A chance post on a social media site from an amateur researcher led to the discovery that the system doubled in brightness at infrared wavelengths some three years before the star started to fade in visible light. "To be honest, this was a complete surprise to me," says Dr. Kenworthy. "When the ASASSN survey shared the light curve of this star with other astronomers, I started watching it with a network of telescopes and observers. Out of the blue, an astronomer on social media pointed out that the star brightened up in the infrared over a thousand days before the optical fading. I knew then that this was an unusual event."
Leiden graduate student Richelle van Capelleveen (who did the work as a master’s student) continues: "I worked on the light curve with Dr. Kenworthy and during our work, we realised that this could be a collision of two planets."
The most likely explanation is that two ice giant exoplanets collided together, producing the infrared glow picked up by the NEOWISE mission, and the resultant expanding debris cloud then moved in front of the star some three years later and caused the star to dim in brightness at visible wavelengths.
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A simulation of a collision between two ice giant bodies showing the simulation particles (top) and density (bottom) in a slice through the midplane of the impact. The scale bar gets smaller during the simulation to follow the expanding post-impact body and debris. (c) Jingyao Dou/Bristol University
"The temperature and size of the glowing material and the amount of time the glow has lasted is consistent with the collision of two ice giant exoplanets, as we infer from our calculations and computer models," says coauthor Dr. Simon Lock (University of Bristol, United Kingdom).
"What’s new is that we think this is the first time we see the glow from the body that is produced by the planetary collision," says Dr. Grant Kennedy (University of Warwick, United Kingdom), also a coauthor on the paper.
"This is really a fantastic opportunity to find out about the interior of giant planets," says Dr. Ludmila Carone from the Space Research Institute of the Austrian Academy of Sciences in Graz. Normally, giant planets hide their heavy elements under thick layers of hydrogen and helium. In this collision, however, material from the interior was ejected or dredged up into the outer regions of the body created by the merger of the two planets. Carone adds: "We can already conclude that a lot of water vapour was released that helped to cool the post-impact body down to 1000 K."
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Animation of a planetary collision at ASASSN-21qj. What happens when two planets collide? Astronomers were fortunate enough to have unique views of the collision and aftermath of two Neptune sized planets colliding. When these ice giant planets crashed, they left behind a hot, loose ball of gas, dust and rocks, with an even hotter core. Over hundreds of years this will thin out around the orbit, the core will grow by accretion and moons may even form from the debris. (c) Alice Hopkinson/Las Cumbres Observatory
Over the next few years, the cloud of dust will start to smear out along the orbit of the collision remnant, and a tell-tale scattering of light from this cloud can be detected with both ground-based telescopes and the James Webb Space Telescope. Astronomers will watch closely what happens next in this system.
Ultimately, the cloud of material around the remnant may condense to form a retinue of moons that will orbit around this new planet.
A planetary collision afterglow and transit of the resultant debris cloud. By: Matthew Kenworthy, Simon Lock, Grant Kennedy, Richelle van Capelleveen, Eric Mamajek, Ludmila Carone, Franz-Josef Hambsch, Joseph Masiero, Amy Mainze, J. Davy Kirkpatrick, Edward Gomez, Zoë Leinhard, Jingyao Dou, Pavan Tanna, Arttu Sainio, Hamish Barker, Stéphane Charbonnel, Olivier Garde, Pascal Le Dû, Lionel Mulato, Thomas Petit, Michael Rizzo Smith. In: Nature, 12 October 2023. [original | preprint (pdf)]
Artist impression of a donut shaped cloud that was formed after two ice giant planets collided. The cloud glows dull red with the heat from the collision. An asteroid and smaller debris from the collision is in front of the donut shaped cloud. The parent star, ASASSN-21qj, is off in the distance to the left of the cloud. (c) Mark Garlick [high resolution]