Does Mercury Have Moons?

Most planets in our solar system have moons. Earth has one. Mars has two. Jupiter has nearly a hundred. Saturn has even more. So when kids first learn about Mercury, the closest planet to the Sun, the natural question is: does Mercury have any moons of its own?

The answer is short: no. Mercury has zero moons. It also has zero rings. The full reason why is a little more interesting.

How Many Moons Do the Other Planets Have?

To put Mercury's zero in context, here's how the other planets stack up. The numbers shift as new moons are discovered, but here are recent counts:

So out of the eight planets, only Mercury and Venus have no moons at all. Everyone else has at least one. Most of the gas giants have dozens, and Saturn now leads the count.

Why Doesn't Mercury Have Any Moons?

Two big reasons.

Reason 1: The Sun's gravity is too strong.

Mercury orbits very close to the Sun. The Sun's gravity is so dominant in that part of the solar system that it makes it really hard for any object to stay in orbit around Mercury. Picture a marble trying to circle a small magnet that's right next to a giant magnet — the giant one will keep pulling the marble away.

Every planet has an invisible region around it called the Hill sphere. That's the area where the planet's gravity wins out over the Sun's gravity. Inside the Hill sphere, an object can orbit the planet stably. Outside, the Sun grabs it. Mercury's Hill sphere is small and pretty close to the planet's surface, which means there's just not much room for a moon to live there safely.

Reason 2: Mercury is small.

Mercury is the smallest planet, with only 38% of Earth's gravity. Smaller planets have weaker gravity, which makes it harder for them to capture passing objects (like asteroids) and turn them into moons. They also can't easily form moons by colliding with other objects, the way Earth's moon may have formed.

Did Mercury Ever Have Moons?

Maybe! It's possible that Mercury had a small moon at some point in its history, billions of years ago. But if it did, the Sun's gravity would have eventually pulled the moon away, or the moon would have crashed into Mercury. Either way, no moons survive there now.

Some scientists have wondered whether the giant Caloris Basin impact might have created chunks of debris that briefly orbited Mercury — effectively temporary moons — but those would have fallen back to the surface or been swept away within a short time. Read more about the Caloris Basin impact.

Does Mercury Have Rings?

Also no. Mercury has zero rings.

Rings are made of dust, rock, and ice that orbit a planet in a flat disc. The four planets in our solar system that have rings are all the gas giants: Jupiter, Saturn, Uranus, and Neptune. Saturn's are by far the most famous and the brightest.

For the same reason that Mercury can't keep a moon, it can't keep a ring system either. The Sun's gravity is too strong, and the Sun's heat would also vaporize ice rings quickly. Any dust or rock that ended up near Mercury would be either pulled away by the Sun, sucked down to the planet's surface, or evaporated.

Could Mercury Ever Get a Moon?

It's not impossible. If a passing asteroid happened to come by Mercury at exactly the right speed and trajectory, the planet's gravity might capture it. That would be a "captured moon," like Mars's tiny moon Phobos, which scientists think is a captured asteroid.

But it would be a long-shot, and even if it happened, the moon probably wouldn't stay in orbit forever. The Sun would tug it away over time. So if Mercury ever does get a moon, it's likely to be a short-term arrangement (in cosmic terms — meaning maybe a few million years).

For now, Mercury is one of only two planets in the solar system with no moons and no rings. The other is its neighbor Venus. Read about Venus.

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