We all have a very rudimentary knowledge regarding moons. These are astronomical objects that are usually smaller than planets and tends to orbit a planet, which can have one, none or many moons. They are objects of fascination to many people. Yet, currently there is no scientific definition of a moon. Then how do we know if an astronomical object is a moon or not?
Our solar system is full of moons of different sizes and shapes. Earth's own solitary moon is rocky, while other planets have icy, ocean-filled moons. Some planets have dozens of satellites, some have none. Astronomers have started to explore these satellites in hopes of finding signs of life- or at least water. They're looking forward to sending probes to visit lunar destinations, such as Jupiter's moon Europa and Saturn's moon Enceladus.
The group of people in charge of deciding what is a moon and what isn't, is the International Astronomical Union (IAU), which approves and certifies the names of celestial objects, informed planetary scientist Francis Nimmo of the
University of California, Santa Cruz. And he was unable to find a place where the group defines the term 'moon'.
The IAU has already faces quite a bit of fallout because of Pluto and its planetary status. The group voted in 2006 to define a planet, which caused the demotion of Pluto to dwarf-planet status. This controversial decision angered some scientists as well as general people, and has remained a sore spot since then.
This is one of the reasons why definitions matter, Nimmo said. "You need to have a common language so that you're not getting at cross-purposes when you're having conversations. If you don't have definitions of things, you can end up with horrible confusion."
However, the fact that we have a simple definition for moon is enough, according to Nimmo. "As far as I'm concerned, a moon is something that orbits a planet or dwarf planet," he concludes. The only distinction between different moons that most researchers make depends on whether they are regular or irregular satellites, Nimmo added.
Some satellites like Jupiter's Io and Ganymede are regular satellites. These moons tend to orbit the same plane around their parent planet. But some satellites like Jupiter's Pasiphae, tend to have weird and eccentric orbits and are therefore dubbed irregular satellites.
The reason for a moon to be regular or irregular tends to be its origin. Regular moons are thought to have formed from the same material and around the same time and place as their planet or to have been carved out of the planet by a massive collision, as is suspected of Earth's moon. Irregular satellites, on the other hand, are usually believed to be asteroids or comets that formed somewhere else and were later captured by a planet's gravitational pull.