Rhea. The status of some moons, like Callisto, Titan, and Rhea, as well as the moons of Uranus and Pluto’s moon Charon, is still not fully known. Other large moons like Io, Europa, Ganymede, and Triton are thought to still be rounded today.

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

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