Ceres (dwarf planet)
Ceres is a dwarf planet in the 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. Ceres was later classified as an asteroid and more recently as a dwarf planet, the only one not beyond the orbit of Neptune and the largest that does not have a moon.
Ceres's diameter is about a quarter that of the Moon. Its small size means that even at its brightest it is too dim to be seen by the naked eye, except under extremely dark skies. Its apparent magnitude ranges from 6.7 to 9.3, peaking at opposition once every 15- to 16-month synodic period. As a result, its surface features are barely visible even with the most powerful telescopes, and little was known about it until the robotic NASA spacecraft Dawn approached Ceres for its orbital mission in 2015.
Dawn found Ceres's surface to be a mixture of water, ice, and hydrated minerals such as carbonates and clay. Gravity data suggest Ceres to be partially differentiated into a muddy mantle/core and a less dense, but stronger crust that is at most thirty percent ice by volume. Although Ceres likely lacks an internal ocean of liquid water, brines still flow through the outer mantle and reach the surface, allowing cryovolcanoes such as Ahuna Mons to form roughly every fifty million years. This makes Ceres the closest known cryovolcanically active body to the Sun. Ceres has an extremely tenuous and transient atmosphere of water vapour, vented from localised sources on its surface.
History
Discovery
In the years between the acceptance of heliocentrism in the 18th century and the discovery of Neptune in 1846, several astronomers argued that mathematical laws predicted the existence of a hidden or missing planet between the orbits of Mars and Jupiter. In 1596, theoretical astronomer Johannes Kepler believed that the ratios between planetary orbits would conform to "God's design" only with the addition of two planets: one between Jupiter and Mars and one between Venus and Mercury. Other theorists, 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 1772, German astronomer Johann Elert Bode, citing Johann Daniel Titius, published a formula later known as the Titius–Bode law that appeared to predict the orbits of the known planets but for an unexplained gap between Mars and Jupiter. This formula predicted that there ought to be another planet with an orbital radius near 2.8 astronomical units, or 420 million km, from the Sun. The Titius–Bode law gained more credence with William Herschel's 1781 discovery of Uranus near the predicted distance for a planet beyond Saturn. In 1800, a group headed by Franz Xaver von Zach, editor of the German astronomical journal Monthly Correspondence, sent requests to twenty-four experienced astronomers, whom he dubbed the "celestial police", asking that they combine their efforts and begin a methodical search for the expected planet. Although they did not discover Ceres, they later found the asteroids Pallas, Juno, and Vesta.
One of the astronomers selected for the search was Giuseppe Piazzi, a Catholic priest at the academy of Palermo, Sicily. Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801. 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". Instead of a star, Piazzi had found a moving starlike object, which he first thought was a comet. Piazzi observed Ceres twenty-four times, the final sighting occurring on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin. 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". In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande. The information was published in the September 1801 issue of the Monatliche Correspondenz.
By this time, the apparent position of Ceres had changed and was too close to the Sun's glare for other astronomers to confirm Piazzi's observations. Towards the end of the year, Ceres should have been visible again, but after such a long time, it was difficult to predict its position. To recover Ceres, the mathematician Carl Friedrich Gauss, then twenty-four years old, developed an efficient method of orbit determination. He predicted the path of Ceres within a few weeks and sent his results to von Zach. On 31 December 1801, von Zach and fellow celestial policeman Heinrich W. M. Olbers found Ceres near the predicted position and continued to record its position. At 2.8 AU from the Sun, Ceres appeared to fit the Titius–Bode law almost perfectly; when Neptune was discovered in 1846, eight AU closer than predicted, most astronomers concluded that the law was a coincidence.
The early observers were able to calculate the size of Ceres only to within an order of magnitude. Herschel underestimated its diameter at in 1802; in 1811, German astronomer Johann Hieronymus Schröter overestimated it at. In the 1970s, infrared photometry enabled more accurate measurements of its albedo, and Ceres's diameter was determined to within ten percent of its true value of.
Name and symbol
Piazzi's proposed name for his discovery was Ceres Ferdinandea: Ceres after the Roman goddess of agriculture, whose earthly home, and oldest temple, lay in Sicily; and Ferdinandea in honour of Piazzi's monarch and patron, King Ferdinand III of Sicily. The latter was not acceptable to other nations and was dropped. Before von Zach's recovery of Ceres in December 1801, von Zach referred to the planet as Hera, and Bode referred to it as Juno. Despite Piazzi's objections, those names gained currency in Germany before the object's existence was confirmed. Once it was, astronomers settled on Piazzi's name.The adjectival forms of Ceres are Cererian and Cererean, both pronounced. Cerium, a rare-earth element discovered in 1803, was named after Ceres.
The old astronomical symbol of Ceres, still used in astrology, is a sickle, ⟨⚳⟩. The sickle was one of the classical symbols of the goddess Ceres and was suggested, apparently independently, by von Zach and Bode in 1802. It is similar in form to the symbol ⟨♀⟩ of the planet Venus, but with a break in the circle. It had various minor graphic variants, including a reversed form typeset as a 'C' with a plus sign. The generic asteroid symbol of a numbered disk, ①, was introduced in 1867 and quickly became the norm.
Classification
The categorisation of Ceres has changed more than once and has been the subject of some disagreement. Bode believed Ceres to be the "missing planet" he had proposed to exist between Mars and Jupiter. Ceres was assigned a planetary symbol and remained listed as a planet in astronomy books and tables for over half a century.As other objects were discovered in the neighbourhood of Ceres, astronomers began to suspect that it represented the first of a new class of objects. When Pallas was discovered in 1802, Herschel introduced the term asteroid for these bodies, writing that "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes". In 1852 Johann Franz Encke, in the Berliner Astronomisches Jahrbuch, declared the traditional system of granting planetary symbols too cumbersome for these new objects and introduced a new method of placing numbers before their names in order of discovery. The numbering system initially began with the fifth asteroid, 5 Astraea, as number 1, but in 1867, Ceres was adopted into the new system under the name 1 Ceres.
By the 1860s, astronomers widely accepted that a fundamental difference existed between the major planets and asteroids such as Ceres, though the word "planet" had yet to be precisely defined. In the 1950s, scientists generally stopped considering most asteroids as planets, but Ceres sometimes retained its status after that because of its planet-like geophysical complexity. Then, in 2006, the debate surrounding Pluto led to calls for a definition of "planet", and the possible reclassification of Ceres, perhaps even its general reinstatement as a planet. A proposal before the International Astronomical Union, the global body responsible for astronomical nomenclature and classification, defined a planet as "a celestial body that has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium shape, and is in orbit around a star, and is neither a star nor a satellite of a planet". Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun, but on 24 August 2006 the assembly adopted the additional requirement that a planet must have "cleared the neighbourhood around its orbit". Ceres is not a planet because it does not dominate its orbit, sharing it as it does with the thousands of other asteroids in the asteroid belt and constituting only about forty per cent of the belt's total mass. Bodies that met the first proposed definition but not the second, such as Ceres, were instead classified as dwarf planets. Planetary geologists still often ignore this definition and consider Ceres to be a planet anyway.
Ceres is designated as a dwarf planet and an asteroid. A NASA webpage states that Vesta, the belt's second-largest object, is the largest asteroid. The IAU has been equivocal on the subject, though its Minor Planet Center, the organisation charged with cataloguing such objects, notes that dwarf planets may have dual designations, and the joint IAU / USGS / NASA Gazetteer categorises Ceres as both asteroid and a dwarf planet.