This is a list of most likely gravitationally rounded objects 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 sizes of these objects range over three orders of magnitude in radius, 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.
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 loosely. Mercury is round but not actually in hydrostatic equilibrium, but 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 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 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.
Many TNOs in the 200–500 km radius range are dark and low-density bodies, like
229762 Gǃkúnǁʼhòmdímà (radius 321±14 km) or (55637) 2002 UX25 (radius 332.5±14.5 km). They have densities too low to be solid mixtures of ice and rock, which the larger TNOs are. It was once considered that this was because they were predominantly icy like some moons of Saturn, but TNOs both above and below this size range contain significant rock fractions, and William Grundy et al. pointed out that there is no evolutionary mechanism that would allow large and small TNOs to be rocky while medium ones would not be. They hypothesise instead that medium-sized TNOs are also rocky, and have low densities because they retain internal
porosity from their formation, and hence are not planetary bodies (as planetary bodies have sufficient gravitation to collapse out such porosity). Ice–rock mixtures at Kuiper belt temperatures are expected to be strong enough to support significant open spaces in objects up to 350 km radius. At 450 km radius, the interior might eventually start to collapse, but the process might not reach the surface, which would remain cold and uncompressed (as collapsing out porosity would shrink an object, likely differentiated objects now in this size range like Orcus were probably once even larger). Dark surfaces indicate that a body has never been resurfaced (in contrast to Orcus and Charon with bright, relatively clean water ice on their surfaces), and thus that it has at most incompletely differentiated (and might not have differentiated at all). This is roughly in agreement with 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.
If this assessment is correct, then only the largest few TNOs could be dwarf planets. This assessment considered
120347 Salacia (radius 423±10.5 km) and
174567 Varda (radius 370±7 km) to also be dark and low-density bodies; later studies suggest nonetheless that their densities might be higher, potentially high enough to be solid.
The only known additional TNOs that have a radius greater than 450 km are Gonggong, Quaoar, Sedna, and probably Orcus. Lowering the cutoff to 400 km increases the certainty for Orcus, and adds Salacia and perhaps also (307261) 2002 MS4, though 2002 MS4's mass is unknown and Salacia's is just below the IAU Q&A's stated mass limit. Astronomers generally agree that the first four are dwarf planets, while disagreeing on smaller bodies.
Quaoar have moons that have allowed their mass and density to be determined using
Kepler's third law, and they are either bright enough (Orcus) to suggest resurfacing and thus planetary geology at least at some point in their past, or are dense enough (Gonggong and Quaoar) that they are clearly solid bodies and thus at least potentially dwarf planets.
Sedna, which is bright but has unknown density, has been included as a strong additional candidate. Salacia, being the only other candidate known to be over 400 km radius, has been included as a sample object at the borderline: it is dark, but might be dense enough to be solid, and is above the IAU Q&A's cutoff radius and just below its cutoff mass.
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.
There are at least 20
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, Pluto, and Eris.
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, Pluto and Eris. 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 satellite of Orcus (
Vanth) is larger than Mimas, and about the size of Proteus. However, it is not included in the table as too little is known about it. It is a dark body in the size range that should allow for internal porosity.
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).
^ 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.
^ Saturn satellite info taken from NASA Saturnian Satellite Fact Sheet.
^ With the exception of the Sun and Earth symbols, astronomical symbols are mostly used by astrologers today; although occasional use of the other planet symbols (and Pluto) in astronomical contexts still exists, it is officially discouraged.
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 other symbols are even rarer in modern astronomy.
The symbol for Ceres was taken from material published by James L. Hilton; it was used astronomically when Ceres was thought to be a major planet, and continues to be used today in astrology.
The second symbol for Uranus was also taken from there; it is more common in astrology than the first symbol.
The symbols for Haumea, Makemake, and Eris appear in a NASA JPL infographic, as does the second symbol for Pluto; they are otherwise mostly astrological symbols.
The symbols for Quaoar, Sedna, Orcus, and Gonggong were taken from Unicode; so far they have only been used in astrology.
The symbol for Salacia was taken from a Unicode proposal, but is not encoded in the Unicode Standard. It has yet to receive widespread adoption amongst astronomers or astrologers.
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.
^ Uranus satellite info taken from NASA Uranian Satellite Fact Sheet.
^ Radii for plutoid candidates taken from material published by John A. Stansberry et al.
^ 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."
^ Natural satellite numbers taken from material published by Scott S. Sheppard.
Manual calculations (unless otherwise cited)
^ Surface area A derived from the radius using , assuming sphericity.
^ Volume V derived from the radius using , assuming sphericity.
^ Density derived from the mass divided by the volume.
^ Calculated using the formula where Teff = 54.8 K at 52 AU, is the geometric albedo, q = 0.8 is the
phase integral, and 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 was replaced with 4 accounting for the difference between circle and sphere. All parameters mentioned above were taken from the same paper.
^ Surface area was calculated using the formula for a scalene
where is the modular angle, or angular eccentricity; and , 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.
^ The ratio between the mass of the object and those in its immediate neighborhood. Used to distinguish between a planet and a dwarf planet.
^ This object's rotation is synchronous with its orbital period, meaning that it only ever shows one face to its primary.
^ 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.
^ "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.
^ Callisto's axial tilt varies between 0 and about 2 degrees on timescales of thousands of years.
abcStansberry, John A.; Grundy, Will M.; Brown, Michael E.; Cruikshank, Dale P.; Spencer, John; Trilling, David; Margot, Jean-Luc (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". The Solar System Beyond Neptune: 161.
^Arimatsu, Ko; Ohsawa, Ryou; Hashimoto, George L.; Urakawa, Seitaro; Takahashi, Jun; Tozuka, Miyako; et al. (October 2019). "New constraint on the atmosphere of (50000) Quaoar from a stellar occultation". The Astronomical Journal. 158 (6): 236.
^Szakáts, R.; Kiss, Cs.; Ortiz, J. L.; Morales, N.; Pál, A.; Müller, T. G.; et al. (November 2022). "Tidally locked rotation of the dwarf planet (136199) Eris discovered from long-term ground based and space photometry". Astronomy & Astrophysics.