Free Astronomy Magazine July-August 2016

SPACE CHRONICLES analysis for the observa- tions. A moon's discovery can provide valuable in- formation on the dwarf- planet system. By mea- suring the moon's orbit, astronomers can calcu- late a mass for the sys- tem and gain insight into its evolution. Uncovering the moon also reinforces the idea that most dwarf planets have satellites. “Makemake is in the class of rare Pluto-like objects, so finding a com- panion is important,” Parker said. “The dis- covery 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.” Finding this moon only increases the parallels be- tween Pluto and Make- make. Both objects are already known to be cov- ered in frozen methane. As was done with Pluto, further study of the satellite will easily reveal the density of Makemake, a key result that will indicate if the bulk compositions of Pluto and Makemake are also similar. “This new discovery opens a new chapter in comparative planetology in the outer solar system,” said team leader Marc Buie of the Southwest Research Institute, Boulder, Colo- rado. The researchers will need more Hubble observations to make accu- rate measurements to determine if the moon's orbit is elliptical or circu- lar. Preliminary estimates indicate that if the moon is in a circular orbit, it completes a circuit around Make- make in 12 days or longer. Determin- ing the shape of the moon's orbit will help settle the question of its is in a special orienta- tion, these dark patches should make the dwarf planet's brightness vary substantially as it rotates. But this amount of vari- ability has never been observed. These previous infrared data did not have sufficient resolu- tion to separate Make- make from MK 2. The team's reanalysis, based on the new Hubble ob- servations, suggests that much of the warmer sur- face detected previously in infrared light may, in reality, simply have been the dark surface of the companion MK 2. There are several possi- bilities that could explain why the moon would have charcoal-black sur- face, even though it is orbiting a dwarf planet that is as bright as fresh snow. One idea is that, unlike larger objects such as Makemake, MK 2 is small enough that it cannot gravita- tionally hold onto a bright, icy crust, which sublimates, changing from sol- id to gas, under sunlight. This would make the moon similar to comets and other Kuiper Belt Objects, many of which are covered with very dark material. When Pluto's moon Charon was dis- covered in 1978, astronomers quick- ly calculated the mass of the system. Pluto's mass was hundreds of times smaller than the mass originally estimated when it was found in 1930. With Charon's discovery, as- tronomers suddenly knew some- thing was fundamentally different about Pluto. “That's the kind of transformative measurement that having a satellite can enable,” Par- ker concluded. T his Hubble Space Telescope image reveals the first moon ever discovered around the dwarf planet Makemake. The tiny moon, located just above Makemake in this image, is barely visible because it is almost lost in the glare of the very bright dwarf plan- et. [NASA, ESA, A. Parker and M. Buie (Southwest Research Insti- tute), W. Grundy (Lowell Observatory), and K. Noll (NASA GSFC)] n origin. 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 is 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. The discovery may have solved one mystery about Makemake. Previous infrared studies of the dwarf planet revealed that while Makemake's sur- face is almost entirely bright and very cold, some areas appear warm- er than other areas. Astronomers had suggested that this discrepancy may be due to the sun warming dis- crete dark patches on Makemake's surface. However, unless Makemake