Researchers initially believed they had identified a dust-covered exoplanet, but this object faded. Shortly after, a new bright spot appeared, indicating a completely different phenomenon.
This artistic illustration shows the violent collision of two massive objects orbiting the star Fomalhaut.
Credit: NASA, ESA, STScI, Ralf Crawford (STScI)
Subsequently, analyses demonstrated that these bright sources were clouds of glowing debris, not planets. Paul Kalas, an astronomer at the University of California, Berkeley, reported that this was the first time such an event was observed outside our Solar System. These collisions involve planetesimals, small rocky objects similar to asteroids, which play a role in the formation of worlds (explanation at the end of the article).
The detection of two distinct impacts in the same system in only twenty years is particularly unusual. Indeed, theoretical models predicted a collision every approximately 100,000 years, making this observation exceptional. This rarity offers a unique window into the dynamic processes that can lead to the birth of new planets, through the study of scattered materials.
This sequence illustrates the events leading to the creation of the dust cloud cs2 around Fomalhaut, showing the approach, collision, and dispersion of debris.
Credit: NASA, ESA, STScI, Ralf Crawford (STScI) The star Fomalhaut, located 25 light-years away, is famous for its dusty debris belts. For example, an object previously named Fomalhaut b, long considered a candidate exoplanet, turned out to be a similar dust cloud. Its gradual disappearance and the sudden appearance of another cloud confirm that these elements are not planets, but rather residues of cosmic shocks.
These results have implications for future exoplanet searches, because debris clouds can mimic a reflection of stellar light. This resemblance could mislead instruments designed to detect planets. Consequently, this requires precise observation methods to distinguish true exoplanets from luminous artifacts in stellar systems.
To deepen these discoveries, scientists plan to use the James Webb Space Telescope. Its NIRCam instrument will allow analysis of the dust grain size and the composition of the debris, including the possible presence of water and ice.
Composite Hubble image showing the debris clouds cs1 and cs2 around Fomalhaut, with the star masked to highlight the structures.
Credit: NASA, ESA, Paul Kalas (UC Berkeley) Planetesimals: fundamental elements of planetary creation
Planetesimals are small rocky or icy bodies that orbit young stars. They form from the dust and gas present in protoplanetary disks, where particles gradually agglomerate under the effect of gravity. These objects, ranging in size from a few yards (a few meters) to several miles (several kilometers), represent the building blocks of future planets.
When planetesimals collide, they can merge to form more massive bodies, a central process in the formation of rocky planets like Earth. These impacts also release heat and debris, which can influence the composition and evolution of the stellar system. Studying these collisions thus helps understand how worlds assemble over time.
In our own Solar System, planetesimals played an essential role billions of years ago. The asteroids and comets we observe today are remnants of that era, offering clues about the initial conditions. Observations of collisions in other systems, like Fomalhaut, allow comparing these processes on a galactic scale.