Astronomers have potentially detected the aftermath of a massive collision between two giant planets, marking the first time such an event has been observed. This collision could eventually lead to the formation of a brand new planet, offering a unique opportunity to witness the birth of a world and gain insights into the formation of planets.
The unusual flickering of a sun-like star caught the attention of astronomers in December 2021. Over the course of a few months, the visible light emitted by the star underwent fluctuations. There were moments when it almost disappeared, only to return to its previous brightness. This star, which is located approximately 1,800 light years away from Earth, was named ASASSN-21qj after the ASASN-SN astronomy survey that initially detected its dimming.
Although dimming stars are not uncommon, as it is often attributed to material passing between the star and Earth, ASASSN-21qj stood out thanks to the observation of an amateur astronomer, Arttu Sainio. Sainio noted on social media that around two and a half years before the star’s light began to fade, the emission of infrared light from its location had increased by approximately 4%. Infrared light is typically emitted by objects at high temperatures, usually a few hundred degrees Celsius. This discovery raised the question of whether these two observations were connected and what exactly was happening around ASASSN-21qj.
In a publication in Nature, the researchers propose that both sets of observations can be explained by a cataclysmic collision between two planets. These giant impacts are believed to be common in the final stages of planet formation, determining the final sizes, compositions, thermal states, and orbital arrangements within planetary systems. In our own solar system, giant impacts are responsible for peculiarities such as Uranus’ odd tilt, Mercury’s high density, and the existence of Earth’s Moon. However, direct evidence of such impacts occurring in other galaxies has been limited so far.
To account for the observed phenomena, the collision between the two planets must have released more energy in the first few hours after impact than the star itself emits. The colliding bodies would have become superheated and melted, potentially even vaporized. This collision would have created a hot and luminous mass of material hundreds of times larger than the original planets. The WISE space telescope, operated by NASA, detected the infrared brightening of ASASSN-21qj. However, since WISE only observes the star every 300 days or so, it likely missed the initial flash of light resulting from the impact.
The expanded planetary body formed by the collision will require a significant amount of time, potentially millions of years, to cool down and shrink into a recognizable new planet. During the initial period when this “post-impact body” reached its largest extent, the emitted light could still be several percent as bright as the light from the star. This luminosity could explain the observed infrared brightening.
The impact would have also ejected large amounts of debris, which would have settled into various orbits around the star. A portion of this debris would have vaporized upon impact, later condensing into clouds of ice and rock crystals. Over time, some of these clumps of material may have passed between Earth and ASASSN-21qj, partially obstructing the visible light emitted by the star and causing the irregular dimming.
If the researchers’ interpretation is correct, studying this star system could provide valuable insights into the mechanisms behind planet formation. Even with the limited observations available thus far, several interesting discoveries have been made. Firstly, the post-impact body must have been several hundred times larger than Earth to emit the observed amount of energy. This suggests that the colliding planets were likely several times the mass of Earth, potentially comparable in size to ice giant planets like Uranus and Neptune. Secondly, the estimated temperature of the post-impact body is around 700°C, indicating that the colliding bodies could not have been solely composed of rock and metal. Instead, the outer regions of at least one of the planets must have contained volatile elements with low boiling temperatures, such as water. This implies that the collision involved two Neptune-like worlds rich in ice.
The time delay between the emission of infrared light and the observation of debris crossing the star suggests that the collision occurred at a considerable distance from the star, further than the distance between the Earth and the Sun. In this regard, the system resembles our own solar system more closely than many other tightly-packed planetary systems commonly observed around other stars.
Excitingly, ongoing observations of this star system could allow for the monitoring of its evolution over several decades, enabling the testing of various hypotheses. Future observations, including those conducted using NASA’s James Webb Space Telescope (JWST), will help determine the sizes and compositions of particles within the debris cloud, identify the chemistry of the upper layers of the post-impact body, and track the cooling process of this hot debris mass. It may even be possible to witness the emergence of new moons.
These observations have the potential to enhance our understanding of how giant impacts shape planetary systems. Until now, the only examples we had were remnants of impacts within our own solar system. By witnessing the birth of a new planet in real time, we can gain valuable insights and refine our theories regarding planet formation.
In conclusion, the recent observations surrounding ASASSN-21qj point towards a monumental collision between two giant planets. If confirmed, this discovery would provide a groundbreaking opportunity to witness the formation of a new world and unravel the mysteries of planetary formation. The ongoing research and future observations hold great promise for advancing our understanding of how planets are born.
