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

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.


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

Hill Sphere

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.


Written by James Pailly.

To read all the articles in the ‘Sciency Words‘ series, visit the Planet Pailly blog.

  • John H Reiher Jr

    For a size comparison and how small Mercury’s Hill Sphere is, check out this graph:

    Using the following calculator you can see that Mercury has a Hill Sphere of 220,619 km in diameter:

    If Mercury was at 1 AU, it would have a Hill Sphere of 570,000 km.

    And for comparison, Ceres has a Hill Sphere of 217,000 km at 2.7675 AU.

  • Ah shucks…nothing to complain about. John beat me to some additional info and a monologue about the mathematical aspect of the Hill Sphera..I learned my lesson. Good one though “Hill Sphere” is indeed a scientific term 😉

  • John Hawkins

    Poor Mercury :C

    • michael pulleine

      no! mercury deserves everything it gets! do you hear me mercury? nobody loves you and you deserve to die! in fact, when the sun expands in 6/5 billion years you will be the first to die and nobody will care about you!