Astronomers say they’ve likely witnessed the collision of two planets orbiting a distant star.
The star in question Gaia20ehk, which is located 11,000 lightyears from Earth.
It’s a relatively normal star, much like our own Sun, but is undergoing an unusual event. When astronomers took a closer look at, they noticed it began to flicker.
The chaos of young star systems
Planets form out of the dusty discs of debris that orbit young stars.
These swirling discs are composed of the leftover ingredients out of which the star itself formed.
As such, the regions around young stars are chaotic places, young planets and protoplanets regularly colliding with each other and flying off into space.
Eventually, however, these chaotic systems calm down and usher in an era of equilibrium, with fully-grown planets safely orbiting their host stars
However, as this team found out, planet collisions can and do still happen.
Gaia20ehk and a strange flickering
The strange flickering of star Gaia20ehk was discovered by astronomer Anastasios (Andy) Tzanidakis while looking through archive telescope data from 2020.
Gaia20ehk is a ‘main sequence‘ star like our Sun, and so should have been emitting a steady beam of light.
“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,” says Tzanidakis, from the University of Washington.
“I can’t emphasise enough that stars like our Sun don’t do that. So when we saw this one, we were like ‘Hello, what’s going on here?'”
The team say the flickering was caused by rocks and dust passing in front of the star, and the likely source of that debris was a collision between two planets.
“It’s incredible that various telescopes caught this impact in real time,” Tzanidakis says.
“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 the Mooon.
“If we can observe more moments like this elsewhere in the Galaxy, it will teach us lots about the formation of our world.”
More chaotic space science
The chances of capturing a planet collision
What is the likelihood of seeing the aftermath of a planet collision around a random star in our Galaxy?
Not only does the observer have to be looking at the right star at the right time, but the star and planet must be positioned just right so we can actually see the effects from Earth.
“Andy’s unique work leverages decades of data to find things that are happening slowly – astronomy stories that play out over the course of a decade,” says the study’s senior author James Davenport, a University of Washington assistant research professor of astronomy.
“Not many researchers are looking for phenomena in this way, which means that all kinds of discoveries are potentially up for grabs.”
Afer the discovery of Gaia20ehk’s flickering behaviour, the team turned to a different telescope to observe it in infrared light.
“The infrared light curve was the complete opposite of the visible light,” says Tzanidakis.
“As the visible light began to flicker and dim, the infrared light spiked. Which could mean that the material blocking the star is hot – so hot that it’s glowing in the infrared.
“That could be caused by the two planets spiraling closer and closer to each other.
“At first, they had a series of grazing impacts, which wouldn’t produce a lot of infrared energy. Then, they had their big catastrophic collision, and the infrared really ramped up.”
Does this sound familiar?
The team say there’s evidence that the planet collision around Gaia20ehk resembles the one that created Earth and the Moon about 4.5 billion years ago.
Our Moon is thought to have formed when a Mars-sized young planet, known as Theia, smashed into Earth.
The dust cloud around Gaia20ehk is orbiting the star at about 1 astronomical unit, which is the average distance between Earth and the Sun.
That means the material might eventually cool down and solidify into something similar to the Earth-moon system.
This could take as long as a few million years to happen.
“How rare is the event that created the Earth and Moon? That question is fundamental to astrobiology,” Davenport says.
“It seems like the moon is one of the magical ingredients that makes the Earth a good place for life. It can help shield Earth from some asteroids, it produces ocean tides and weather that allow chemistry and biology to mix globally, and it may even play a role in driving tectonic plate activity.
“Right now, we don’t know how common these dynamics are. But if we catch more of these collisions, we’ll start to figure it out.”