Which characteristic defines a dwarf planet?

A dwarf planet is defined primarily by its ability to orbit the Sun and possess a nearly round shape due to its own gravity. This characteristic distinguishes dwarf planets from smaller celestial bodies, such as asteroids, which may not be spherical. The definition established by the International Astronomical Union (IAU) includes that a dwarf planet does not clear its neighboring region of other objects, which is a key difference from full-fledged planets. The requirement for it to orbit the Sun and have a round shape ensures that it is considered a significant entity within the solar system.

The other options do not accurately represent the definition of a dwarf planet. For example, while a planet that cannot support life may be true for some celestial bodies, this does not specifically pertain to the criteria for dwarf planets. Similarly, a moon or a large asteroid with an atmosphere does not align with the characteristics that define dwarf planets, which are distinct from moons and are specifically classified based on their orbital dynamics and physical shape.