Could two giant planetary building blocks really meet in the same star system in just two decades? In the star system around the bright southern star Fomalhaut, scientists have just encountered exactly this scenario not once, but twice in a sequence of events which is challenging our understanding of how young star systems with planets evolve and how planets are detected.
Fomalhaut, just 25 light-years away within Piscis Austrinus constellation, has an impressive dust ring that measures 133 astronomical units around its star. The giant ring named the “Eye of Sauron” due to its hot looks hosts a tumultuous environment where ice and rock planetesimals measuring up to tens of miles near collisions and breakages. In 2004, NASA’s Hubble Space Telescope captured an illuminating spot named Fomalhaut b that assumed to be an obscured planet. However by 2014, that illuminating spot disappeared and was later replaced with another spot within close range that emerged in 2023 and named Fomalhaut cs1 and cs2; these have been described to be giant debris clouds formed by disastrous collisions of objects up to 37 miles (60 km) in size four to six times larger than an asteroid known for destroying dinosaurs.
Events like these should, according to current theories, only happen once every 100,000 in a system such as this. But here, in only 20 years, two have been caught in the act. “If you had a movie of the last 3,000 years, and it was sped up so that every year was a fraction of a second, imagine how many flashes you’d see over that time. Fomalhaut’s planetary system would be sparkling with these collisions.” Based on what has been seen, scientists have estimated that over the lifetime of the star, tens of millions of encounters could have occurred. It has a lifetime estimated at 440 million years.
Such effects would be gigantic by comparison with anything occurring within our own planetary system. In fact, says Northwestern University’s Jason Wang, the Fomalhault belt may hold 1.8 Earth masses of planetesimals, or about 300 million objects similar to cs1 and cs2, along with a second 1.8 Earth masses of smaller debris below 0.3 km. These would be constantly replacing fines on a dust scale of just a few 10,000ths of an inch, which would otherwise be lost to the star’s strong star wind or be gravitationally captured by star itself.
These collisions are more than just random occurrences of fireworks. This is because the high-resolution images provided by the ALMA observations of Fomalhaut’s warping debris disk show that the eccentricity of this eccentric disk is a decreasing function with respect to the distance measured from the star. This is the first time that such a trend has been validated for a debris disk. Additionally, the closeness of the collision points for cs1 and cs2 enhances the suspicions that a possible unseen world could be disturbing the planetesimals into the collision zones.
Today, it is now imperative to distinguish between dust clouds and actual planets. “Fomalhaut cs2 looks exactly like an extrasolar planet reflecting starlight,” says Kalas. “What we learned from studying cs1 is that a large dust cloud can masquerade as a planet for many years.” he explains. Such consideration assumes more significance in the context of next-generation observatories, such as the upcoming Habitable Worlds Observatory at NASA, designed to image Earth-like planets in reflected light.
Technically speaking, the identification of such collisions was possible via multi-orbit high-contrast imaging with Hubble’s optical cameras, with further verification via independent tests to eliminate artifacts of the data itself. In future work, the James Webb Space Telescope’s Near-Infrared Camera (NIRCam) instrument will determine cs2 cloud color, which will further refine dust grain size and composition parameters – including the presence of water ice. Infrared observations can then supplement available Hubble observations from visible light to provide multi-spectral characterization of cloud development. Scientists will monitor cs2 brightness, extent, and orbital changes for the coming three years to detect any belt-related secondary dust cascades.
The Fomalhaut system has also provided a natural lab for investigating planetesimal collisions, debris disks, and the dangers of planet detection through direct images. For anyone fascinated with space and planetary exploration, both events are not only rare occurrences in the cosmos but also serve as living experiments for understanding the harsh processes that once occurred in our own solar system.
