Chapter 1: The Collision Revelation

In an intriguing twist of cosmic fate, astronomers have unraveled the mystery of an exoplanet that was believed to exist but has now been shown to be a massive dust cloud resulting from a cosmic collision. A research team from the University of Arizona has revealed that Fomalhaut b, located in the Fomalhaut star system, approximately 25 light-years from Earth, never truly existed as a planet. Instead, what was observed was an expanding cloud of dust left behind from a significant collision between two planetesimals—icy bodies that often serve as building blocks for planets.

This groundbreaking discovery arose from a reevaluation of data collected by the Hubble Space Telescope (HST) between 2004 and 2012. The researchers had access to additional unpublished data from 2013 and 2014, which proved crucial. Within these later datasets, they noticed discrepancies concerning Fomalhaut b, which had been initially identified as an exoplanet in 2008 based on earlier observations. Notably, the supposed planet appeared to have vanished.

The Mysterious Gap In Exoplanet Population explores the implications of this discovery, delving into how astronomers have reinterpreted the data surrounding Fomalhaut b.

Andras Gáspár, part of the James Webb Space Telescope teams at the University of Arizona and lead author of the study, explains, “One of the projects I’m involved in is observing the extensive debris disk surrounding Fomalhaut using the Mid-Infrared Instrument (MIRI). While preparing, I reviewed all archival HST data on the system and noticed something unusual with Fomalhaut b—it wasn’t present in the latest images. Analyzing the data revealed a pattern: it was diminishing.”

This serendipitous find encouraged Gáspár and his colleague George Rieke, a professor of Astronomy at Steward Observatory, to investigate further, especially since their research primarily deals with modeling cosmic collisions and the resultant dust.

Chapter 2: Observations and Insights

The team's analysis of Fomalhaut b yielded three notable findings over the years: the object was fading, it was growing large enough for HST to resolve, and its trajectory was more consistent with an object being ejected from a system rather than a planet in orbit.

“All these observations suggest a singular conclusion: it is a dust cloud formed from a colossal collision between two large bodies, expanding and being expelled from the system due to stellar radiation pressure,” Gáspár elaborates. He acknowledges that while others had previously suggested the object might be a dust cloud, their research was the first to substantiate this as a valid model.

Rieke adds, “The Fomalhaut star system serves as a critical testing ground for our theories regarding the evolution of exoplanets and star systems. We have evidence of such collisions in other systems, but none on this scale have been observed in our own solar system.”

The team’s findings have larger implications, suggesting that such collisions are part of a broader narrative about planetary systems' evolution.

Strange and Intriguing Exoplanets discusses the peculiar characteristics of Fomalhaut b and the implications of this discovery on our understanding of exoplanetary systems.

The initial identification of Fomalhaut b as a planet was puzzling due to its bright optical emissions paired with an absence in infrared wavelengths. Gáspár notes, “This was a secure detection of something—there was no doubt. Planets don’t emit light on their own in these wavelengths.” The anomaly regarding its infrared invisibility raised questions about its true nature. “Given its size and relative youth, it should have been emitting thermal energy,” he adds. “Hence, the concern was not about its existence but its classification. It’s certainly not an exoplanet.”

The narrative continues to evolve as the astronomers reflect on the nature of Fomalhaut b. Gáspár points out, “The collision occurred shortly before our first data collection, making the object appear point-like. As the fragments of the collision followed a trajectory consistent with an orbital path, they initially resembled a planet.”

As time progressed, these particles diverged from their orbital trajectory, which was captured in the 2013/14 datasets. This means that the object initially appeared planet-like under optical observation. “The announcement in 2008 was based on the best analysis available at that time,” Gáspár emphasizes. “Science adapts when new evidence emerges.”

The narrative does not conclude with the reclassification of Fomalhaut b. There remain numerous unanswered questions regarding the objects involved and the collision itself, which excite Gáspár and his team.

Chapter 3: The Role of Dust Clouds

The astronomers estimate that the observed dust cloud originated from a collision between two bodies each around 100 km in diameter. While most of the mass still resides in the larger fragments, the number of dust particles produced is vastly greater. Gáspár analogizes, “Consider someone smoking in a room with a light beam—it’s easy to see the smoke due to light scattering, despite its small mass.” The challenge lies in extrapolating the “smoke” to the sizes of the colliding bodies.

The knowledge of collision outcomes, primarily derived from laboratory experiments, plays a critical role in understanding this event. As Fomalhaut b resides within a vast ring of icy debris, the colliding bodies were likely composed of ice and dust, akin to those found in the Kuiper Belt at the outer edge of our solar system.

The significance of their findings extends beyond Fomalhaut; they hint at other dynamic events within the system that could lead to increased collision likelihood, such as the migration of a giant planet. Gáspár anticipates further exploration with the James Webb Space Telescope (JWST) after it becomes operational in 2021. “The Fomalhaut system has a significant debris disk, and we will be observing it using NIRCam and MIRI,” he explains. “There’s potential to discover a genuine planet within the system that could elucidate the dynamic activities and eccentric debris ring.”

Witnessing large-scale collisions, which we know statistically occur to create observable debris disks, remains a thrilling prospect in the study of planetary system evolution.

Special thanks to Andras Gáspár. Source: Gáspár, A., Rieke, G. H., “New HST data and modelling reveal a massive planetesimal collision around Fomalhaut,” Proceedings of the National Academy of Sciences, [2020].

Rob is a freelance science journalist from the UK, specializing in physics, astronomy, cosmology, quantum mechanics, and obscure comic books.