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A means of uniquely identifying each minor planet or similar body within the Solar System. These designations were initially applied to asteroids, but as more objects were discovered, they came to be used for trans-Neptunian objects, dwarf planets, and host of other types of object. In fact, any body within the Solar System that is not a major planet, moon or comet will have an associated minor planet designation.
Minor planet designations consist of a simple number, starting with 1 for Ceres, the first asteroid to be discovered. Objects are formally assigned a designation, not necessarily in discovery order, but rather in the order in which their orbit is confirmed. To date, close to a million such designations have been assigned (at the time of writing, the value is 895,910), with another approximately half-million objects awaiting an assigned number.
For objects with an assigned name or provisional designation, the object is identified in full by giving the object number first, and then its name or designation. So, for example, the second asteroid to be discovered is fully '2 Pallas', while later additions to the list include dwarf planets such as '90482 Orcus'. Strictly, the minor planet designation should be given in brackets so, especially in technical literature, the examples here would be given as '(2) Pallas' or '(90482) Orcus'. When the context is obvious, these brackets are often omitted, but in more formal use they are preferred.
Origins and History
The first known asteroid, Ceres, was discovered on New Year's Day in the year 1801, and the second, Pallas, was discovered in the following year. With so few asteroids known at that time, there was no immediate need for any kind of formal numbering system, and that situation persisted for approximately half a century. The year 1851 saw two asteroids discovered in the same year - Irene and Eunomia - bringing the total to fifteen. By this time it was becoming clear that many more asteroid discoveries could be expected, and that some kind of regularised method of identifying them would be needed.
The first such system was devised by Johann Encke, who proposed numbering asteroids sequentially, and signifying those asteroid numbers by writing them in a circle. Encke's original system differed in several ways from the modern approach: he had the numbering system follow the order of discovery, and omitted the four most important asteroids discovered to that date - specifically, Ceres, Pallas, Juno and Vesta. Some practical problems with this approach, such as possible duplicate or mistaken discoveries, were resolved by Benjamin Apthorp Gould, who developed essentially the system still in use today. Encke's original notion of circling asteroid numbers - which was practical when only fifteen asteroids were known, but rather less so for numbers in the hundreds of thousands - is the origin of the convention of using brackets for these numbers.
System of Assignment
Minor planet designations are controlled by the IAU's Minor Planet Center. They are applied in chronological order of orbital determination, starting with the first asteroid discoveries, so the sequence starts 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, and so on. Note that all the early discoveries have been given formal names ('Ceres', 'Pallas' and so on). By no means all minor planets are given names like this: a little over 26,000 of these bodies have been allocated official names, with the rest being assigned provisional designations based on the date of assignment (for example, '1930 BM' refers to the discovery of Pluto in 1930).
The assignment of minor planet designations has not always been strictly sequential. The discovery of the large trans-Neptunian object Varuna in the year 2000 was considered significant enough that it was assigned a memorable designation out of sequence, so it became 20000 Varuna. Similarly, the ringed dwarf planet Quaoar, discovered two years later, was given the out-of-sequence designation 50000 Quaoar. As more such important objects were discovered, it was decided that this approach was potentially confusing, and such 'skips' in the numbering have been discontinued.
An exception was considered in one unique case. When Pluto was reassigned as a dwarf planet in 2006, it qualified for a minor planet designation, and there was some consideration of placing it out of sequence as 100000 Pluto. Ultimately the strict numerical sequence was maintained, and the newly-assigned dwarf planet was instead formally designated 134340 Pluto. (The number 100000 was instead assigned to a small asteroid of the inner Asteroid Belt named Astronautica.)
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