Nature World News reported this week the discovery of an “alien” moon orbiting the dwarf planet Makemake. The moon, 100 miles in diameter,is also 1300 times dimmer than the planet it circles. An “alien” moon is one that is not detected immediately on the discovery of the primary planet. Considering that Makemake is in the Kuiper Belt, fifty times further from the Earth than the Earth is from the Sun, and is sheathed in methane ice, making it extremely bright, perhaps it is understandable that it took four years after the planet was discovered to be found.
The moon has yet to be named something more characteristic than “MK2”, probably because the tradition in which it is to be named has to be researched. Makemake, discovered in 2006, is named for the god of fertility of Easter Island. It is one of four dwarf planets in the Kuiper Belt. Haumea, discovered the year before Makemake, is named for the Hawaiian goddess of childbirth and has two moons. Eris, the third dwarf planet, is named for a Greek deity, a companion of Ares who specialized in chaos and excessive blood-letting in battle. Discovered in 2005, it has one moon.
The fourth dwarf planet in the Kuiper Belt is Pluto. Discovered in 1930 and once believed to be the ninth planet in the Solar System, it has its own orbital path around the Sun However, its path exits the Kuiper Belt and crosses the orbit of Neptune. Also, its size is more in keeping with the other dwarfs, being in fact smaller than Earth’s Moon both in dimension and mass. So, when the classification of “dwarf planet” was established in 2006, Pluto was assigned that designation. Pluto has five moons.
The Fifth dwarf planet in our System is Ceres. It was actually discovered first, in 1801, in the Asteroid Belt between Mars and Jupiter, and was considered the largest asteroid until the new classification was developed. It has no moons.
The presence of moons circling these planets allows for calculations regarding the planets’ mass and density, said Mark Hammergren, an astronomer at the Adler Planetarium in Chicago. “Knowing the size of the orbit, and its rotation period, we can calculate the mass and density of both the planet and the moon,” he said. “This in turn will give us information about the internal composition, how much of its volume is ice. And that will contribute to our knowledge of how our solar system was formed.”
“The darkness of the moon’s surface indicates far less ice than coats the planet,” said Dean Regas, of the Cincinnati Observatory. “A spectral analysis of the light reflected from the moon will also give us data” as to the composition of the moon, and how it was formed. We know so little of the history of our Solar System that whatever we can gain here materially increases our knowledge of the whole System and the far-off Kuiper Belt, Regas said.
Other resident objects in the Kuiper Belt are mostly icy comets that follow extremely narrow oval orbits to whip around the Sun inside the orbit of Mercury.