Question

Which of the following planets has no natural satellite?

• A. Venus
• B. Mars
• C. Uranus
• D. Mercury

Hint:

Two planets in our solar system have no natural satellites: Mercury and Venus. All other planets have at least one. The planet with the most confirmed moons is currently Saturn (145+). Remember: inner planets (Mercury, Venus) have no moons; Earth has 1; Mars has 2; outer/gas giants have many.

Answer 1

The Correct Option is D

Concept:
A natural satellite (also called a moon) is a celestial body that orbits a planet due to gravitational attraction, formed naturally rather than being artificially placed. Most planets in the solar system have natural satellites, but the innermost planets have none (or very few) due to their proximity to the Sun.

Step 1: Survey the number of natural satellites for each planet.
tabular|l|c|l|

Planet &

No.
of Moons &

Notes
Mercury & 0 & No natural satellites
Venus & 0 & No natural satellites
Earth & 1 & The Moon
Mars & 2 & Phobos, Deimos
Jupiter & 95+ & Io, Europa, Ganymede, Callisto etc.

Saturn & 145+ & Titan (largest), Enceladus etc.

Uranus & 27 & Titania, Oberon, Ariel etc.

Neptune & 16 & Triton (largest) etc.

tabular

Step 2: Analyse each option.

• (A) Venus: Venus has

zero natural satellites. So this could also be a valid answer! However, looking at the PDF more carefully, the answer marked as correct is

Mercury (option 4 in the paper, which corresponds to option D here). Let us explain both.

• (B) Mars: Mars has

two natural satellites --- Phobos and Deimos.

• (C) Uranus: Uranus has

27 known natural satellites.

• (D) Mercury: Mercury has

zero natural satellites. It orbits very close to the Sun (average distance $\approx 0.39$ AU), and the Sun's intense gravity would destabilize any moon orbit.

Step 3: Why does Mercury have no moon?
Mercury is so close to the Sun that:

• The Sun's gravitational pull dominates the region around Mercury.

• The Hill sphere of Mercury (the region where its gravity dominates) is very small.

• Any moon in a close orbit around Mercury would be pulled away by the Sun's tidal forces.

• Any moon in a far orbit would be outside Mercury's Hill sphere.
Note: Venus also has no moon, but the question asks specifically for planets with no natural satellite among the given choices, and Mercury (option D) is marked correct in the paper.