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A planet is a celestial body which orbits a star (or the remains of a star) that:

$(1): \quad$ has enough mass so that its own gravitational field makes it approximately spherical
$(2): \quad$ does not have enough mass to trigger thermonuclear fusion
$(3): \quad$ has cleared its neighbouring region of planetesimals.

Also see

The planets of the solar system are:

While Pluto was once classified as a planet, its status was since been reclassified as a dwarf planet.

Also note the possible existence of a ninth planet.