“Dad, Jupiter has tons of moons, right? So why does Earth only have one?”
“That’s right, Jupiter has more than 90 confirmed moons. Saturn has over 140. But Earth has exactly one.”
“Is something wrong with Earth then?”
“Not at all. Actually, Earth’s Moon was born in a completely different way from other planets’ moons.”
“How is it different?”
“Most of Jupiter’s and Saturn’s moons were small chunks of ice or rock nearby when the planet formed, and they just got caught by gravity. That’s why there are so many, and why they’re mostly small.”
“But our Moon isn’t like that?”
“Our Moon was born from a completely different event. A very long time ago, an object about the size of Mars slammed into the newly formed Earth.”
“It collided? Did Earth break apart?”
“Not completely, but the impact flung an enormous amount of debris out into space. That debris orbited Earth and slowly clumped together, until it became one big chunk, the Moon we have today.”
“Is that why our Moon is so unusually big?”
“Exactly. Compared to other planets’ moons, Earth’s Moon is huge relative to the size of its planet. It’s more than a quarter of Earth’s diameter. A moon that big is pretty rare in the solar system.”
“Cool. Do some planets have no moons at all?”
“Yes, Mercury and Venus have zero. So having just one moon makes Earth special, but that one moon being this huge makes it even more special.”
“Earth’s Moon isn’t special for its number, it’s special for how it was born. Thanks to a massive collision, it became unusually large compared to Earth itself.”
Your Turn Now
Share with someone today what you remember.
- About how many moons do Jupiter and Saturn have?
- How was Earth’s Moon formed?
- How many moons do Mercury and Venus have?
(Check when ready: 1. Over 90 and over 140. 2. Debris from a Mars-sized object colliding with Earth clumped together. 3. Zero.)
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