The star Gaia20ehk was unremarkable.
Telescope scans showed it shining steadily about 11,000 light-years from Earth, near the constellation Puppis.
But when Anastasios Tzanidakis was looking through data from 2020, he noticed the star’s light dimming and then fluctuating wildly.
“I'm saying this all very casually, but I cannot stress enough that stars like our sun don't do that,” said Tzanidakis, a doctoral student in astronomy at the University of Washington.
Tzanidakis and his colleagues developed a hypothesis: two planets had collided in front of Gaia20ehk, blocking the star’s light. Witnessing such events could have major implications for our understanding of planet formation — as Tzanidakis put it, “how worlds are created.”
To observe activity light-years away, astronomers monitor light through telescopes. The result is called a light curve. “Almost think of it as, like, the stock market,” said Tzanidakis, who, like many in his field, enjoys analogies. “We have daily prices every day, and then that fluctuates up and down.”
To verify the star’s light curve reading, the team looked at data from a NASA space telescope, which measures infrared light, that is, heat. The NASA data showed that as the visible light dimmed, something was creating a lot more heat near the star.
“What the infrared light was telling us is that the material must actually be glowing, and it must be hot. Really hot. Like 900 degrees Kelvin,” he said — about the temperature of molten lava. They think that heat was produced by the planetary collision, which in turn blocked the star’s light.
Tzanidakis and UW assistant research professor of astronomy James Davenport published their study last month in The Astrophysical Journal Letters.
In the beginning
If the activity around Gaia20ehk is a collision, astronomers may be witnessing “one of the last pieces of the puzzle” in the process of planet formation, Tzanidakis said.
Planets form when gravity forces together matter orbiting a new star — stuff like gas or dust. Eventually, those masses grow, and can cross paths and collide.
But what happens then?
One theory is that the debris from such an impact eventually circles the bigger body, creating rings or a solid mass. That is central to prevailing theories for how the moon formed. Among these: An object about the size of Mars hit ancient Earth 4.5 billion years ago, creating a huge cloud of material, which was caught in orbit and eventually created the Earth-moon system. Another model has the two bodies colliding and vaporizing, then reforming into the Earth and moon. Whatever the case, the moon seems integral to making Earth hospitable to life, and such an event would be part of how Earth as we know it came to be.
The resulting dust now orbiting Gaia20ehk may be about the same distance to the star as the Earth is to the sun, astronomers believe, and there is a possibility that it could eventually solidify into a system like the one between Earth and the moon.
They might be waiting a while. The process "could take decades, it could take millennia. We don’t know!" Davenport said. "We keep getting images of the system, even a few weeks ago, and the dust is still there."
But learning about such collisions could reveal more about the specific astrophysics of this process — what the planets are made of, for instance, and how long it will take the dust to cool down, but also how often collisions occur.
“If we can understand truly how frequently they happen, this might tell us something about the fundamental process of how planets are put together,” he said.
Evidence of planetary collisions is rare, but not unprecedented. “There have been a few of these stars that have this very similar property,” Tzanidakis said. They dim, but in the infrared light, abruptly brighten. “What that suggests to many researchers around the world is that we're witnessing planets running into each other and colliding.”
As astronomers research the role collisions play in planet formation, they are turning to facilities like the Vera C. Rubin Observatory in Chile, which has a massive telescope that just embarked on a 10-year project to scan the night sky. This will help identify variations — whether something moves like an asteroid, flashes like a supernova, or dims like obstructions in front of a star.
"It's going to take us a while to start discovering new objects like this," Davenport said. "That's where I think having an example that we can look at now, from before the survey, is really helpful."
‘The very slow universe’
The brightness variations caused by planetary collisions take anywhere from decades to millions of years to play out. Tzanidakis said astronomers are starting to understand what they call the “very slow universe.”
As Tzanidakis and his colleagues research how planets form, he wants to tap data archives such as the one used for this research.
“I am really fascinated with using public telescope data,” he said. “There's many telescopes around the world that are constantly staring at the sky. So I like to scour the archives and put these stories together.”
Davenport said witnessing evidence of such a collision is a treat. By understanding the origin of planets such as Earth, we can better understand how life begins.
“A lot of us are after the story of: Is there life in the universe? How common is it? When does it arise, and where did we come from,” he said. “And maybe, ultimately, where are we going?”
