The observation of small galaxies locked in a strange dance around a larger one presents a challenge to current theories of how all galaxies form and evolve, they said.
"When we looked at the dwarf galaxies surrounding Andromeda, we expected to find them buzzing around randomly, like angry bees around a hive," Geraint Lewis from the University of Sydney's School of Physics said.
"Instead, we've found that half of Andromeda's satellites are orbiting together in an immense plane, which is more than a million light-years in diameter but only 30,000 light years thick," he said in a university release. "These dwarf galaxies have formed a ring around Andromeda."
These small galaxies, many times fainter than their bright hosts, were thought to trace a path around the big galaxy that was independent of every other dwarf galaxy, astronomers said.
To find them all tracing the same "ring" path around the Andromeda Galaxy came as a surprise, they said.
"This was completely unexpected -- the chance of this happening randomly is next to nothing," Lewis said. "It really is just weird."
Computer models used to predict how dwarf galaxies should orbit large galaxies have always given the result that dwarfs should be scattered randomly through space.
"Now that we've found that the majority of these dwarf galaxies orbit in a disc around the giant galaxy Andromeda, it looks like there must be something about how these galaxies formed or subsequently evolved that has led them to trace out this peculiar coherent structure," Lewis said.