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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. [74] Pallas (radius 255.5 ± 2 km ), the third-largest asteroid, appears never to have completed differentiation and likewise has an irregular shape.
For the giant planets, the "radius" is defined as the distance from the center at which the atmosphere reaches 1 bar of atmospheric pressure. [11] Because Sedna and 2002 MS 4 have no known moons, directly determining their mass is impossible without sending a probe (estimated to be from 1.7x10 21 to 6.1×10 21 kg for Sedna [12]).
Ceres (minor-planet designation: 1 Ceres) is a dwarf planet in the middle main asteroid belt between the orbits of Mars and Jupiter. It was the first known asteroid , discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily , and announced as a new planet .
The Sun, planets, moons and dwarf planets (true color, size to scale, distances not to scale) The following outline is provided as an overview of and topical guide to the Solar System: Solar System – gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly.
Giuseppe Piazzi, discoverer of Ceres, the largest object in the asteroid belt: Ceres was known as a planet, but later reclassified as an asteroid and from 2006 as a dwarf planet. On January 1, 1801, Giuseppe Piazzi, chairman of astronomy at the University of Palermo, Sicily, found a tiny moving object in an orbit with exactly the radius ...
The dwarf planet Ceres, as imaged by NASA's Dawn spacecraft. As of 2024, only two missions have targeted and explored dwarf planets up close. On March 6, 2015, the Dawn spacecraft entered orbit around Ceres, becoming the first spacecraft to visit a dwarf planet. [87] On July 14, 2015, the New Horizons space probe flew by Pluto and its five moons.
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Among the other dwarf planets, Ceres has no known moons. It is 90 percent certain that Ceres has no moons larger than 1 km in size, assuming that they would have the same albedo as Ceres itself. [6] Eris has one large known moon, Dysnomia. Accurately determining its size is difficult: one indicative estimate of its radius is 350 ± 57.5 km. [7]