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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The two parts of a formal designation are
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.
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.