Why does Earth have one moon?
Show answer & explanation
Answer: Giant impact formed it
Gravity allows only one — Wrong. Gravity doesn't limit moon number—Mars has two, Jupiter has 95! Earth has one because of how it formed—giant impact hypothesis.
Giant impact formed it ✓ — Correct! Giant Impact Hypothesis: ~4.5 billion years ago, Mars-sized object (Theia) collided with proto-Earth. Impact ejected massive debris into orbit. This debris coalesced into the Moon. Evidence: Moon's composition matches Earth's mantle, low iron content, Earth-Moon system angular momentum. This explains Moon's unusual size relative to Earth (largest moon-to-planet ratio for rocky planets). Dramatic formation!
Earth captured it passing by — Wrong. Capture is possible (Neptune's Triton), but our Moon formed from debris after a giant impact, not capture.
More Astronomy & Space questions
- Earth's atmosphere slowly leaks to space. Which gas escapes fastest?
- Why is Earth's day getting slightly longer every century?
- Why was Earth's day stuck at 19.5 hours for 1.5 billion years?
- Why might several small units beat one giant Moon reactor?
- Why is fission likelier than fusion for first Moon bases?
- Why put a lunar reactor away from the habitat?