Astronomy Practice Exam

What defines a planet?

1. A large luminous ball that generates heat

2. An icy object that orbits a star

3. A moderately large object that shines by reflected light

4. A small and rocky object in a galaxy

The correct answer is: A moderately large object that shines by reflected light

A planet is defined as a moderately large object that orbits a star and has sufficient mass to assume a nearly round shape. It also must be large enough to clear its orbital path of other debris, which is an important criterion established by the International Astronomical Union (IAU). The choice highlighting that a planet shines by reflected light is particularly relevant. This is because planets do not produce their own light but instead reflect the light emitted by their host star. For instance, Earth and other planets in the solar system are visible from a distance because they reflect sunlight. The other descriptions do not encompass the full criteria that define a planet. A large luminous ball that generates heat typically describes stars, not planets. An icy object that orbits a star may refer to comets or ice-covered objects in the outer solar system but does not capture the essence of planet classification. Similarly, a small and rocky object in a galaxy suggests asteroids or meteoroids rather than planets, which are generally larger and must meet specific criteria regarding their gravitational influence and orbital characteristics.