Why do some planets have moons? A physics expert explains why Earth has only one moon while other planets have hundreds
- Written by Nicole Granucci, Instructor of Physics, Quinnipiac University
![Why do some planets have moons? A physics expert explains why Earth has only one moon while other planets have hundreds](https://images.theconversation.com/files/597535/original/file-20240530-17-5lri0f.jpg?ixlib=rb-4.1.0&rect=6%2C10%2C1033%2C914&q=45&auto=format&w=496&fit=clip)
Small planets like Mercury and Venus have a tiny Hill sphere radius, since they can’t exert a large gravitational pull. Any potential moons would likely get pulled in by the Sun instead.
Many scientists are still looking to see whether these planets may have had small moons in the past. Back during the formation of the solar system, they may have had moons[9] that got knocked away by collisions with other space objects.
Mars has two moons, Phobos and Deimos. Scientists still debate whether these came from asteroids that passed close into Mars’ Hill sphere radius and got captured by the planet, or if they were formed at the same time as the solar system. More evidence supports the first theory[10], because Mars is close to the asteroid belt.
Jupiter, Saturn, Uranus and Neptune have larger Hill sphere radii, because they are much larger than Earth, Mars, Mercury and Venus and they’re farther from the Sun. Their gravitational pulls can attract and keep more natural satellites such as moons[11] in orbit. For example, Jupiter has 95 moons[12], while Saturn has 146[13].
Moons forming with a solar system
Another theory suggests that some moons formed at the same time as their solar system.
Solar systems start out with a big disk of gas rotating around a sun. As the gas rotates around the sun, it condenses into planets and moons that rotate around them[14]. The planets and moons then all rotate in the same direction.
This animation shows how the planets in our solar system formed. The dark rings in the disk represent the formation of the planets and moons. Eventually, the gas condenses into planets, natural satellites and asteroids.But only a few moons in our solar system were likely created this way. Scientists predict that[15] Jupiter’s and Saturn’s inner moons formed during the emergence of our solar system[16] because they’re so old. The rest of the moons in our solar system, including Jupiter’s and Saturn’s outer moons, were probably gravitationally captured by their planets.
Earth’s Moon is special because it likely formed in a different way. Scientists believe that long ago, a large, Mars-sized object collided with the Earth. During that collision, a big chunk flew off the Earth and into its orbit[17] and became the Moon.
This animation from NASA shows a simulation of how our Moon was formed during the collision.Scientists guess that the Moon formed this way because they’ve found a type of rock called basalt in soil on the Moon’s surface. The Moon’s basalt looks the same[18] as basalt found inside the Earth.
Ultimately, the question of why some planets have moons is still widely debated, but factors such as a planet’s size, gravitational pull, Hill sphere radius and how its solar system formed may play a role.
Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com[19]. Please tell us your name, age and the city where you live.
And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.
References
- ^ Curious Kids (theconversation.com)
- ^ curiouskidsus@theconversation.com (theconversation.com)
- ^ I’m a physics instructor (www.qu.edu)
- ^ satellites as objects in space (www.nasa.gov)
- ^ Hill sphere radius (www.asteroidmission.org)
- ^ gravitational force of attraction (spaceplace.nasa.gov)
- ^ we all stay grounded to Earth (theconversation.com)
- ^ the Hill sphere radius (doi.org)
- ^ may have had moons (doi.org)
- ^ More evidence supports the first theory (doi.org)
- ^ attract and keep more natural satellites such as moons (ssd.jpl.nasa.gov)
- ^ Jupiter has 95 moons (science.nasa.gov)
- ^ Saturn has 146 (science.nasa.gov)
- ^ condenses into planets and moons that rotate around them (science.nasa.gov)
- ^ Scientists predict that (doi.org)
- ^ inner moons formed during the emergence of our solar system (science.nasa.gov)
- ^ flew off the Earth and into its orbit (doi.org)
- ^ looks the same (science.nasa.gov)
- ^ CuriousKidsUS@theconversation.com (theconversation.com)
Authors: Nicole Granucci, Instructor of Physics, Quinnipiac University