As we move away from the sun in the solar system,
2021
As we move away from the sun in the solar system,
- A.
time taken by planets to complete one orbit around the sun increases.
- B.
time taken by planets to complete one orbit around the sun decreases.
- C.
size of the planets decreases gradually.
- D.
planets very close to the sun are made up of gases and liquids and the planets far away from the sun are made up of rocks.
Show answer & explanation
Correct answer: A
Kepler's Third Law of planetary motion states that the square of a planet's orbital period (T2) is directly proportional to the cube of its average orbital distance from the Sun (a3). Physically, gravitational pull weakens with distance, so a planet farther out moves at a lower orbital speed while also covering a much longer path — both effects make its revolution take longer.
Consider the planets in order of increasing distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Mercury, the closest planet (about 0.39 AU from the Sun), completes one orbit in about 88 days.
Earth, at 1 AU, takes 1 year (365 days); Jupiter, at about 5.2 AU, takes about 12 years; and Neptune, the farthest planet (about 30 AU), takes about 165 years.
The orbital period rises steadily at every step outward, exactly as Kepler's Third Law predicts.
Cross-check with the formula directly: for Mars at a ≈ 1.524 AU, T2 ≈ (1.524)3 ≈ 3.54, so T ≈ 1.88 years — matching the planet's known orbital period and confirming that period grows with distance.
Hence, as we move away from the Sun, the time taken by planets to complete one orbit increases.