Astronomers might have witnessed worlds in collision

A 10-million-kilometer-wide debris cloud may form a second-generation planet after a planetary collision.

Key Takeaways

1. Astronomers may have observed a rare planet-planet collision that left behind a molten core and a vast debris cloud.
2. The star ASASSN-21qj, located 1,800 light-years away, showed unusual brightening and dimming consistent with such an event.
3. The collision likely involved two ice giant planets and resulted in a synestia, a unique cosmic doughnut of vaporized material.
4. The aftermath may lead to the formation of a new planet and potentially moons over millions of years.
5. Alternative explanations, such as disintegrating comet fragments, are being explored alongside the giant impact hypothesis.

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Evidence of a Cosmic Catastrophe

Astronomers may have witnessed an extraordinary cosmic event: a planet-planet collision around a distant sunlike star named ASASSN-21qj, located 1,800 light-years from Earth. This discovery, described in Nature, is a rare glimpse into the violent processes that can shape planetary systems.

The saga began in late 2021 when astronomer Matthew Kenworthy of the Leiden Observatory observed a near-total dimming of ASASSN-21qj. This unusual dimming, flagged by the All-Sky Automated Survey for Supernovae (ASAS-SN), sparked interest due to its extreme nature. Further investigation revealed an infrared brightening event two and a half years earlier, observed by NASA’s NEOWISE telescope. This combination of dimming and brightening led researchers to hypothesize a catastrophic planetary collision.

Using data from NEOWISE and other observatories, researchers estimated the collision’s heat emission to be around 1,000 kelvins, sufficient to melt aluminum. The collision likely involved two ice giant planets, which formed a synestia—a doughnut-shaped structure of vaporized material. Within hours, the planets’ molten cores merged, creating the beginnings of a new planet.

Implications and Alternative Theories

This event offers valuable insights into how planetary systems form and evolve. ASASSN-21qj is estimated to be 300 million years old, a relatively young star still experiencing the tumultuous interactions common in early planetary systems. The collision’s aftermath—a 10-million-kilometer-wide debris cloud—may eventually condense into a second-generation planet with possible moons.

However, alternative explanations challenge the planetary collision hypothesis. Jonathan Marshall, an astronomer at the Academia Sinica Institute, suggests the data might better align with the disintegration of interstellar comet fragments. Infrared spectrum measurements indicate the dust’s chemical makeup may resemble comet material, and different age estimates for ASASSN-21qj further complicate the picture.

To confirm the collision scenario, researchers plan to gather more data using advanced telescopes like NASA’s James Webb Space Telescope and the European Southern Observatory’s Extremely Large Telescope. These observations could help resolve uncertainties and deepen understanding of giant impacts and their role in planetary evolution.