More moons in sight
JUST last week, I was mentioning to Planetarium audiences that Jupiter had 69 known moons. But then came the announcement that 10 more had been discovered, making the total 79. It was a significant jump in the total.
You may wonder how it is that astronomers have only relatively recently found 10 moons that were not previously known to exist. After all, our own moon is the second brightest object in the sky, and many moons of the other planets have been observed since the very early days of the use of telescope to study the night sky.
The reason is that these moons are very, very small indeed: at most a few kilometres across, possibly as small as one kilometre, which is about the size of only a few city blocks. Painstaking observations are required to detect them.
The first moons other than our own to be found, in the early 17th century, were the four brightest moons of Jupiter. These are named Io, Europa, Ganymede and Callisto. Essentially, they are bright because of their size. Europa, the smallest, has a diameter of 3122km, and the largest, Ganymede, is 5262km across.
Like our own moon, we see them simply because they reflect sunlight, just like the planets do. They are so bright that they are even visible in a good pair of binoculars, appearing like starlike points of light close to Jupiter. They can be seen very easily indeed through a small telescope.
Currently, in the evenings, you will find Jupiter very high in the north, looking like a bright yellowish-white star. If you hold your binoculars steadily, watch for these moons, although you may not see all four if one or more is placed too close to Jupiter at the time, or is in front of, or behind, the planet.
The fifth moon of Jupiter, Amalthea, was discovered much later, in 1892. By the time my own interest in astronomy was building up, in the 1960s, there were 12 known moons, which had been the case since 1951. It was very exciting to learn, therefore, of the discovery of a 13th, named Leda, in September 1974.
Many later discoveries of Jupiter’s moons were made through images returned by spacecraft in the vicinity of Jupiter, but now, more are found using highly sensitive imaging devices on Earth-based telescopes, together with sound analytical techniques.
Much the same has happened with Saturn, which had nine known moons up to the mid-1960s, but now has 62.
No doubt there are more to be found. As our instrumentation becomes better and better, we shall surely find sub-kilometre-sized objects around the outer planets, especially Jupiter.
It makes me wonder where it will all end, and whether a line will be drawn to specify the minimum size for an object to be called a moon. After all, the rings of Saturn are made up of countless pieces of ice and rock, the diameters of which have been estimated as being between about one centimetre and several metres. Essentially, each of these particles is a moon of Saturn!
This does not mean to say that discoveries of previously unknown moons do not have a place in solar system research. It is important to learn the history of the sun and its family — the more information we can gather, the more we may be able to understand about its formation and evolution.
Perhaps a number of Jupiter’s moons were originally a single object that suffered a devastating collision. We can’t tell for sure, but it’s still exciting when discoveries like this are announced.