An ordinary, Sun-like star called Gaia20ehk -- eleven thousand light years away in the constellation Puppis -- had, up until 2016, a nearly flat energy output. This is more or less what our Sun would look like from that distance; yes, there are minor fluctuations, but (fortunately for us) it's pretty stable over short time intervals.
Then... well, here it is in Tsanidakis's words: "The star's light output was nice and flat, but starting in 2016 it had these three dips in brightness," he said. "And then, right around 2021, it went completely bonkers. I can't emphasize 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 chaotic fluctuations in energy output were across the electromagnetic spectrum, but strongest in the infrared region. And stranger still, a more detailed analysis showed that the peculiar behavior was not from the star itself, but because there was -- suddenly -- a huge, irregular debris cloud surrounding it. This rock and dust eclipsed the star's light, but some of it was apparently radiating itself, accounting for the wild yo-yoing in the infrared. "The infrared light curve was the complete opposite of the visible light," Tzanidakis said. "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."
Tsanidakis and his team figured out that there was only one phenomenon that fit all the observations; two of Gaia20ehk's planets had collided with each other.
"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."
Artist's rendition of the collision of the two planets in the Gaia20ehk system [Image credit: A. Tsanidakis et al.]
Tsandiakis and his colleagues are particularly interested in watching how this all plays out, because -- as he mentioned -- it is very similar to the process that is thought to have formed the Moon. The collision between the proto-Earth and a Mars-sized planet astronomers call Theia, something like 4.5 billion years ago, triggered the remelting of the entire combined mass; the energy of the collision sheared off a chunk of Theia, which collapsed into what would eventually become the Moon. Now that we've actually seen something similar happening in another star system, astronomers will be on the lookout for more events like this.
"How rare is the event that created the Earth and Moon? That question is fundamental to astrobiology," said James Davenport, senior author of the paper, which was published three days ago in Astrophysical Journal Letters. "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."
Tsanidakis explains that while collisions are probably common in the early history of a stellar system, they can still occur in systems with stable, middle-aged stars like Gaia20ehk. Near passes by other stars, or by rogue exoplanets, could destabilize planetary orbits, causing one of the system's planets either to be ejected, or (in this case) gradually to spiral inward. This could explain the three dips in brightness that was his first clue something odd was happening -- they represent grazing passes as the two planets' orbits overlapped more and more. But eventually, they got close enough that there was a head-on impact, and all hell broke loose.
Considering the quantity of data that missions like Gaia produce, I find it astonishing that Tsanidakis and his colleagues even picked up on it. You have to wonder what other wonders might be hidden in the enormous hauls from JWST, Hubble, and (soon) the Vera Rubin Telescope. Fortunately, a sharp-eyed astronomer caught this one, and as a result we've learned a huge amount about exoplanetary collisions.
It's staggering to think about. The awe-inspiring vistas we're seeing through our best telescopes are only now being studied and analyzed, and who knows what else the astronomers will find?
All from following astrophysicist Neil deGrasse Tyson's adjuration -- "Keep looking up."
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