Gaia mission shows that many asteroids have moons, and it’s not clear why

Researchers using data from the Gaia mission’s third data release have found 3,038 binary asteroid candidates. These celestial pairs include an asteroid and a smaller moonlet. Using a novel detection method that records periodic “wobbling” in the astrometric data, the team showcased a previously unexplored population of binary asteroids, particularly among small and intermediate-sized objects.

The Gaia spacecraft, launched by the European Space Agency (ESA) in 2013, has revolutionized astronomy by providing unprecedentedly accurate measurements of the positions, distances, and motions of over a billion stars in the Milky Way. And, in addition to stars, Gaia has observed thousands of asteroids in our solar system. Now, astronomers can track their movements with extraordinary precision.

The new study looked at binary asteroids — asteroids with moons.

Binary asteroids are systems in which two asteroids are gravitationally bound and orbit a common center of mass. These pairs can vary widely in size. The primary body is typically larger, with the secondary body (moonlet) orbiting around it. Binary asteroids are significant because they offer unique insights into the collisional and dynamic processes that shape the solar system.

However, they’re tough to spot.

“Binary asteroids are difficult to find as they are mostly so small and far away from us,” says Luana Liberato of Observatoire de la Côte d’Azur, France, lead author of the new study. “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.”

New ways to find binary asteroids

Before the advent of the Gaia mission, binary asteroids were primarily discovered using a combination of techniques. High-resolution imaging obtained from ground-based or space telescopes could identify asteroid companions that are widely separated from their primaries. However, this method is biased toward larger, brighter asteroids with substantial separation between the two bodies.

Another method involves photometry, where variations in the light curve of an asteroid indicate the presence of a companion. This technique is particularly effective for compact systems where the companion passes in front of or behind the primary. However, photometry struggles to detect binary systems with larger separations or when the components are of similar brightness. Radar ranging is another powerful tool for studying near-Earth asteroids but its range is limited.

While these methods have collectively identified several hundred binary asteroids, they all have their own limitations. This is where the Gaia mission’s astrometry comes in.

The key to astrometry lies in identifying periodic variations in the position of an asteroid that could indicate the gravitational influence of a companion. These variations, known as “wobbling,” occur because the observed position of an asteroid represents the center of light (photocenter), which does not always coincide with the center of mass of a binary system.

To detect these wobbles, the researchers developed a sophisticated period detection method that searches for periodic signatures in the residuals of orbit fits to Gaia’s astrometric data. By applying strict statistical criteria, they filtered out noise and spurious signals, leaving behind a list of promising binary candidates.

Mapping asteroids in the solar system

The researchers applied their method to a sample of 30,030 asteroids observed by Gaia to see how many of them have moons. After rigorous filtering and validation, they identified 3,038 binary asteroid candidates. This number far exceeds the current population of known binary asteroids

“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.”

This suggests that astrometry is a useful tool for finding more asteroids with moons — and the study is already raising questions about the solar system.

The presence of so many binary systems among small and intermediate asteroids suggests that these bodies may form through different mechanisms than larger asteroids.

One possible formation scenario is through rotational fission, where an asteroid’s rapid rotation causes it to break apart, with the fragments re-accumulating into a binary system. Another possibility is that these binaries form through gravitational interactions in asteroid families, where the debris from a collision leads to the formation of binary systems.

To solve this mystery, more research is required — and the European Space Agency (ESA) is working on just that. ESA will launch Hera mission later this year. Hera will follow up on NASA’s DART mission, which collided with a moonlet orbiting an asteroid.

The study “Binary asteroid candidates in Gaia DR3 astrometry” was published in Astronomy & Astrophysics.