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To get a definite answer to ‘how many moons are in the Solar System’, we will have to narrow our scope to the number of moons orbiting accepted planets. Within that scope, there are 166 natural satellites in our solar system.
Here is the breakdown of where those moons orbit:
Mercury and Venus-0.
The number of known moos has been steadily growing with the improvement of technology. The number has nearly doubled from NASA’s 2003 information.
Ganymede is the largest moon in the solar system and our Moon is the fifth largest. The smallest is a tie between S/2003 J 9 and S/2003 J 12, in orbit around Jupiter. Scientist theorize that there were moons around Mercury and Venus, but may have impacted the surface of their primaries in ancient history.
The reason that we narrowed the scope of our answer to ‘how many moons are in the Solar System, is the sheer numbers of natural satellites. If you include moons that orbit dwarf planets, Trans-Neptunian Objects, Trojan moons, and asteroids the number would jump to 336 classified moons. In addition to those, there have been another 150 very small objects observed within the rings of Saturn. At least one moon of Saturn(Rhea) is thought to have a moon of its own. Some of the known moons may someday be reclassified as dwarf planets. The total number of moons may be a fluid quantity for years to come.
Some of the moons in the solar system are known to have volcanoes, cryptovolcanos, and tectonic activity. Some are thought to have sub-surface oceans. Io is the most volcanically active body that scientists know of. At least four moons still have active tectonic plates. A few have observed atmospheres containing oxygen. Europa, among others is thought to be capable of supporting life as we know it, although this is unproven as yet. Iron metallic cores are not uncommon along with moons that have their own magnetic fields.
NASA has a great deal of information about each of the moon in the Solar System. At any time in the future, the answer to ‘how many moons are in the Solar System’ may, and probably will, change. The factor affecting that change is the ongoing improvement of technology. Based on that, we are lucky that the number of moons is not a known constant.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.