how astronomers caught two planets smashing into each in real time

As citizen scientist caught two large planets slamming into each other. A team of astronomers then watched what happened over the next three years.

While most of us have been using social media to doomscroll, citizen scientist Arttu Sainio saw blips in the light curve of the star ASASSN-21qj, indicating an infrared flare that shouldn’t have been there. Curious astronomers took a second look and watched this flare fade and turn into a dimming over roughly three years while orbiting the star. Running the numbers, the only plausible explanation for what they saw was two small gas giants like Neptune, or large rocky worlds, slamming into each other. That infrared flare was the burning hot remnant of the collision. The dimming? A debris cloud which spread from the remnant over the span of nearly two and a half years.

Obviously, this is pretty amazing. We observed a planetary collision in real time, even though it happened during the construction of the Pyramid of the Sun in Teotihuacan, as the star is 1,850 light years away in the southern constellation Puppis — which yes, does translate to poop deck, or stern, thanks to the names of constellations around it — and the light from the event is just now reaching us. But we did see it from very far away, mostly through heat signatures and a massive eclipse that drastically dimmed the doomed worlds’ parent star. What would the impact look like if we could see it up close, in a hypothetical flyby on a warp capable spaceship?

You may be picturing a giant smash followed by a blast of shrapnel the size of islands and continents flying out into the rest of the solar system because that’s what we see in movies. In reality, planets are massive. They’re thousands of kilometers across and tip the scales at trillions of trillions of tons. As they would approach each other, their gravities would bend, warp, and crack their surfaces if they were rocky worlds, or heat up and disrupt the flow of their already turbulent clouds if they were gas giants. How they would behave at the exact moment of impact would also depend on their overall compositions, but the collision itself could last for up to a day.

Rocky planets would act like superheated, dried out putty, merging into an elongated egg slowly shedding clouds of rock and dust rising like a shroud around the planets, not just immediately blasting out away from the impact. They would be held back by the gravity of the combined object and look relatively slow due to the sheer scale of the event, unable to suddenly accelerate to relativistic speeds even from the energy released as the planets started to combine, their mutual gravity kneading and mixing mountain ranges worth of rock and metal with every passing second, raising surface temperatures to the point where they glow as brightly as a small star.

Gas giants would behave somewhat similarly because while they wouldn’t have solid surfaces, the pressure below the topmost layers of their atmospheres would force the gas to act like a liquid churning around their solid metallic cores. They would also act a lot like superheated putty, but their collision would emit clouds of gas and diamonds formed by the denser atmospheric layers as underneath, their cores melted into each other, creating an elongated remnant glowing like a small sun. The energy released in the process would melt and liquefy rock and metal no matter its density, and turn gas into plasma. This is what created the brilliant infrared flash seen by Sainio.

Over the next few months, the dust and debris released by the impact and exceeding the escape velocity of the newly created world would disperse into the solar system, creating a vast, low density cloud which would blot out the sunlight for any observers who happen to be watching the star from the right angle. Meanwhile the new object’s surface will cool and start to stabilize as it regains a spherical shape. Over the coming years, its combined innards will start to steadily differentiate, heavier metals sinking to the core while lighter rocks float closer to the surface or get picked up by the winds of a gas giants’ turbulent clouds now fueled by the heat of the collision.

Fast forward a few million years and you’ll see a seemingly normal world, most likely a gas giant calmly orbiting a nondescript star. You’d never know that it’s the result of an apocalyptic merger of two planets seen from 17 quadrillion kilometers away. And if it sounds a little too spectacular and dramatic to you, it shouldn’t. We think that it’s very common for young solar systems to lose planets to collisions and migrations, and we both live on, and look up at, the aftermath of such an impact. But even though this is a pretty common event in our universe, it’s still exciting that we were close enough, had the right tools, and were looking at the right time to see it with our own eyes.

See: Kenworthy, M. et al. (2023) A planetary collision afterglow and transit of the resultant debris cloud. Nature, DOI: 10.1038/s41586-023-06573-9