Is the Earth gaining a second moon?

I was amused to read on the BBC News site ‘ Get ready for a cosmic surprise this autumn - Earth is about to get a second moon, according to scientists.’ I named my first science quiz book How Many Moons does the Earth Have? precisely because of this kind of silliness. My answer was ‘one’ and it still will be while a chunk of passing rock is briefly caught by Earth’s gravitational pull.

Apart from anything, the suggestion that something is a moon just because it is briefly pulled off track by a planet’s gravity seems to reflect an underlying misunderstanding of how gravity works. Everything with mass distorts spacetime and causes moving objects to be displaced from their natural straight line path. This would make practically everything in space a moon if you took this argument to its limit, because they are all being pulled of track by the planets.

The news item is also amusing because back in 2013, the TV show QI claimed that the Earth had a large number of moons (it was QI’s various attempts at numbering our moons at more than one that inspired my book in the first place). If this were true, the current story isn’t even news. But it’s not really: I reproduce below what I wrote at the time.

QI excuse their latest, 20,000 (that's from memory - it was some large number) value [for the number of the Earth’s moons] by saying there are lots of little lumps of rock that get captured by Earth's gravitational field for a few days and while captured they are natural satellites, which makes them moons. But this is the excrement of the male cow. You might as well say the Sun has many thousands of planets, because of all the asteroids, as a planet is a satellite of the Sun. However, we all know there are just eight planets.

Now to be fair, with planets there are clearer rules. To be a planet the body has to (in my wording):

Orbit the Sun
Be roughly spherical
Have swept its orbit clean of minor debris

... this last one being the rule that did for Pluto. But I would suggest, whether or not there is an IAU definition of 'moon' as there is for 'planet' there are still clear intended consequences of using the word 'moon' as opposed to just 'satellite'. These are that the body in question should be:

Long lasting - I suggest staying in orbit for at least 1,000 years
Sizeable - say at least 5 kilometres across

This would still allow moon status for the pretty dubious companions of Mars, Phobos and Deimos, which are about 20 kilometres and 10 kilometres across. For comparison, the ‘moon’ described by BBC News is 10 metres across.

Clearly such rules are there implicitly when we talk about moons. If the time rule didn't exist, then every lump of rock that spent 5 minutes in our company would be a moon, while without the size rule, we would have to count every tiny piece of debris in Saturn's rings as a moon - every one of them is, after all, a natural satellite.

Image (which really is a moon) from Wikipedia by Gregory H. Rivera reproduced under CC3.0

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