For the second instance on record, an impact event between two celestial bodies within an extrasolar system has been documented, occurring in proximity to our Solar System.
Said star is Fomalhaut, a stellar infant merely 440 million years of age, still encircled by a disc of remnants from its genesis. Situated at a distance of just 25 light-years, Fomalhaut serves as an exceptional proving ground for examining the disc processes that precede planetary accretion.
Now, the Hubble Space Telescope has illuminated an occurrence that may exemplify one such process: two rocky fragments, each estimated to span approximately 60 kilometers (37 miles) in diameter. Had they not been annihilated upon impact, these nascent celestial building blocks could have potentially evolved into planets orbiting the star.
“This is unequivocally the inaugural instance where I have witnessed a point of luminescence materialize spontaneously within an exoplanetary system,” opines astronomer Paul Kalas, affiliated with the University of California, Berkeley.
“Its absence in all prior Hubble imagery signifies that we have just observed a cataclysmic collision involving two substantial entities, resulting in an expansive debris field, a phenomenon incomparable to anything presently observed within our own Solar System. Quite astonishing!”
This is not Fomalhaut’s inaugural noteworthy event. Back in 2004, observers identified an object within its orbital path that appeared sufficiently luminous to suggest a planet. Subsequent observations, including direct imaging conducted in 2012, seemed to substantiate this. The presumed gas giant, Fomalhaut b, was even assigned a designation – Dagon.
However, by the time new observations were procured in 2014, Dagon had entirely dissipated. Astronomers concluded that the most plausible explanation for the vanishing entity was that it was never a planet, but rather a brilliant, expanding cloud of dust originating from a violent collision between two asteroids.
Fast forward to 2023, when Hubble reoriented its observational focus toward Fomalhaut to ascertain if the capricious star had engaged in further unusual activities. Spoiler alert: Unusual activities were indeed prevalent. A luminous anomaly had emerged in close proximity to the star, bearing a striking resemblance to Dagon.
“With these observations, our initial objective was to track Fomalhaut b, which we had initially categorized as a planet,” states astronomer Jason Wang from Northwestern University.
“We presumed the luminous spot was Fomalhaut b, as it represented the known source within the system. Nevertheless, upon meticulous comparison of our recent imagery with historical records, we discerned that it could not possibly be the identical source. This realization was both exhilarating and perplexing.”
Kalas and his collaborators have designated the luminous anomaly as Fomalhaut cs2, signifying “circumstellar source 2”; Dagon, concurrently, has been reclassified as Fomalhaut cs1. Rest in peace, Dagon.
“Fomalhaut cs2 exhibits characteristics precisely mirroring an extrasolar planet reflecting stellar illumination,” Kalas elucidates. “Our investigation of cs1 revealed that a substantial dust cloud can be erroneously identified as a planet for an extended period. This serves as a significant cautionary insight for future missions aiming to detect extrasolar planets via reflected light.”
Based on the Hubble observations, in addition to prior evidence of cs1’s temporal variations, the research team deduced that both diffuse clouds were likely the outcome of impacts involving smaller, similarly proportioned bodies. Intriguingly, both incidents occurred within a comparable region on the periphery of the Fomalhaut disc.
“Previous theoretical models posited a collision frequency of once every 100,000 years, or even longer. Here, within a two-decade span, we have witnessed two,” Kalas remarks.
“If you were to view a cinematic representation of the past three millennia, accelerated such that each year constituted a mere fraction of a second, envision the multitude of flashes you would perceive over that duration. Fomalhaut’s planetary system would be scintillating with such impacts.”
A solitary collision, an isolated data point, merely indicates that such an event is plausible under the specific conditions present. A second collision, however, unlocks a vastly expanded understanding. A second impact provides crucial statistical data.
“The compelling aspect of this observation is its capacity to enable researchers to quantify both the dimensions of the colliding entities and their abundance within the disc, data that is exceptionally challenging to acquire through alternative methods,” states astronomer Mark Wyatt of the University of Cambridge in the UK.
“Our estimations indicate that the planetesimals responsible for generating cs1 and cs2 were approximately 37 miles or 60 kilometers in diameter, and we infer the presence of 300 million such objects in orbit within the Fomalhaut system.”
The stellar environment immediately surrounding Fomalhaut is undoubtedly intriguing. More recent investigations have disclosed concentric gaps within the disc – a potential indicator of a planetary body currently in formation, actively clearing its orbital path of debris. Nevertheless, the planets themselves remain undetected.
Concurrently, observations conducted by the JWST in 2023 revealed a conspicuous aggregation of dust within the same outer ring where cs1 and cs2 materialized. At that juncture, astronomers attributed this phenomenon to yet another impact event, although this interpretation awaits definitive confirmation.
While Fomalhaut presents numerous unresolved questions, the accumulating evidence points towards a dynamic environment suggestive of nascent planet formation.
“The system serves as a natural laboratory for investigating the behavior of planetesimals during collisions, thereby offering insights into their composition and formation processes,” Wyatt asserts.
The research contingent will persist in their utilization of both the Hubble and JWST instruments to monitor cs2 and observe its subsequent evolutionary trajectory over the forthcoming years.
“We will meticulously track cs2 for any alterations in its morphology, luminosity, and orbital path over time,” Kalas states. “It is conceivable that cs2 may begin to exhibit a more elongated or cometary shape as the dust particles are propelled outward by the pressure exerted by starlight.”
