Why do some planets have moons? A physics expert explains why Earth has only one moon while other planets have hundreds

🌈 Abstract

Why some planets have moons while others don't.

🙋 Q&A

[01] Reasons for Planets Having Moons

1. What are the two main theories for why some planets have moons?

• Moons are gravitationally captured if they are within a planet's Hill sphere radius
• Moons are formed along with the solar system

2. What is the Hill sphere radius and how does it determine whether a planet can keep a satellite in orbit?

• The Hill sphere radius is the minimum distance for a planet to keep a satellite in orbit, based on the masses of the planet and the satellite.
• Larger planets like Jupiter and Saturn have larger Hill sphere radii, allowing them to gravitationally capture and keep more moons in orbit.
• Smaller planets like Mercury and Venus have tiny Hill sphere radii, so potential moons would likely get pulled in by the Sun instead.

3. How do moons form along with the solar system?

• The solar system starts with a rotating disk of gas that condenses into planets and moons rotating in the same direction.
• This is likely how Jupiter's and Saturn's inner moons formed.

4. How did Earth's Moon likely form differently?

• Scientists believe Earth's Moon formed from a large, Mars-sized object colliding with the Earth long ago, with a chunk of Earth flying off into orbit to become the Moon.

[02] Unanswered Questions

• Scientists are still debating whether Mars' moons Phobos and Deimos were captured from asteroids or formed with the solar system.
• There are still many open questions about why some planets have moons while others don't.