Astronomers say they’ve spotted evidence of a cataclysmic collision between two distant planets.
In addition to sending a shiver up our spines, the resulting study published in the journal The Astrophysical Journal Letters could also provide new insight into the evolution of our own solar system — and perhaps even how the Earth got its Moon.
The clues to this cosmic tragedy come from a main sequence star known as Gaia20ehk. It by all accounts seemed thoroughly ordinary, burning at a steady brightness like our Sun. That is, until it began flickering out of control.
“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,” said lead author Anastasios Tzanidakis, an astronomer at the University of Washington, in a statement about the work.
“I can’t emphasize enough that stars like our Sun don’t do that,” he added. “So when we saw this one, we were like ‘Hello, what’s going on here?’”
The star, it turned out, wasn’t flickering like a giant dying lightbulb. Instead, the dark patches were caused by huge streams of rock and dust passing in front of it. The quantities would have to be enormous to even partially blot out starlight, so the astronomers say the most likely explanation was that they’re the debris from a planetary collision.
Observations taken with another telescope corroborated this theory. In the infrared data, the light curve spiked while the visible light dimmed. This “could mean that the material blocking the star is hot — so hot that it’s glowing in the infrared.”
A collision between two massive bodies would produce these levels of heat. Before a brutal bodyslam, the planets would’ve been locked in a morbid dance.
“That could be caused by the two planets spiraling closer and closer to each other,” Tzanidakis said. “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.”
“It’s incredible that various telescopes caught this impact in real time,” Tzanidakis said. “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 Moon. If we can observe more moments like this elsewhere in the galaxy, it will teach us lots about the formation of our world.”
It’s not the first sign of a planetary dust-up astronomers have found. In 2023 study, a team said they spotted the grisly aftermath of a collision between two ice giants in a young star system, which created a red-hot, torus-shaped debris cloud.
But the latest findings bear deep parallels to an ancient catastrophe much closer to home. Around four and half billion years ago, astronomers believe that the Earth was also battered by another world. Theia, as the hypothetical planet has come to be known, is speculated to have been roughly the size of Mars. When it smashed into the Earth, it obliterated itself in the process, with some of its remains gradually coalescing in Earth’s orbit to form the Moon.
Encouragingly, the astronomers found that the dust cloud around Gaia20ehk orbits at roughly the same distance as the Earth does the Sun, or one astronomical unit, where it could also cool and coalesce to form a rocky satellite.
The serendipitous similarities could have implications for astrobiology. Our Moon is unusually large compared to its host planet, and it seems to be “one of the magical ingredients that makes the Earth a good place for life,” Tzanidakis said, shielding us from asteroids, producing tides, and affecting the weather.
If collisions that lead to an outsized moon are rare, perhaps life is all the more rarer.
“Right now, we don’t know how common these dynamics are,” Tzanidakis said. “But if we catch more of these collisions, we’ll start to figure it out.”
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