Astronomers have discovered something new while observing the nearby star Fomalhaut A: Around 20 years after a point of light on the star’s dust ring appeared seemingly out of nowhere, another point of light is now shining – but not in the same place. The first point of light is no longer visible. What’s behind it? According to astronomers, the data from the Hubble telescope suggests that there were two collisions of kilometer-sized planetary building blocks around this young star. This duplication of planetesimal collisions at close spatial and temporal intervals is extremely unusual, the team explains. In principle, such collisions are normal in young star systems – our solar system also went through a phase of such planetesimal collisions in its early days.
Only 25 light-years from Earth, Fomalhaut A is one of the brightest stars in the night sky. It lies in the constellation Piscis Austrinus and is more massive, more luminous and significantly younger than the Sun. At the same time, Fomalhaut A is unusual in several respects. Because it forms an extremely widely spread triple system with a red dwarf and another dwarf star. The red dwarf Fomalhaut C is located 2.5 light-years from Fomalhaut A – further than any other known star system. From our perspective, it even belongs to a different constellation. Another special feature is the fact that two of the three stars in this triple system are surrounded by a belt of dust and ice – similar to the Kuiper belt in our solar system.
Puzzle about a point of light “from nowhere”
But even more unusual was a bright spot of light that astronomers using the Hubble Space Telescope discovered in 2004 on the inner edge of Fomalhaut A’s dust ring. At that time, they interpreted this object, known as Fomalhaut b, as an exoplanet about three times the mass of Jupiter. What remained a mystery, however, was why this planet candidate shone brightly in visible light but left hardly any traces in the infrared – this is difficult to explain for an extrasolar gas giant. The solution to the mystery emerged a few years later: Further Hubble images revealed that this object was becoming increasingly faint, but at the same time increasingly larger and more diffuse. Astronomers concluded that the Fomalhaut b spot of light could not be an exoplanet. Instead, two ice-rich planetary building blocks around 200 kilometers in size must have collided in this star system. This collision first caused a bright spot of light, then a rapidly spreading cloud of dust and debris. This fits with common models according to which planetesimals, asteroids and comets constantly collide with one another in the protoplanetary disks of young stars. The young solar system also went through such a phase.
Now there is news from the Fomalhaut system. To find out what became of the debris cloud from the planetesimal collision dubbed cs1, astronomers led by Paul Kalas from the University of California at Berkeley used the Hubble Telescope to target the system again in 2023 and 2024. “Our original intention was to continue observing Formalhaut b,” reports co-author Jason Wang from Northwestern University in Illinois. In fact, the researchers continued to detect a point of light near Fomalhaut A’s dust belt. “But after comparing our new images with the old ones, we realized that it could not be the same source. The point of light was in a slightly different location in the system.” However, nothing could be seen at the previously identified position of the light point cs1. “It was exciting and puzzling at the same time,” says Wang. Two different points of light, but similar in their optical characteristics, appeared around the star Fomalhaut A seemingly out of nowhere.
Collision of planetesimals
But what are these strange points of light on Fomalhaut A? After the astronomers checked their data several times and made sure that there was no observational error, they looked for an explanation. “Our strongest guess is that we saw two different collisions of planetesimals over the last 20 years,” says Wang. That is extremely unusual.
According to theoretical models, such collisions should only occur about once every 100,000 years. But at Fomalhaut A, astronomers have now observed two within 20 years – and there could have been significantly more: “Hundreds of such collisions could have taken place that went unnoticed,” explains co-author Bin Ren from the Max Planck Institute for Astronomy in Heidelberg. “Only these two were bright enough to be visible with Hubble. Fomalhaut’s proximity to Earth helped us detect these faint bursts of light in the first place.”
But even for the Hubble telescope’s high-resolution optics, the chunks that caused these collisions are not directly visible. However, based on the light signature of the debris clouds, the astronomers were able to determine that these planetesimals must have been around 30 kilometers across. They suspect that there could still be at least 300 million objects of this size in the dust ring around Fomalhaut A. “The Fomalhaut system is a natural laboratory for us where we can research how planetesimals behave during collisions in young systems,” says co-author Mark Wyatt from the University of Cambridge. “This can also give us clues about what they are made of and how they were formed.”
However, it remains a mystery why the two collisions occurred so close to each other. If collisions between asteroids or planetesimals happen randomly, cs1 and cs2 would be more likely to occur in more distant locations. But they are strikingly close to each other on the inner edge of Fomalhaut’s outer dust ring. Further observations with the Hubble Space Telescope, but especially with the James Webb Telescope, should now provide more information about the processes in this young star system. While Hubble can only capture the second collision cloud, which is still visible, in visible light, the Webb telescope can create high-resolution images and spectral data in the infrared range. Together, the images could reveal how the object cs2 will change over the next few years, but also how large the dust particles produced by the collision are and what they are made of.
Source: Paul Kalas (University of California, Berkeley) et al., Science, doi: 10.1126/science.adu6266