In December 2021, astronomers made an extraordinary observation of a flickering star that led to the discovery of a possible massive collision between two giant planets. The star, known as ASASSN-21qj, showed variations in visible light over a few months, capturing the attention of the scientific community. What made this observation even more intriguing was the previous emission of infrared light from the same location, suggesting a connection between the two events. The discovery, made possible by an amateur astronomer named Arttu Sainio, opens a window into the formation of planets and the processes involved in their evolution.
The proposed explanation for the observations is a cataclysmic collision between two planets, known as a giant impact. Such collisions are believed to be common in the final stages of planet formation and have significant effects on their characteristics, including size, composition, and thermal state. Giant impacts are responsible for several peculiar features in our solar system, such as the tilt of Uranus, the high density of Mercury, and the formation of Earth’s Moon. However, direct evidence of ongoing giant impacts in other planetary systems has been scarce.
To account for the observations, the collision must have released more energy in its initial hours than the star itself emits. The collision would have caused the colliding bodies to become superheated, melted, vaporized, or a combination of these effects. The result would have been a hot, glowing mass of material hundreds of times larger than the original planets. The emission of infrared light, captured by NASA’s WISE space telescope, corresponds to the initial expansion of this “post-impact body.” It will take millions of years for this body to cool and shrink, eventually forming a new planet.
The collision would have also generated plumes of debris, consisting of vaporized and condensed material, orbiting the star in various paths. Some of this debris passed between ASASSN-21qj and Earth, leading to intermittent dimming of the star’s visible light. The presence of debris clouds composed of tiny ice and rock crystals suggests that a fraction of the colliding bodies’ mass was vaporized upon impact. The erratic dimming pattern offers valuable insights into the dynamics of the collision and the interaction of debris with the star system.
By studying the ASASSN-21qj star system, scientists hope to gain a better understanding of the mechanisms involved in planet formation. The observations already provide valuable information regarding the size and composition of the post-impact body. It is estimated to be several hundred times the size of Earth and composed of elements with low boiling temperatures, indicating the presence of ice-rich Neptune-like worlds. The distance between the collision site and the star suggests a planetary system similar to our own, with ice giants located far from the central star.
The exciting prospect is that scientists can continue observing the ASASSN-21qj system for decades, allowing them to test and refine their theories. Future observations, including those from NASA’s James Webb Space Telescope (JWST), will provide more detailed insights into the composition and characteristics of the debris cloud, as well as the cooling process of the post-impact body. It is even possible that new moons may emerge as the system evolves and stabilizes. These ongoing observations contribute to our understanding of how giant impacts shape planetary systems and the formation of new worlds.
The detection of a massive collision between planets and the subsequent study of its aftermath offers a unique opportunity to witness the birth of a new world. By analyzing the observations, scientists can gain insights into the complex processes involved in planet formation and the factors that mold their properties. The ASASSN-21qj system serves as a captivating example of a giant impact, shedding light on the dynamics and consequences of such events in the cosmos. As our knowledge continues to grow, so does our appreciation for the awe-inspiring wonders of the universe.