The light of a new world: the discovery of a planet born from a cosmic collision
Light left over from an explosive collision between planetary giants may have been detected for the first time in a distant star system. The debris from the collision could cool and form an entirely new planet. If the observation is confirmed, it will offer an extraordinary opportunity to witness the birth of a new world in real time and open a window on planet formation.
Oddities in starlight
In December 2021, astronomers observing an otherwise not particularly notable Sun-like star noticed it beginning to flicker. For a few months, the visible light (the light we can see with our eyes) from this star changed. At times it would almost disappear, only to return to its previous brilliance.
A mystery to solve
The star, which is located about 1,800 light years from Earth, was identified as ASASSN-21qj, based on the ASASN-SN astronomical study that observed the dimming of the star for the first time. Seeing stars dimming like this is not unusual. It is generally attributed to material passing between the star and Earth. ASASSN-21qj could simply have been added to a growing list of similar observations if it weren’t for an amateur astronomer, Arttu Sainio. Sainio noted on social media that about two and a half years before the star’s light was seen fading, infrared light emission from its location had increased by about 4%.
We propose that both sets of observations could be explained by a catastrophic collision between two planets. Giant impacts, as these collisions are known, are believed to be common in the final stages of planet formation. They determine the sizes, compositions, and final thermal states of planets and shape the orbits of objects in those planetary systems.
To explain the observations, a collision would have to release more energy in the first hours after the impact than the star would have emitted. The material from the colliding bodies would have been superheated and melted, vaporized, or both. The impact would have formed a bright mass of material hundreds of times larger than the original planets.
Gaining insights from the collision
If our interpretation of events is correct, studying this star system could help us understand a key mechanism of planet formation. Even from the limited set of observations we have so far, we have learned some very interesting things.
The outer regions of at least one of the planets must have contained elements with low boiling temperatures, such as water. Therefore, we believe we have witnessed a collision between two ice-rich, Neptune-like worlds. The observed delay between the emission of infrared light and the observation of debris passing through the star suggests that the collision occurred quite far from the star, farther than Earth is from the Sun.
The most interesting aspect of all this is that we can continue to observe the evolution of the system for many decades and test our conclusions. Future observations, using telescopes like NASA’s JWST, will determine the sizes and compositions of particles in the debris cloud, identify the chemistry of the upper layers of the post-impact body, and track how this hot mass of debris cools. We may even see new moons emerge.