A planet is can be one of two related things:
A kind of Azimuth Urbit identity that sits at the bottom of the identity hierarchy. Planets act as independent identities for use by individual humans as peers on the Arvo network. Planets are issued by stars, and receive services such as peer-to-peer routing and over-the-air software updates from the star that sponsors them.
A ship on the Arvo network whose identity is a planet in the former definition. Planets, in the second sense, have the power to issue moons. Moons are Arvo ships that are meant for use with devices, are completely subordinated from their parent planet, and have no Azimuth presence.
Planets have addresses that are representable by 32 bits. This means that there are nearly 4.3 billion possible planets. Planets have four-syllable names like ~morzod-ballet or ~rovnys-ricfer.