List of gravitationally rounded objects of the Solar System

This is a list of objects in our Solar System that are rounded because of their own gravity. These objects are called gravitationally rounded objects (GRO). They have shapes that are close to spheres, but not perfect circles, because their gravity pulls them into a rounded form. Most of these objects could be called planets based on how scientists define a planet by its physical properties.

The sizes of these objects vary a lot. Some are small, like dwarf planets and certain moons, while others are much larger, like the eight main planets and even the Sun. This list does not include very small objects that stay rocky and irregular in shape. Instead, it focuses on those that are big enough for gravity to shape them.

Understanding these objects helps us learn about how gravity shapes worlds. It also helps scientists decide which space bodies might be interesting to explore. From tiny moons to giant planets, each one tells us more about the vast and wonderful Solar System.

Star

Main article: Sun

The Sun is a bright star that gives us light and warmth. It is very big and heavy, holding almost all the mass in our Solar System. The Sun is what we call a G-type main-sequence star, which means it is 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	m^3/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 defined a planet as a body that orbits the Sun and is big enough to be round because of its own gravity. This also means the planet is big enough to control the space around its path.

There are eight planets in our Solar System. Four are called terrestrial planets (Mercury, Venus, Earth, and Mars), and the other four are giant planets. The giant planets are split into two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). Together, these four giant planets make up more than 99% of the Solar System’s mass, not counting the Sun.

Key
* Terrestrial planet
° Gas giant
^× Ice giant

		*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

Dwarf planets are objects that orbit the Sun and are big enough to pull themselves into round shapes, but they haven’t cleared out other objects from their paths. Since 2008, five dwarf planets have been officially recognized. Pluto is the only one confirmed to be perfectly round, while Ceres, which orbits between Mars and Jupiter in the asteroid belt, is almost round. The other three orbit beyond the planet Neptune.

Scientists think several more objects beyond Neptune could be dwarf planets, but they need more observations to be sure. Some of these objects, like Sedna, Gonggong, and Quaoar, show signs of having melted inside, like bigger dwarf planets. Others might have been round in the past but aren’t anymore because of how they spin. In the asteroid belt, only Ceres is agreed to be a dwarf planet, though some think others like Vesta and Pallas might also count because they share some similarities with Earth’s rocky planets.

Key
^† Asteroid belt
^‡ Kuiper belt
^§ Scattered disc
^× Sednoid

		^†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	m^3/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, CH_4, 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 our Solar System that are big enough to have a rounded shape due to their own gravity. These include seven around Saturn, five around Uranus, four around Jupiter, and one each around Earth, Neptune, and Pluto. The smallest of these rounded moons is Mimas, a moon of Saturn, while larger moons like Titan and Rhea are still believed to maintain this rounded shape. Other large moons such as Io, Europa, Ganymede, and Triton are also thought to remain rounded today.

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

		^🜨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