“Makemake was the odd one out,” said Alex Parker, a research scientist at the Southwest Research Institute in Boulder, Colo. “Makemake was the only one that didn’t have a moon.”

Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology who led the team that discovered Haumea, Makemake and Eris, and the moons of Haumea and Eris, had looked for moons around Makemake, too. He did not find any, but that was not necessarily the end of the story. “I’ve always been suspicious in the back of my mind,” he said.

Astronomers led by Marc W. Buie of the Southwest Research Institute decided to use the Hubble Space Telescope to take a closer look and see what might have been missed. They found a dot next to Makemake — the moon, which, for now, is designated S/2015 (136472) 1 and nicknamed MK 2.

“When the moon turned up in the data, it was not actually that faint,” said Dr. Parker, who performed the image analysis. (For the astronomers, 1/1,300th the brightness of Makemake is considered not that faint.)