Astronomers have seen asteroids colliding around a distant star for the first time.
These catastrophic collisions round the nearby star system were seen by the Hubble Space Telescope.
The star in question is Fomalhaut, and Hubble was able to give astronomers a front-row seat to observe massive objects smashing into each other around it.
Fomalhaut is a relatively young star: about 200–400 million years old, which makes it a stellar infant when compared to our 4.5 billion-year-old Sun.
The star system seems to be in a state of chaos, similar to what our own Solar System would have been like in its infancy.
"This is certainly the first time I’ve ever seen a point of light appear out of nowhere in an exoplanetary system,” says study principal investigator Paul Kalas of the University of California, Berkeley.
"It’s absent in all of our previous Hubble images, which means that we just witnessed a violent collision between two massive objects and a huge debris cloud unlike anything in our own Solar System today. Amazing!"
A well-studied star system
Fomalhaut is relatively close to Earth, at just 25 lightyears distant.
It's much more massive and brighter than our Sun, and previously astronomers have observed belts of dusty debris surrounding it.
These belts are the leftover ingredients from which the star formed, and out of them, planets may eventually develop.
In fact, astronomers can already see planetesimals around Fomalhaut. These are large bodies that have formed out of the surrounding dust, and will likely eventually grow to produce fully-formed planets.
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In 2008, astronomers using the Hubble Space Telescope were able to discover a potential planet around Fomalhaut, making it the first system with a possible planet found using visible light.
However, that object, called Fomalhaut b, is now thought to be a dust cloud masquerading as a planet, and the result of colliding planetesimals.
More recently, astronomers using Hubble found a second point of light at a similar location around the star.
This is called 'circumstellar source 2' or cs2' while the first object is known as 'cs1'.
Fomalhaut's many mysteries
What remains unclear is why we're seeing two debris clouds so close to each other around the same star.
Astronomers say that, if collisions between asteroids and planetesimals were random, cs1 and cs2 should appear by chance at unrelated locations.
Instead, they're located close to each other along the inner portion of Fomalhaut’s outer debris disk.
Also, why have astronomers been able to see two such events in a short space of time?
"Previous theory suggested that there should be one collision every 100,000 years, or longer. Here, in 20 years, we've seen two," says Kalas.
"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."
"The exciting aspect of this observation is that it allows researchers to estimate both the size of the colliding bodies and how many of them there are in the disk, information which is almost impossible to get by any other means," says co-author Mark Wyatt at the University of Cambridge, UK.
"Our estimates put the planetesimals that were destroyed to create cs1 and cs2 at just 30 kilometres in size, and we infer that there are 300 million such objects orbiting in the Fomalhaut system."
"The system is a natural laboratory to probe how planetesimals behave when undergoing collisions, which in turn tells us about what they are made of and how they formed," says Wyatt.
A lesson learned?
The fact that these point sources of light might be mistaken for a planet in orbit around the star is something to consider, astronomers say, when analysing data from future exoplanet missions.
"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.
"This is a cautionary note for future missions that aim to detect extrasolar planets in reflected light."
What next?
Kalas says he and his team will use the Hubble Space Telescope to further study cs2 over the next three years.
"We will be tracing cs2 for any changes in its shape, brightness, and orbit over time," he says.
"It’s possible that cs2 will start becoming more oval or cometary in shape as the dust grains are pushed outward by the pressure of starlight."
And the team say they'll also use the NIRCam (Near-Infrared Camera) instrument on the James Webb Space Telescope to observe cs2.
NIRCam can provide colour information that will help the team determine the size of the cloud’s dust grains and their composition. It may even discover whether the cloud contains water ice.
It seems that the Fomalhaut system is only just starting to give up its secrets, and astronomers have much to learn.
