Two giant planets collided and vaporised in a distant star system

A star system 1800 light years from our own may have been the scene of a cataclysmic collision, as two giant planets crashed together and were incinerated, leaving behind a glowing-hot doughnut. It is the first time we have seen a planetary collision, and its aftermath, as it happened.

In 2021, astronomers spotted a strange event in which a sun-like star, dubbed ASASSN-21qj, dimmed by as much as 95 per cent. When Matthew Kenworthy at Leiden University in the Netherlands and his colleagues looked at past observations of the star, they found it had previously doubled in brightness three years before the dimming.

The cause of that brightening and subsequent dimming, they think, was two giant planets crashing together in an explosive event, with a resultant doughnut-shaped disc of heated dust and gas orbiting in place of the planets and obscuring our view of the star years later.

“We went through a whole series of possible ideas,” says Kenworthy. “The one that seems to fit all the data we have is a collision of two ice giants. It’s the first time this has been seen.”

The two planets would have each been several tens the mass of Earth, comparable to Neptune, and orbited the star at a distance similar to that of Jupiter around our sun. As they smashed together they would have been “pulverised, totally reduced to molten muck”, says Kenworthy, leaving behind a “giant ball of silica vapour” about seven times as wide as our sun.

Up close, an observer would have seen a “bright red glowing collision”, says Kenworthy, with rock and debris being blasted out from the planets’ solid cores.

A white-hot remnant would have burned at the centre of this ball, eventually forming into a torus-shaped ring orbiting the star, with a scorching temperature of some 700°C. That’s about half as hot as what would have been expected if the two planets were rocky, leading the researchers to surmise the planets were rich in water vapour, making them ice giant planets like Neptune and Uranus. The remains may eventually condense into a new planet surrounded by multiple moons in a few thousand years.

How the two planets collided is unclear. They may have been perturbed in their orbits by a passing star or another planet before crashing into each other, releasing the equivalent energy in an instant as a small star burning for two years.

“We have good evidence that planetary collisions do occur,” says Jonathan Marshall at the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan – for example, the moon is thought to have been created when a Mars-sized object called Theia smashed into Earth. Marshall, however, has previously proposed that the dimming of ASASSN-21qj was due to comets breaking apart in the system, not a planetary collision. “We didn’t feel there was enough mass to justify more than small bodies involved,” says Marshall.

André Izidoro at Rice University in Houston, Texas, says the idea of a giant impact in this system is “not out of the question”, noting that “super-Earths and mini-Neptunes are super common close to other stars, so giant impacts among them should also be super common”.

However, such events should become less frequent as a star system ages. In the solar system, it is thought that this tumultuous period ended around 100 million years after the birth of the sun, but Kenworthy and his colleagues believe ASASSN-21qj is 300 million years old. If correct, it would show that giant impacts can happen later, says Izidoro.

Further observations of the system, perhaps with the James Webb Space Telescope, could tell us whether the planetary collision idea is correct. “My prediction is in five to 10 years we’ll start seeing extra light from the system bouncing from the dust cloud,” says Kenworthy. “If it doesn’t do that, something else is going on.”