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He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet". He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin. Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his work. Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet. He was searching for "the 87th of the Catalogue of the Zodiacal stars of Mr la Caille", but found that "it was preceded by another". Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801. One of the astronomers selected for the search was Giuseppe Piazzi, a Catholic priest at the Academy of Palermo, Sicily. Although they did not discover Ceres, they later found the asteroids 2 Pallas, 3 Juno and 4 Vesta. In 1800, a group headed by Franz Xaver von Zach, editor of the German astronomical journal Monatliche Correspondenz (Monthly Correspondence), sent requests to 24 experienced astronomers (whom he dubbed the "celestial police"), asking that they combine their efforts and begin a methodical search for the expected planet. The Titius–Bode law got a boost with William Herschel's discovery of Uranus near the predicted distance for a planet beyond Saturn. The pattern predicted that there ought to be another planet with an orbital radius near 2.8 astronomical units (AU), or 420 million km, from the Sun. In 1772, German astronomer Johann Elert Bode, citing Johann Daniel Titius, published a numerical procession known as the Titius–Bode law (now discredited) a formula that appeared to predict the orbits of the known planets but for an unexplained gap between Mars and Jupiter. Other theoreticians, such as Immanuel Kant, pondered whether the gap had been created by the gravity of Jupiter in 1761 astronomer and mathematician Johann Heinrich Lambert asked, "And who knows whether already planets are missing which have departed from the vast space between Mars and Jupiter? Does it then hold of celestial bodies as well as of the Earth, that the stronger chafe the weaker, and are Jupiter and Saturn destined to plunder forever?" In 1596, theoretical astronomer Johannes Kepler believed that the ratios between planetary orbits would only conform to God's design with the addition of two planets: one between Jupiter and Mars and one between Venus and Mercury. In the years between the acceptance of heliocentrism and the discovery of Neptune, several astronomers argued that mathematical laws predicted the existence of a hidden or missing planet between the orbits of Mars and Jupiter. This makes Ceres the closest known cryovolcanic body to the Sun, and the brines provide a potential habitat for microbial life. It is not completely frozen, however: brines still flow through the outer mantle and reach the surface, allowing cryovolcanoes such as Ahuna Mons to form at the rate of about one every 50 million years.
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Ceres's small size means that any internal ocean of liquid water it may once have possessed has likely frozen by now. Gravity data suggest Ceres to be partially differentiated into a muddy (ice-rock) mantle/core and a less-dense but stronger crust that is at most 30% ice by volume. Its surface features are barely visible even with the most powerful telescopes, and little was known of them until the robotic NASA spacecraft Dawn approached Ceres for its orbital mission in 2015.ĭawn found Ceres's surface to be a mixture of water ice and hydrated minerals such as carbonates and clay. Its apparent magnitude ranges from 6.7 to 9.3, peaking at opposition (when it is closest to Earth) once every 15- to 16-month synodic period. This was unexpected because asteroids typically do not emit vapor, a hallmark of comets.Ĭeres's small size means that even at its brightest it is too dim to be seen by the naked eye, except under extremely dark skies. In January 2014, emissions of water vapor were detected around Ceres, creating a tenuous, transient atmosphere known as an exosphere. In 2006, it was reclassified again as a dwarf planet – the only one always inside Neptune's orbit – because, at 940 km (580 mi) in diameter, it is the only asteroid large enough for its gravity to make it plastic and to maintain it as a spheroid. Originally considered a planet, it was reclassified as an asteroid in the 1850s after the discovery of dozens of other objects in similar orbits. Ceres was the first asteroid discovered, on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily. Ceres ( / ˈ s ɪər iː z/ minor-planet designation: 1 Ceres) is the largest object in the asteroid belt between the orbits of Mars and Jupiter.