What are the necessary characteristics of a planet?

The necessary characteristics of a planet, as defined by the International Astronomical Union (IAU), include three primary criteria: it must orbit a star, be large enough to assume a nearly round shape due to its own gravity, and clear its orbital zone of other objects.

The first criterion, orbiting a star, is essential as it distinguishes planets from other celestial bodies like moons or asteroids. The second criterion ensures that the object is sufficiently massive so that sufficient gravitational force gives it a nearly spherical shape, which is a fundamental characteristic of celestial bodies in equilibrium. The third criterion, clearing its orbit, means that the planet has become the dominant gravitational force in its vicinity, having either consumed or ejected other debris in its orbital path.

The other choices do not encompass all necessary criteria or include characteristics that are not universally applicable to all planets. For example, having a solid surface or being in the habitable zone might apply to some planets but not to gas giants. Characteristics like possessing rings or having a moon are not requirements for a planet's identity. Thus, the comprehensive definition provided in the correct choice aligns accurately with established astronomical standards.