| abstract | - The ordering is different depending on whether one chooses radius or mass, because some objects are denser than others. For instance Uranus is bigger than Neptune but less massive, and although Ganymede and Titan are larger than Mercury, they have less than half its mass. Some objects in the lower tables, despite their small radii, are more massive than objects in the upper tables because they have a higher density. Several new trans-Neptunian objects (TNOs) have been discovered of significant size. While their radius remains provisional due to the recency of discovery, and is often expressed as a range, the approximate locations in this list are shown. All Solar System objects more massive than 1021 kilograms (one yottagram [Yg]) are known or expected to be approximately spherical. Astronomical bodies relax into rounded shapes (ellipsoids), achieving hydrostatic equilibrium, when the gravity of their mass is sufficient to overcome the structural strength of their material. However, objects made of ice become regular more easily than those made of rock, and many icy objects are spheroidal at far lower masses. The cutoff boundary for regularity appears to roughly coincide with the 200 km radius. The larger objects in the mass range between 1018 kg to 1021 kg (1 to 1000 Zettagrams (Zg)) such as Tethys, Ceres, and Mimas, have relaxed to an equilibrium oblate spheroid due to their gravity, while the less massive rubble piles (e. g. Amalthea and Janus) are roughly rounded, but not spherical, dubbed "irregular". Spheroidal bodies typically have some polar flattening due to the centrifugal force from their rotation, but a characteristic feature of the "irregular"-shaped bodies is that there is a significant difference in the length of their two equatorial diameters. There appears to be difficulty in figuring out the diameter (within a factor of about 2) for typical objects beyond Saturn. (See 2060 Chiron as an example.) For TNOs there is some confidence in the diameters, but for non-binary TNOs there is no real confidence in the "unreferenced wiki-assumed" masses/densities. Many TNOs are just assumed to have a density of 2.0 g/cm³, though it is just as likely that they have a comet like density of only 0.5 g/cm³. Thus most provisional TNOs are not given a MEarth value to prevent from cluttering the list with too many assumptions that could be off by an order of magnitude. For example if a TNO is poorly assumed to have a mass of 3.59×1020 kg based on a radius of 350 km with a density of 2 g/cm³ and is later discovered to only have a radius of 175 km with a density of 1 g/cm³, the mass estimate would be only 2.24×1019 kg. The sizes and masses of many of the moons of Jupiter and Saturn are fairly well known due to numerous observations and interactions of the Galileo and Cassini orbiters. But many of the moons with a radius less than ~100 km, such as Jupiter's Himalia, still have unknown masses with assumed densities. Again, as we get further from the Sun than Saturn, things get less clear. There has not yet been an orbiter around Uranus or Neptune for long-term study of the moons. For the small outer irregular moons of Uranus, such as Sycorax, which were not discovered by the Voyager 2 flyby, even different NASA web pages, such as the National Space Science Data Center and JPL Solar System Dynamics, have somewhat contradictory size and albedo estimates depending on which research paper is being cited. Data for those objects smaller than Miranda are less reliable due to uncertainties in the figures for mass and radius, and irregularities in the shape and density of the objects listed.
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