How Astronomers Know — Evidence Lab
Gravity Keeps Worlds in Orbit
- Estimated time
- ~18 min
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Learning goals
- Planets closer to the Sun feel a stronger gravitational pull, so they must move faster to stay in orbit. Distant planets need less speed.
- MS-ESS1-2 — Students relate orbital motion to gravity and distance.
Help & glossary
- Use the hint ladder before checking your answer — hints get more specific.
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Question 1 of 3
Look at the table of orbital speeds. What pattern do you see as you move outward from the Sun?
Orbital speed is how fast a planet moves along its orbit. All these planets are staying in stable orbits.
| Planet | Distance (AU) | Orbital speed (km/s) |
|---|---|---|
| Mercury | 0.39 | 47.4 |
| Venus | 0.72 | 35 |
| Earth | 1 | 29.8 |
| Mars | 1.52 | 24.1 |
| Jupiter | 5.2 | 13.1 |
| Saturn | 9.58 | 9.7 |
| Uranus | 19.2 | 6.8 |
| Neptune | 30.05 | 5.4 |
Table of orbital speeds: Mercury 47.4 km/s, Venus 35.0, Earth 29.8, Mars 24.1, Jupiter 13.1, Saturn 9.7, Uranus 6.8, Neptune 5.4.
Sources
- Planetary Fact Sheet — NASA (Goddard) — NASA Goddard · NASA Goddard Space Flight Center · Accessed 2026-09-01