Question

Which one of the following planets lost its planet status recently?

• A. Moon
• B. Neptune
• C. Pluto
• D. Saturn

Answer 1

The Correct Option is C

Step 1 — Recall the list of planets in the Solar System:
For a long time, schoolbooks listed nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Pluto was discovered in 1930 and considered the ninth planet for decades.

Step 2 — Why was Pluto’s status questioned?
As astronomy advanced, scientists discovered other small celestial bodies beyond Neptune, similar in size to Pluto (e.g., Eris, Haumea, Makemake). This raised questions: should Pluto still be called a planet, or should the definition of a planet be revised?

Step 3 — The 2006 IAU decision:
In 2006, the International Astronomical Union (IAU) redefined the term “planet.” According to this definition, a planet must:
1. Orbit the Sun,
2. Be spherical in shape (due to its own gravity),
3. Have “cleared the neighborhood” around its orbit (i.e., be gravitationally dominant).
Pluto failed the third criterion because its orbit overlaps with other icy bodies in the Kuiper Belt.

Step 4 — Result of the reclassification:
Pluto was reclassified as a dwarf planet, not a full-fledged planet. The Solar System officially has 8 planets now.

Step 5 — Elimination of other options:
• Mercury, Venus, Earth, and other planets still retain full planet status.
• Only Pluto has been officially downgraded from “planet” to “dwarf planet.”

Final Answer:
The correct option is (C): Pluto.