There are plenty of moons orbiting around the planets in our solar system — Saturn, for example, has well over 100 known moons — and astronomers believe that planets outside our solar system almost certainly have moons too, called exomoons. Those are particularly hard to spot, but now researchers have a new bevy of a different type of moon to explore: those that orbit around asteroids.
It seems funny to think of it, but it’s actually quite common for asteroids to have tiny moons of their own. These form what is called a binary asteroid, like the famous pair of Didymos and Dimorphos, which NASA crashed a spacecraft into in 2022. The new data about binary asteroids comes from the European Space Agency’s Gaia mission, a space-based telescope that has spotted new potential moons around 350 asteroids.
“Binary asteroids are difficult to find as they are mostly so small and far away from us,” said lead researcher Luana Liberato of Observatoire de la Côte d’Azur, France in a statement. “Despite us expecting just under one-sixth of asteroids to have a companion, so far we have only found 500 of the million known asteroids to be in binary systems. But this discovery shows that there are many asteroid moons out there just waiting to be found.”
Gaia is on a mission to map the entire Milky Way in 3D, but during its observations it also collects a large amount of data about objects within our solar system. It is this solar system data that has turned up the many new potential asteroid binaries. If these are confirmed, it would almost double the number of known asteroid binaries for astronomers to explore.
Gaia is able to detect hints of these binaries by looking for a distinctive “wobble” in the orbits of larger asteroids, which implies the existence of a smaller binary partner. Gaia is still collecting data, with another large-scale data release coming in 2026 that will uncover even more asteroid orbits.
“Gaia has proven to be an outstanding asteroid explorer, and is hard at work revealing the secrets of the cosmos both within and beyond the Solar System,” says Timo Prusti, project scientist for Gaia at ESA. “This finding highlights how each Gaia data release is a major step up in data quality, and demonstrates the amazing new science made possible by the mission.”
The research is published in the journal Astronomy & Astrophysics.
