Astronomers classify planets based on their position relative to Earth’s orbit around the Sun. This classification helps understand how we observe them. One category is inferior planets, distinguished by their specific orbital relationship to Earth.
What Defines an Inferior Planet
An inferior planet is any planet whose orbit around the Sun lies entirely within Earth’s orbit. The term “inferior” refers purely to their orbital position, not to any lesser quality. This positioning influences how these planets appear when viewed from Earth.
Their orbit inside Earth’s creates specific geometrical relationships with the Sun and Earth. Because their orbital path is closer to the Sun, Earth can never come between an inferior planet and the Sun.
The Planets Classified as Inferior
Only two planets in our solar system fit the definition of an inferior planet: Mercury and Venus. Mercury is the innermost planet, orbiting closest to the Sun. Venus is the second planet from the Sun, positioned between Mercury and Earth.
Observing Inferior Planets from Earth
Observing inferior planets from Earth presents unique phenomena due to their orbital positions. Like our Moon, Mercury and Venus exhibit phases, showing varying amounts of their sunlit surfaces as they orbit the Sun. When an inferior planet is between Earth and the Sun, its unilluminated side faces us, appearing as a “new” phase and making it difficult to see. As it moves in its orbit, more of its sunlit side becomes visible, transitioning through crescent, quarter, and gibbous phases.
These planets never appear far from the Sun in the sky, visible only shortly before sunrise or after sunset. When an inferior planet is to the east of the Sun, it appears as an “evening star,” setting after the Sun. Conversely, when it is to the west of the Sun, it rises before the Sun and is known as a “morning star.” The maximum angular separation from the Sun, known as greatest elongation, is when these planets are best observed. For Mercury, this angle ranges from 18° to 28°, while for Venus, it is between 45° and 47°, with variations due to their elliptical orbits.
Occasionally, an inferior planet passes directly between Earth and the Sun, appearing as a small, dark disk moving across the Sun’s face. This event is called a transit. Transits are rare occurrences because the orbital planes of Mercury and Venus are slightly tilted relative to Earth’s orbit, meaning they usually pass above or below the Sun from our perspective.
Comparing Inferior and Superior Planets
To understand inferior planets, it is helpful to contrast them with superior planets. Superior planets are those whose orbits lie outside Earth’s orbit, meaning they are farther from the Sun than Earth. Examples include Mars, Jupiter, Saturn, Uranus, and Neptune.
The primary difference in their orbital positions leads to distinct observational characteristics. Unlike inferior planets, superior planets do not exhibit a full range of phases when viewed from Earth. They generally appear full or gibbous (more than half illuminated) because we always see a significant portion of their sunlit side from Earth. Superior planets can also appear at any angular distance from the Sun, including directly opposite the Sun, a configuration known as opposition. Inferior planets, by definition, can never be at opposition.