ASK A SCIENTIST
Question: Why do we only have one moon?
This is a great question that opens up many of the ideas involved in how a planetary system works. A planetary system is a star surrounded by some planets and their moons.
If you look up "planet facts" in the back of some science books, you find that the number of moons the planets in our Solar System have is zero to sixty-two (or more). These moons all have different sizes from chunks of rock a few miles wide to round bodies larger than the planet Pluto. The conclusion that scientists have made based on this information is that there must be several ways a planet can get a moon. So let's talk about these ways.
A moon can form out of stuff that didn't become part of the planet. The stuff would be gases, dust, rock, and maybe ice. This stuff circles (orbits) the planet and comes together making larger and larger chunks until most of it has formed into a moon. This is probably how the larger moons of Jupiter, Saturn, Uranus, and Neptune were formed. Some of the smaller moons of Jupiter and Saturn may be some of the larger chunks that didn't become part of one of the big moons.
Another way that a planet can get a moon is to capture it. An object, that is most likely what we would call an asteroid, is traveling around our Sun and comes too close to a planet. The gravitational attraction between the planet and the object pulls the object into an orbit around the planet. Just like that, the planet has a new moon. The smaller moons of Jupiter, Saturn, Uranus, Neptune, Mars, and Pluto are most likely captured objects. Pluto's moon, Charon, is about half the diameter of Pluto.
The third way that a planet can get a moon is important to us because it is the way scientists think the Earth got its moon. Originally, the Earth did not have a moon. Sometime in the past, probably around 3.5 billion years ago, a large object traveling through our Solar System crashed into the Earth. This object could have been about the size of Mars. It broke off a piece of the Earth. This piece of the Earth became a bunch of small pieces (think about what happens if you throw a snow ball real hard against the sidewalk) that went into orbit around the Earth. These pieces bumped into each other and became larger and larger pieces until just about all the pieces came together into Earth's moon. It took a while for this to happen.
It is possible that sometime in the future a large asteroid can come by the Earth and be pulled into orbit. Then the Earth would have two moons. What do you think life on the Earth would be like today if it had no moon?
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