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List of possible dwarf planets

The meaning of «list of possible dwarf planets»

The number of dwarf planets in the Solar System is unknown. Estimates have run as high as 200 in the Kuiper belt[1] and over 10,000 in the region beyond.[2] However, consideration of the surprisingly low densities of many dwarf-planet candidates suggests that the numbers may be much lower, possibly only nine or ten among bodies known so far.[3] The International Astronomical Union (IAU) required dwarf planets to be in hydrostatic equilibrium, and notes five in particular: Ceres in the inner Solar System and four in the trans-Neptunian region: Pluto, Eris, Haumea, and Makemake, the last two of which were accepted as dwarf planets for naming purposes. Only Pluto is actually confirmed to be in hydrostatic equilibrium, due to the results of the New Horizons mission. Other trans-Neptunian objects have been called dwarf planets if they appear to at least be solid bodies: astronomers generally include at least Orcus, Quaoar, Gonggong, and Sedna.

Beside directly orbiting the Sun, the qualifying feature of a dwarf planet is that it have "sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape".[4][5][6] Current observations are generally insufficient for a direct determination as to whether a body meets this definition. Often the only clues for trans-Neptunian objects (TNO) is a crude estimate of their diameters and albedos. Icy satellites as large as 1500 km in diameter have proven to not be in equilibrium, whereas dark objects in the outer solar system often have low densities that imply they are not even solid bodies, much less gravitationally controlled dwarf planets.

Ceres, which has a significant amount of ice in its composition, is the only accepted dwarf planet in the asteroid belt, though there are unexplained anomalies.[7] 4 Vesta, the second-most-massive asteroid and one that is basaltic in composition, appears to have a fully differentiated interior and was therefore in equilibrium at some point in its history, but no longer is today.[8] The third-most massive object, 2 Pallas, has a somewhat irregular surface and is thought to have only a partially differentiated interior; it is also less icy than Ceres. Michael Brown has estimated that, because rocky objects such as Vesta are more rigid than icy objects, rocky objects below 900 kilometres (560 mi) in diameter may not be in hydrostatic equilibrium and thus not dwarf planets.[1] The question remains open if the two largest icy outer-belt asteroids 10 Hygiea and 704 Interamnia are also dwarf planets.[7][9]

Based on a comparison with the icy moons that have been visited by spacecraft, such as Mimas (round at 400 km in diameter) and Proteus (irregular at 410–440 km in diameter), Brown estimated that an icy body relaxes into hydrostatic equilibrium at a diameter somewhere between 200 and 400 km.[1] However, after Brown and Tancredi made their calculations, better determination of their shapes showed that Mimas and the other mid-sized ellipsoidal moons of Saturn up to at least Iapetus (which, at 1471 km in diameter, is approximately the same size as Haumea and Makemake) are no longer in hydrostatic equilibrium; they are also icier than TNOs are likely to be. They have equilibrium shapes that froze in place some time ago, and do not match the shapes that equilibrium bodies would have at their current rotation rates.[10] Thus Rhea, at 1528 km in diameter, is the smallest body for which gravitational measurements are consistent with current hydrostatic equilibrium. Ceres, at 950 km in diameter, is close to equilibrium, but some deviations from equilibrium shape remain unexplained.[11] Much larger objects, such as Earth's moon and the planet Mercury, are not near hydrostatic equilibrium today,[12][13][14] though the Moon is composed primarily of silicate rock and Mercury of metal (in contrast to most dwarf planet candidates, which are ice and rock). Saturn's moons may have been subject to a thermal history that would have produced equilibrium-like shapes in bodies too small for gravity alone to do so. Thus, at present it is unknown whether any trans-Neptunian objects smaller than Pluto and Eris are in hydrostatic equilibrium.[3]

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