Far, far away beyond the planet Neptune lies the Kuiper Belt, a vast reservoir of frozen orbs left over from the construction phase of the solar system 4.5 billion years ago. Four of them are dwarf planets, the most famous of which is Pluto with its system of five moons. Another is Makemake (MAH-kee MAH-kee), located more than a billion miles beyond Pluto. It’s a cold little world just 870 miles wide where the temperature hovers around 400° below zero F (-238° C).
But at least it’s so lonely any more. NASA’s Hubble Space Telescope has spotted a small, dark moon orbiting the dwarf planet, named for a creation deity of the Rapa Nui people of Easter Island.
The moon — provisionally designated S/2015 (136472) 1 and nicknamed MK 2 — is more than 1,300 times fainter than Makemake. MK 2 was seen approximately 13,000 miles from the dwarf planet, and its diameter is estimated to be 100 miles (160 kilometers) across. Prior to the discovery, Makemake was the only outer solar system dwarf planet without a moon. The other three — Pluto, Haumea and Eris — all have at least one.
Now that we know all four possess satellites, there’s a message here. Astronomers believe that collisions among objects in the outer solar system, the likely source of the moons, may be common. They still have to determine the shape of MK2’s orbit to be sure. A tight circular orbit means that MK 2 is probably the product of a collision between Makemake and another Kuiper Belt Object. If the moon is in a wide, elongated orbit, it’s more likely to be a captured object from the Kuiper Belt. Either event would have likely occurred several billion years ago, when the solar system was young.
MK2 appears to orbit its host edge-on, the reason it was so tricky to find. Most of the time, it’s lost in Makemake’s glare as it orbits either directly in front or behind the dwarf planet. Now that Hubble’s tracked it down, further observations will define the moon’s orbit, allowing astronomers to calculate a mass for the system, determine Makemake’s density and nail down the moon’s origin.
“Makemake is in the class of rare Pluto-like objects, so finding a companion is important,” said Alex Parker of Southwest Research Institute, Boulder, Colorado. “The discovery of this moon has given us an opportunity to study Makemake in far greater detail than we ever would have been able to without the companion.”
Both Pluto and Makemake are known to be covered in frozen methane. Once we know the dwarf planet’s density based on the orbit of MK2, we’ll be able to determine if the bulk compositions of Pluto and Makemake are also similar.
I think the coolest, or maybe the “warmest” discovery about the new moon is that it solves a peculiar temperature discrepancy recorded on the dwarf planet. Previous studies revealed that while Makemake’s surface is almost entirely bright and very cold, some areas appear warmer than others. Astronomers thought maybe sunlight was warming dark patches on the surface. But if darker areas were really there, we’d see Makemake’s brightness vary a lot as it rotates. We don’t.
The team’s reanalysis, based on the new Hubble observations, suggests that much of the warmer surface detected previously in infrared light may, in reality, simply have been the dark surface of the companion MK 2. Mystery solved. But why would the moon have such a charcoal-black surface so different from its host in the first place? One idea floating around is that unlike larger Makemake, MK2 has too little gravity to hold onto its icy crust, which may long ago have vaporized in sunlight, changing from solid to gas and escaping to space. This would make the moon similar to comets and other Kuiper Belt Objects, many of which are covered with very dark material.
Distant, frigid and dark we nonetheless extend a warm welcome to the solar system’s 182nd moon!