Astronomers searching for a planet beyond Pluto discovered instead a dozen new moons orbiting Jupiter. That swells to 79 or so the number found circling the giant planet since Galileo spotted the first of them with a homemade telescope more than 400 years ago.

The finds were announced Tuesday in a bulletin from the International Astronomical Union’s Minor Planet Center at the Smithsonian Astrophysical Observatory in Cambridge, Mass., which serves as the global clearinghouse for the study of moons, asteroids and comets.

“It was serendipity,” said astronomer Scott S. Sheppard at the Carnegie Institution for Science in Washington, D.C., who led the team.

The 12 small moons turned up last year as the astronomers were searching for an undiscovered world called “Planet Nine” that some scientists suspect lurks at the far edge of the solar system. “We were able to observe around Jupiter at the same time we were looking beyond Pluto,” Dr. Sheppard said.

The astronomers first saw the Jovian moons using the Blanco telescope at Cerro Tololo Inter-American Observatory in Chile, which is operated by the U.S. National Optical Astronomy Observatory, and the Subaru telescope at the summit of Mauna Kea, Hawaii, operated by the National Astronomical Observatory of Japan. Four other observatories helped confirm the observations.