Why Doesn’t Mercury Have a Moon? (Sciency Words: Hill Sphere)

In 2012, NASA announced the discovery of a moon orbiting the planet Mercury. Sadly, this turned out to be an April Fool’s Day prank. In reality, Mercury does not and probably cannot have a moon. Why? Because the Sun is a bully.

Fe06 Flick

For a more complete answer, let’s get to this week’s edition of Sciency Words. Every Friday, we take a look at some new and interesting scientific term so we can all expand our scientific vocabularies together. Today’s word is:


Named after American astronomer George William Hill, a planet’s Hill sphere is the region of space where that planet’s gravity has more influence over orbiting bodies than the gravity of the Sun.

In general, if a moon’s orbit lies within a planet’s Hill sphere, the moon will remain in orbit of the planet. Otherwise, the moon will probably escape the planet’s gravity and begin to orbit the Sun.

To determine the size of a planet’s Hill sphere, we must consider two factors against each other: the planet’s gravity vs. the proximity of the Sun. Even a large, Jupiter-sized planet will have a small Hill sphere if it’s too close to the Sun. Conversely, tiny planets can have surprisingly large Hill spheres if they’re far enough away.

Maybe at some point in the distant past, Mercury did have a moon. But Mercury is too small and way too close to the Sun to have a substantial Hill sphere. Sooner or later, this hypothetical moon would have been yanked away from Mercury and launched into a highly unstable orbit around the Sun.

P.S.: Pluto may not be considered a planet anymore, but it still has a Hill sphere, and because Pluto is so far away from the Sun, its Hill sphere is much larger than the Hill spheres of Mercury, Venus, Earth, or Mars. Large enough to hold onto at least five moons.

Fe06 Pluto's Moons


The Moon That Went Up a Hill but Came Down a Planet from Bad Astronomy

File: Hill Sphere of the Planets from Wikipedia

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