OK, WHAT I鈥檓 about to say may cause some distress. You remember how it was with Pluto? In 2006, the International Astronomical Union (IAU) voted to downgrade the much-loved ninth planet to a mere dwarf planet. More than a decade later, it seems some people still haven鈥檛 gotten over it. But I鈥檓 still going to say it: things are getting a little out of hand with the solar system鈥檚 moons.
On 7 October, astronomers announced the discovery of 20 moons around Saturn. This took the total number of moons in the solar system to 214. The thing is, these new satellites are all about 5 kilometres in diameter. It would take me less than 4 hours to walk the entire circumference of one of them. Compared with some of the objects we currently call moons, which would qualify as planets if they weren鈥檛 orbiting another planet (like Earth鈥檚 moon), these are really just large rocks.
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For all the controversy about its redefinition of a planet, the IAU has, as yet, no official definition of a moon. Generally, planetary scientists take a moon to be any natural object orbiting a planet or dwarf planet. Some add that other, smaller objects can have moons as well, including moons themselves, whose theoretical satellites have been dubbed 鈥�moonmoons鈥�. Some use the term 鈥渕oonlet鈥� for small objects, for example those orbiting within Saturn鈥檚 rings.
It is time for us to find a means of distinguishing between real moons and these smaller rocks. That isn鈥檛 just because we are discovering smaller and smaller orbiting objects, but also because the importance of moons is growing. Three moons in our solar system 鈥� Enceladus, Titan and Europa 鈥� are widely agreed to be the best places to look for signs of life beyond Earth. What is possible on these larger objects just isn鈥檛 feasible on a 5-kilometre boulder with simple chemistry and little surface diversity.
So where to draw the boundary? The simplest way would be to set a minimum moon size. Planetary scientists often shy away from that kind of thing, however, because the sizes of objects can be difficult to determine from afar. If we ever detect an exomoon 鈥� a moon orbiting a planet outside our solar system 鈥� it is likely that we will be able to determine its mass but not its radius.
Then perhaps we should define a moon based on its mass. One of the IAU鈥檚 requirements for an object to be a planet is that it has enough mass for its own gravity to have pulled it into an approximately spherical shape. Icy objects become rounded more easily than rocky objects, so icy moons could be as small as 400 kilometres across, while rocky ones would have to be closer to 600 kilometres in diameter.
That would exclude many of the objects that we currently consider moons, like Saturn鈥檚 ravioli-shaped moons Pan and Atlas. We would be left with only 19 known moons in our solar system: four orbiting Jupiter, seven around Saturn, five of Uranus, one each for Neptune and Pluto and, of course, Earth鈥檚 moon. The rest of the things we currently call moons would be moonlets, or perhaps dwarf moons.
This number of moons would be a lot more manageable, and would also mean that the group of objects called moons would have much more in common with one another than they do now. Job done. Now I will just sit back and wait for the hate mail saying I just demoted someone鈥檚 favourite moon.
