They spotted it orbiting around the distant dwarf planet Makemake. Makemake sits way out beyond Neptune in a region called the Kuiper Belt. Makemake itself is already hard to spot because it's so small and so far away, but its moon, called MK 2, is much smaller and appears about 1,300 times fainter.
"Our preliminary estimates show that the moon's orbit seems to be edge-on, and that means that often when you look at the system you are going to miss the moon because it gets lost in the bright glare of Makemake," Alex Parker, the scientist who led the image analysis, said in a statement.
Here you can make out MK 2 in the glare from Makemake:
Makemake is one of several dwarf planets in the Kuiper Belt:
Scientists say that this discovery means that Makemake is more like Pluto than we ever knew. Both dwarf planets are made of frozen methane and now we know that they both have satellites.
Scientists will need more images and data before they can figure out why MK 2 is so dark and what its orbital path is. Mapping its orbital path around Makemake is critical to helping scientists figure out how the moon formed.