List of gravitationally rounded objects of the Solar System

This is a list of most likely gravitationally rounded objects (GRO) 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 radii of these objects range over three orders of magnitude, 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.

Planets

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". 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 more loosely as a requirement for rounding by gravity. Mercury is round but not actually in hydrostatic equilibrium; it is universally regarded as a planet nonetheless.

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.

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 (Ceres is close to equilibrium, though some anomalies remain unexplained). Ceres orbits in the asteroid belt, between Mars and Jupiter. The others all orbit beyond Neptune.

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. 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 800 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. 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).

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.

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. If so, this would resemble the situation of Saturn's moon Iapetus, which is too oblate for its current spin. Iapetus is generally still considered a planetary-mass moon nonetheless, though not always.

The table below gives Orcus, Quaoar, Gonggong, and Sedna as additional consensus dwarf planets; slightly smaller Salacia, which is larger than 400 km radius but is not often given as a confident dwarf planet candidate, has been included as a borderline case for comparison (and is therefore italicized).

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. 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. 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. 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.

Satellites

There are 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 and all of the moons listed for Saturn apart from Titan and Rhea. The status of Callisto, Titan, and Rhea is uncertain, as is that of the moons of Uranus and Pluto's moon Charon. 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, 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.

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).

See also

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

Notes

Unless otherwise cited [z]

1. ^ The planetary discriminant for the planets is taken from material published by Stephen Soter. 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.
2. ^ Saturn satellite info taken from NASA Saturnian Satellite Fact Sheet.
3. ^ With the exception of the Sun and Earth symbols, astronomical symbols are mostly used by astrologers today; although occasional use of the other symbols in astronomical contexts still exists, it is officially discouraged. All symbols encoded in Unicode have been included. 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. The symbol for Ceres, as well as the second symbol for Uranus, was taken from material published by James L. Hilton. 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. His symbols for Quaoar, Sedna, Orcus, and Gonggong were taken from Unicode; his symbol for Salacia is mentioned in two Unicode proposals, but has not been included. 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.
5. ^ Radii for plutoid candidates taken from material published by John A. Stansberry et al.
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."
7. ^ Natural satellite numbers taken from the list published by the Jet Propulsion Laboratory.

Manual calculations (unless otherwise cited)

1. ^ Surface area A derived from the radius using A = 4 π r 2 {\textstyle A=4\pi r^{2}}, assuming sphericity.
2. ^ Volume V derived from the radius using V = 4 3 π r 3 {\textstyle V={\frac {4}{3}}\pi r^{3}}, 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 T = T eff ( 1 − q p ν ) 1 / 4 2 52 / r , {\textstyle T\ =\ {\frac {T_{\textrm {eff}}(1-qp_{\nu })^{1/4}}{\sqrt {2}}}{\sqrt {52/r}},} where Teff = 54.8 K at 52 AU, p ν {\displaystyle p_{\nu }} is the geometric albedo, q = 0.8 is the phase integral, and r {\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, where emissivity and beaming parameter were assumed to equal unity, and π {\displaystyle \pi } 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: 2 π ( c 2 + b a 2 − c 2 E ( α , m ) + b c 2 a 2 − c 2 F ( α , m ) ) , {\textstyle 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),} where α = arccos ⁡ ( c a ) {\textstyle \alpha =\arccos \left({\frac {c}{a}}\right)} is the modular angle, or angular eccentricity; m = b 2 − c 2 b 2 sin ⁡ ( α ) 2 {\textstyle m={\frac {b^{2}-c^{2}}{b^{2}\sin(\alpha )^{2}}}} and F ( α , m ) {\textstyle F(\alpha ,m)}, E ( α , m ) {\textstyle E(\alpha ,m)} 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.