In a groundbreaking observation, astronomers have potentially detected the afterglow of a colossal collision between two massive planets, a phenomenon that could lead to the formation of an entirely new planet. If confirmed, this discovery not only offers a remarkable opportunity to witness the birth of a world in real time but also provides invaluable insights into the process of planetary formation. The event unfolded in December 2021 when astronomers noticed a peculiar flickering in the light emitted by an otherwise unremarkable sun-like star residing approximately 1,800 light years away from Earth, aptly named ASASSN-21qj. The previously unseen light fluctuations prompted further investigation into the nature of this celestial occurrence.
The initial assumption for the star’s dimming was the presence of intervening material obstructing the light’s path from the star to Earth, a fairly common explanation for these phenomena. However, it was an amateur astronomer, Arttu Sainio, who noticed and shared on social media that the emission of infrared light from the star’s location had increased by approximately 4% around two and a half years prior to the observed dimming. This crucial observation raised intriguing questions: Could these two occurrences be connected, and if so, what was happening in the vicinity of ASASSN-21qj?
In an article published in Nature, the researchers propose that the explanation lies in a cataclysmic collision between two planets. Giant impacts of this nature are believed to be prevalent during the final stages of planetary formation, shaping the sizes, compositions, and orbits of planets within a system. Similar past collisions are thought to be responsible for the peculiar aspects of Uranus’ tilt, Mercury’s high density, and the presence of Earth’s Moon. However, direct evidence of ongoing giant impacts in the galaxy has remained elusive until now.
To account for the observations, the collision between the two planets would have emitted more energy in the initial hours than the star itself. This substantial release of energy would have heated, melted, and even vaporized the colliding bodies, resulting in a blazing mass of material many times larger than the planets themselves. The brightening of ASASSN-21qj in the infrared spectrum was detected by Nasa’s WISE space telescope. Although the initial flash of light from the impact may have been missed due to the telescope’s infrequent observations of the star, the extended planetary body resulting from the collision will take millions of years to cool and shrink into a recognizable new planet. During the peak of its expansion, this “post-impact body” would have emitted a significant amount of light compared to the star, explaining the observed infrared brightening.
The colossal impact would have expelled vast quantities of debris into various orbits surrounding the star. A portion of this debris, vaporized by the shock of the collision, would have condensed into clouds of icy and rocky particles. Over time, some of these fragmented clusters would have obstructed the visible light from the star, leading to the irregular dimming observed. If this interpretation is correct, the extensive study of this star system could shed light on a crucial mechanism in the formation of planets and provide insights that challenge existing theories.
Even from the preliminary observations, researchers have gleaned fascinating insights about this enigmatic system. The estimated size of the post-impact body suggests it was several hundred times larger than Earth, implying that the colliding planets themselves were likely comparable in mass to the ice giant planets Uranus and Neptune. Additionally, the low temperature of approximately 700°C indicates that the colliding bodies contained elements with low boiling points, such as water. This discovery points to a collision between two Neptune-like worlds rich in ice. The delay between the infrared emission and the observation of debris crossing the star suggests that the collision occurred at a considerable distance from the star, much farther than the Earth is from the Sun. This configuration resembles our own solar system, with the presence of ice giants positioned further away from the star, in contrast to densely packed planetary systems typically observed.
The most thrilling aspect of this discovery lies in the ability to continue observing this evolving star system for decades, enabling scientists to test and refine their conclusions. Future observations, including those made by NASA’s James Webb Space Telescope (JWST), will provide critical data to determine the composition and sizes of the debris cloud particles, examine the chemistry of the post-impact body’s upper layers, and track the cooling process of this hot debris mass. The potential emergence of new moons within the system adds further intrigue. By analyzing these observations, scientists can further enhance their understanding of how giant impacts shape planetary systems and refine existing theories. Until now, our understanding of impact events has been limited to echoes within our own solar system, making this discovery a significant leap forward in unraveling the mysteries of planetary formation.
The detection of the afterglow resulting from a monumental collision between two planets presents an unprecedented glimpse into the birth of a new world. Thanks to the keen observations of astronomers and the collaborative efforts of researchers, this extraordinary discovery not only deepens our understanding of celestial events but also holds the promise of refining our knowledge of planetary formation processes. As we continue to explore the vast expanses of the universe, each new revelation brings us closer to unraveling the enigmatic origins of the cosmos.