minor planet

minor planet

minor planet: see asteroid.

Any of the many rocky small bodies that orbit the Sun mainly in a flat ring, the asteroid belt, between the orbits of Mars and Jupiter. It is thought that the gravitational influence of what became Jupiter kept the asteroids from aggregating into a single planet while the solar system was forming. Also called minor planets, asteroids are smaller than any of the solar system's major planets; only about 30 are more than 125 mi (200 km) across. Ceres is the largest known asteroid. Millions of boulder-sized asteroidal fragments are thought to exist in the solar system. Asteroids or their fragments regularly strike Earth, plunging through the atmosphere as meteors to reach its surface (see meteorite). Asteroids appear to be composed of carbonaceous, stony, and metallic (mainly iron) materials. Seealso Earth-crossing asteroid; Trojan asteroids.

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Formal minor-planet designations are number-name combinations overseen by the Minor Planet Center, a branch of the IAU. They are used for dwarf planets and small Solar System bodies such as asteroids, but not comets. They are assigned to a body once its orbit is secured, and are unrelated to provisional designations, which are automatically assigned when an object is discovered.

The two parts of a formal designation are

  • a number, historically assigned in approximate order of discovery, now assigned only after the orbit is secured, coupled with
  • a name, either the name assigned by the discoverer, or, more commonly, the provisional designation.

The syntax is (number) Name, for example (90377) Sedna or . The parentheses are now often dropped, as in 90377 Sedna, according to the preference of the astronomer or journal. In practice, however, for any reasonably well-known object the number is mostly a catalogue entry, and the name or provisional designation is generally used in place of the formal designation: Sedna, .

The convention for satellites of minor planets, such as the formal designation (87) Sylvia I Romulus for the asteroid moon Romulus, is an extension of the Roman numeral convention that had been used, on and off, for the moons of the planets since Galileo's time.

Comets are also managed by the Minor Planet Center, but use a different cataloguing system.


By 1851 there were 15 asteroids, all but one with their own symbol. The symbols grew increasingly complex as the number of objects grew, and, as they had to be drawn by hand, astronomers found some of them difficult. This difficulty was addressed by Benjamin Apthorp Gould in 1851, who suggested numbering asteroids in their order of discovery, and placing this number in a circle as the symbol for the asteroid, such as ④ for the fourth asteroid, Vesta. This practice was soon coupled with the name itself into an official number-name designation, "④ Vesta", as the number of minor planets increased. By ca 1858, the circle had been simplified to parentheses, "(4)" and "(4) Vesta", which was easier to typeset. Other punctuation such as "4) Vesta" and "4, Vesta" was also used, but had more or less completely died out by 1949.

The major exception to the convention that the number tracks the order of discovery or determination of orbit is the case of Pluto. Since Pluto was initially classified as a planet, it was not given a number until a 2006 redefinition of "planet" that excluded it. At that point, Pluto was given the formal designation (134340) Pluto.

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