Not every moon is a moon, most are captured objects

9 January 2014

Here’s a 2010 photo, taken by the European Space Agency’s Mars Express probe, of Phobos, one of two… moons orbiting Mars. But that’s not a moon. And nor is Deimos, Mars’ second so-called moon. In reality they’re merely random rocks captured by the Red Planet at some point in the past.

Take a look at Earth’s moon. The Moon. It’s elegant, sizeable, and spherical. The same cannot be said of the rocks orbiting Mars, a couple of unfortunate asteroids that once strayed a tad too close to the fourth planet. Most of the outer planets of the solar system have moons similar in stature to Earth’s satellite, but they also host a bunch of minuscule, oddly shaped rocks, called moons simply because they orbit the planet in question.

It makes me think it is time to consider what really constitutes a moon. If Pluto can no longer be regarded as a planet, why then must every last rock that has been pulled into orbit by a planet, be called a moon? Surely such bodies should adhere — like planets, real planets — to some sort of criteria before being called a moon.

Being pretty much spherical, and of a certain size and mass, could form basic benchmarks, and anything under a certain size should be referred to as a captured object rather than a moon. Sorry Mars, but both your orbiting companions, Phobos and Deimos, are captured objects, not moons.

Originally published on Thursday 9 January 2014.