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Difficulty: HardThe Solar System and Planets

Planetary orbital mechanics dictate that a planet's mean orbital speed depends directly on its distance from the Sun. Based on their orbital distances and Keplerian motion, arrange the following planets in order of their average orbital velocity around the Sun, from fastest to slowest.

  1. 1Mercury
  2. 2Mars
  3. 3Jupiter
  4. 4Neptune

Answer

Mercury, Mars, Jupiter, Neptune (from fastest to slowest orbital velocity)
The correct sequence from fastest to slowest average orbital velocity is Mercury, Mars, Jupiter, and Neptune. In heliocentric orbits, orbital speed is inversely related to the square root of the distance from the Sun. Mercury is closest to the Sun (~0.39 AU) and travels fastest at ~47.4 km/s. Mars is the next closest (~1.52 AU) at ~24.1 km/s. Jupiter is farther out (~5.20 AU) at ~13.1 km/s, and Neptune is the farthest (~30.07 AU), traveling slowest at ~5.4 km/s.

Step-by-Step Solution

1
Apply the physical relationship between semi-major axis (distance from the Sun) and planetary orbital speed.
Planets closer to the Sun require higher orbital velocities to maintain stable orbits against stronger gravitational attraction, meaning orbital speed decreases monotonically as solar distance increases.
According to Kepler's Third Law and Newton's law of universal gravitation, orbital speed v≈GM⊙rv \approx \sqrt{\frac{GM_{\odot}}{r}}.
2
Determine the relative distances of the given planets from the Sun in astronomical units (AU).
Mercury (~0.39 AU) < Mars (~1.52 AU) < Jupiter (~5.20 AU) < Neptune (~30.07 AU).
Planets must be sequenced by their spatial position from the Sun outward to establish speed ranking.
3
Order the planets from closest to farthest to determine the sequence from fastest to slowest orbital speed.
Mercury (~47.4 km/s) → Mars (~24.1 km/s) → Jupiter (~13.1 km/s) → Neptune (~5.4 km/s).
Shortest distance corresponds to highest velocity, and greatest distance corresponds to lowest velocity.

Key Concept

Planetary orbital velocity decreases with increasing distance from the Sun due to gravitational mechanics.
Estimated Time:1m 30s
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Topics

The Solar System and Planets

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