Astronomers observed a planetary collision 11,000 light-years away, capturing the event in real time as two planets smashed into each other, leaving a debris cloud that dimmed their star’s light and emitted a bright infrared flare. The discovery, linked to the star Gaia20ehk, offers insights into how planetary systems form and evolve.
When Anastasios Tzanidakis, a University of Washington doctoral candidate, noticed the star Gaia20ehk behaving unusually in 2016, he had no idea he was witnessing a cosmic collision. For years, the star’s light had been stable, but by 2021, it began flickering chaotically. The star’s light output was nice and flat, but starting in 2016, it had these three dips in brightness. And then, right around 2021, it went completely bonkers,
Tzanidakis said. The irregularities, paired with an infrared spike, hinted at a planetary collision, a rare event that could explain the debris cloud now orbiting the star.
A Citizen Scientist’s Discovery and the Role of Infrared Signatures
While Tzanidakis’s work focused on Gaia20ehk, another discovery by citizen scientist Arttu Sainio highlighted the same phenomenon. Sainio spotted an unusual infrared flare from the star ASASSN-21qj, which astronomers later linked to a planetary collision. Over three years, the debris cloud from the impact dimmed the star’s light, creating a massive eclipse
that astronomers tracked. The event, though distant, was observed in real time, a feat made possible by advanced telescopes and the collaborative efforts of professionals and amateurs alike.
The infrared signature was critical. As visible light from Gaia20ehk dimmed, infrared light spiked, suggesting the blocking material was hot — so hot that it’s glowing in the infrared.
This contrast between visible and infrared light curves confirmed the collision’s aftermath. The infrared light curve was the complete opposite of the visible light,
Tzanidakis explained. The heat signature, reaching 900 K, indicated the collision’s energy was sufficient to melt rock and metal, creating a “glowing dust” cloud that now orbits the star.
The Collision’s Signature: Visible and Infrared Light
The collision’s timeline aligns with a series of events. Starting in 2016, Gaia20ehk experienced three brief dips in brightness, likely due to grazing impacts as the planets spiraled toward each other. By 2021, the final, catastrophic crash occurred, producing the infrared flare and debris cloud. The debris, estimated to have a mass similar to Saturn’s moon Enceladus, is now orbiting the star at a distance of one astronomical unit — the same as Earth’s distance from the Sun. This similarity makes the system a perfect laboratory
for studying how terrestrial planets and moons form, much like the collision that created Earth’s moon.
James Davenport, a UW assistant research professor and senior author of the study, noted that the event’s rarity and visibility could help answer fundamental questions about planetary dynamics. How rare is the event that created the Earth and moon? That question is fundamental to astrobiology,
he said. The findings, published in The Astrophysical Journal Letters, highlight the importance of long-term astronomical surveys. It’s incredible that various telescopes caught this impact in real time,
Davenport added, emphasizing the role of “slow” astronomy that tracks changes over decades.
A Cosmic Analogy to Earth’s Moon Formation
The Gaia20ehk collision mirrors the cataclysmic impact that formed Earth’s moon 4.5 billion years ago. A Mars-sized object, called Theia, smashed into early Earth, ejecting debris that coalesced into the Moon. Similarly, the Gaia20ehk debris cloud is now orbiting its star, potentially forming new moons or planets. There are only a few other planetary collisions of any kind on record, and none that bear so many similarities to the impact that created the Earth and moon,
Tzanidakis said. This connection underscores the universality of such events in shaping planetary systems.
The debris cloud’s current state offers a unique opportunity to study planetary formation. We think that it’s very common for young solar systems to lose planets to collisions and migrations,
Tzanidakis noted. The discovery reinforces the idea that planetary systems are dynamic, with collisions playing a key role in their evolution.
Implications for Understanding Planetary Systems
The event’s significance extends beyond its immediate spectacle. By studying such collisions, astronomers can better understand the processes that shaped our own solar system. Davenport emphasized that the upcoming Vera C. Rubin Observatory could detect 100 similar collisions over the next decade, revolutionizing our view of planetary dynamics. If we catch more of these collisions, we’ll start to figure it out,
he said. The Gaia20ehk discovery is a stepping stone toward uncovering how often such events occur and what they reveal about the formation of planets and moons.
The collaboration between professional astronomers and citizen scientists like Sainio highlights the power of collective observation. As technology advances, the ability to detect and analyze such rare events will improve, offering deeper insights into the universe’s most violent and transformative processes. For now, Gaia20ehk’s collision remains a rare, vivid example of the cosmic forces that shape worlds — and a reminder that even the most dramatic events in the universe can be observed, studied, and understood.