From the rocky, motley moon of the Earth to the icy oceans, our solar system is full of moons. Some planets have dozens of them; others do not. Astronomers find these satellites very interesting – geologically and potentially, astrobiologically – and eagerly send probes to visit lunar destinations such as Jupiter's Moon, Europe and Saturn's Moon Enceladus.
So it may surprise you to find that there is currently no scientific definition of the moon.
The scientists responsible for such an endeavor will be the International Astronomical Union (IAU), which approves and certifies the names of celestial objects, planet scientist Francis Nimmo of the University of California, Santa Cruz, told Live Science. And he failed to find a place where the group defined the term.
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It is possible that the AIU may be a little cautious in providing a moon definition, given the elimination of the planet vote in 2006 to designate a planet that reduced Pluto to status dwarf planet. The controversial decision has angered some scientists and members of the public, and to this day it remains a sore spot.
Pluto's decline even created some mapping headaches, Nimmo said. Pluto's length determination system has been updated to change the planetary status of the world, as the dwarf planets use a different coordinate system from the planets. So all maps from before 2006 are essentially the opposite of those created after re-categorization, according to Planetary Society .
That's one of the reasons definitions matter, Nimmo said. "You must have a common language so that you do not intersect when making calls. If you do not have definitions of things, you can get terribly confused."
What's in a Name?
However, Nimo said he did not think there was an official reason for the moon yet. The unofficial definition that he, and probably the majority of people have in his head, works just fine. "For me, a moon is something that orbits a planet or a dwarf planet."
The only distinction between the various moons that most researchers make depends on whether they are regular or irregular satellites, Nimmo added. Regular moons, such as the moons of Jupiter Io and Ganymede, usually orbit in the same plane around their parent planet, while non-moons, such as Jupiter's Passover, tend to have strange and eccentric orbits.
Usually this is considered to be a consequence of the satellite origin story. Regular moons are thought to be formed of the same material and at about the same time and place as their planet, or carved out of the planet in a massive collision, as suspected by the Earth's moon. Irregular satellites, on the other hand, are usually considered asteroids or comets that have formed elsewhere and are later captured by the gravitational pull of the planet.
Of course, the unofficial definition of the Nimmo moon may itself require an update one day. Recently, some researchers have speculated about the possibility of moons orbiting other moons, objects that the Internet has naturally summarized lunar moons . They could challenge Nimmo's definition above. But the lunar months remain hypothetical, so perhaps an informal definition can stand.
"I think it may be clear to everyone what the moon is," Nimmo said.
Originally published by Live Science .