Lesson 5: Planetary Rotation

🎯 Objectives

• Explain how tides brake a planet's rotation
• Measure in a simulation how long tidal locking takes
• Recognise when a result comes from a hypothetical experiment

🧪 Exercise

1. Simulate TRAPPIST-1e and Proxima b for 1 Gyr and compare the final rotation period with the orbital one.
2. Calculate how many times the simulated locking time fits into the age of TRAPPIST-1 (about 7.6 billion years).
3. Explain why we always see the same face of the Moon.

⚠️ Model limits

• The initial 1-day rotation of TRAPPIST-1e and Proxima b is hypothetical: it is used to show the braking process, and is not a measured value.
• The model uses tides with constant efficiency (constant Q). The times are order-of-magnitude estimates.
• A dense atmosphere could prevent tidal locking through thermal tides (Leconte et al. 2015); the engine does not include this by default.
• Mercury has tides switched off in the engine to preserve its 3:2 resonance, which is not yet simulated.
• Venus: the model drives it to synchronous, prograde rotation even with atmospheric tides enabled. It does not reproduce its current slow, retrograde rotation.