Evidence of cataclysmic planetary collision 1 800 light years from Earth

In December 2021, astronomers observed unprecedented fluctuations in the light of a star identified as ASASSN-21qj, located 1 800 light years away from Earth. Now, a study published in Nature suggests these fluctuations were the result of a colossal collision between two giant planets, potentially providing a rare opportunity to witness the birth of a new planet.

Arttu Sainio, an amateur astronomer, noted that the star’s infrared light emission had increased by approximately 4% around two and a half years prior to the observed dimming. Infrared light, typically emitted by objects at temperatures of several hundred degrees Celsius, led researchers to investigate the connection between these two phenomena.

The team of astronomers suggested that a massive collision between two giant planets could account for both the infrared brightening and the subsequent dimming of ASASSN-21qj. Such giant impacts are not uncommon in the late stages of planetary formation and have been responsible for various features in our own solar system, like the tilt of Uranus and the existence of Earth’s Moon.

Calculations indicate that the collision must have released an enormous amount of energy, sufficient to temporarily outshine the star itself. The resultant mass would have been hundreds of times larger than Earth, with an estimated temperature of around 700 °C (1 292 °F). NASA’s WISE telescope had captured the infrared brightening, although it likely missed the initial burst of light due to its observation cycle of roughly 300 days.

Over an extended period, fragments of the impact would have created a dispersed cloud of material, some of which passed between Earth and ASASSN-21qj. This is believed to be the cause of the star’s erratic dimming in visible light.

The planets involved in the collision are estimated to have been several times the mass of Earth, potentially as large as Neptune or Uranus. At least one of these planets is thought to have contained elements with low boiling points, such as water, indicating that they were ice giants.

Observations suggest that the collision occurred further from the star than Earth is from the Sun. This resembles our own solar system more closely than the compact planetary systems commonly found around other stars. Further observations using advanced telescopes could offer invaluable insights into the properties of the debris and the cooling mass, potentially confirming the monumental discovery.

The collision provides a chance for scientists to observe and understand a critical mechanism in planetary formation. While current data has already contributed to our understanding of these colossal events, ongoing observations will be crucial for confirming the theory.


1 A planetary collision afterglow and transit of the resultant debris cloud – Kenworthy, M., Lock, S., Kennedy, G. et al. – Nature – October 11, 2023

2 The afterglow of an explosive collision between giant planets may have been detected in a far-off star system – The Conversation – October 11, 2023 – https://doi.org/10.1038/s41586-023-06573-9

Featured image credit: The Watchers


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