What is the definition of a planet?

A planet is defined as a celestial body that orbits the Sun, which is consistent with the criteria established by the International Astronomical Union (IAU). This definition emphasizes the relationship between the planet and the star it orbits, specifically the Sun in our solar system.

The importance of this definition lies in its focus on the gravitational influence and orbital dynamics, which are fundamental characteristics that distinguish planets from other celestial objects. To be classified as a planet, an object must not only orbit a star but also have sufficient mass for its self-gravity to overcome rigid body forces, allowing it to assume a nearly round shape.

The other choices do not align with the established characteristics of a planet. For instance, the option referring to a large mass of gas surrounding a star describes the structure of a star or its surrounding nebulae, which is not applicable to planets. The choice mentioning a minor object that orbits a planet describes moons or satellites, not planets themselves. Finally, defining a planet as a solid body that has no atmospheric layers overlooks those planets, like Earth and Venus, that do possess atmospheres and therefore does not accurately represent the diversity found among different planets.