Giant Ice Planets Smashed Together in Blaze of Light—'Complete Surprise'

For the first time ever, scientists have discovered evidence of two planets crashing into each other in a spectacular collision.

These two distant exoplanets are thought to have been ice giants—like cold Neptunes—orbiting around a star about 1,800 light years from Earth, sending out huge plumes of dust and piercing light as they smashed into one another, a new study in the journal Nature has found.

The collision was first spotted by an amateur astronomer who noticed that the star system—named ASASSN-21qj—doubled in brightness at infrared wavelengths, before the star started to fade in visible light three years later. An international team of astronomers then studied the system for several years, concluding that it must have been caused by two planets colliding.

"To be honest, this observation was a complete surprise to me. When we originally shared the visible light curve of this star with other astronomers, we started watching it with a network of other telescopes," co-lead author Matthew Kenworthy, an astronomer from Leiden University, in the Netherlands, said in a statement. "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 this was an unusual event."

Exoplanets are planets orbiting other stars, of which 5,523 have been found as of September 2023. These exoplanets are spread across 4,112 planetary systems, with 932 systems having more than one planet orbiting the star.

The astronomers aren't 100 percent sure that what they're observing is a planetary collision, but it fits the story best of any option.

"We don't know for sure what we are seeing but giant impact fits best," Zoe Leinhardt, an associate professor of astrophysics at the U.K.'s University of Bristol, and paper co-author, told Newsweek. "The giant impact is the only event that we found that could simultaneously explain the large emission of infrared light (from the hot, extended planetary body formed by merger of the colliding bodies) and the transit of material across the star blocking the visible light (a load of debris produced in the collision)."

The researchers think that after the planets collided, the cloud of debris floated across the star after a few years, causing the observed dimming of the visible wavelengths. They hope to carry on studying the star system to learn more about the collision, what the planets were made from, and to watch what happens to the planetary remnants.

"It will be fascinating to observe further developments. Ultimately, the mass of material around the remnant may condense to form a retinue of moons that will orbit around this new planet," Leinhardt said in the statement.

"That would be very exciting," she said.

Do you have a tip on a science story that Newsweek should be covering? Do you have a question about exoplanets? Let us know via science@newsweek.com.