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The illustration shows two planets colliding around the star Gaia20ehk. |Image credit: Andy Chanidakis
Astronomers have collected evidence of a violent collision between two planets in a distant star system. The first clue to this cataclysmic event came when a rather boring star started behaving very strangely. The collision appears to be similar to events in our history where a planetary body struck the Earth and created the moon.
The star in question is Gaia20ehk, a normally stable main sequence star, e.g. sun It is about 11,000 light-years away and has a stable and predictable light output. Until 2016, something very strange started happening. “
The star’s light output was good and flat, but starting in 2016, its brightness dipped three times. Then, right around 2021, things got completely crazy. “Team leader Anastasios Tzanidakis, a researcher at the University of Washington said in a statement. “I can’t stress this enough, stars like our Sun don’t do this. So when we see this, we’re like ‘Hello, what’s going on here?'”
Tzanidakis and colleagues found that Gaia20ehk’s twinkling is not intrinsic to the star itself, but is the result of large amounts of rock and dust passing in front of it as it orbits the star.
Where did these fragments come from? Two planets orbiting Gaia20ehk collide.
“Incredibly, various telescopes captured the impact in real time,” Chanidakis explained. “There are very few recorded planetary collisions of any kind, and none bear so much resemblance to the impacts that created the Earth and Moon. If we can observe more moments like this elsewhere in the galaxy, it will teach us a lot about the formation of our world.”
The right kind of planetary impact
Planets form from collisions and mergers between increasingly larger masses of material called planetesimals around young stars. Such effects are common during chaotic periods that represent the infancy of planetary systems. However, over the course of hundreds and millions of years, these turbulent environments gradually stabilized, forming stable solar systems like ours.
Although planetary collisions may be common, seeing them in distant planetary systems is no easy task and requires a lot of patience and a lot of luck. The colliding planet would also have to orbit its star directly between its star and our field of view to observe the debris from the collision, resulting in a dimming event, which can take many years to unfold.
“Andy’s unique work leverages decades of data to discover something that happens slowly—the story of astronomy that unfolds over a decade,” said James Davenport, a scientist at the University of Washington and a member of the team. “Not many researchers are looking for phenomena in this way, which means all kinds of discoveries are likely to be up for grabs.”
Therefore, the discovery of such an event is extraordinary to say the least. In fact, seeing such events is so rare that when Tzanidakis and his team first saw fluctuations in Gaia20ehk’s brightness, they couldn’t explain the brief dimming period and subsequent chaotic fluctuations. This is something that has never been seen before.
Gaia20ehk’s location is home to the remains of two colliding planets. |Image source: NASA/NSF NOIRLab
Researchers were able to solve the mystery only after studying Gaia20ehk using infrared light with different telescopes.
“The infrared light curve is exactly the opposite of visible light,” Chanidakis said. “When visible light starts to flicker and dim, there’s a surge in infrared light. This could mean that the material blocking the star is hot — so hot that it glows in the infrared.” Two planets slamming together could produce this heat, and the right kind of collision could create enough material to cause a dip in brightness.
“This was probably caused by the two planets getting closer to each other,” Chanidakis explained. “At first, they had a series of side impacts that didn’t produce a lot of infrared energy. Then they had a catastrophically large collision where the infrared energy did increase.”
There are some clues that this collision was similar to the one our planet experienced about 4.5 billion years ago, which produced debris that eventually coalesced to form the moon. In fact, this fuzzy dust cloud orbits the star 20ehk Gaia at a distance of about 93 million miles, about the distance between the Earth-Moon system and the Sun. Therefore, as the material around Gaia20ehk cools, it has the potential to form an exomoon and a planet-moon system similar to ours.
But it could take millions of years to happen. While astronomers may not have a chance to study this process to draw conclusions, other similar collisions are still being looked for. This may help reveal how common the events that created the Moon were. Since Earth’s main natural satellite is thought to be integral to the development of life on Earth, discovering the frequency of such events could shed light on the possibility of life elsewhere in the galaxy.
“How rare were the events that created the Earth and Moon? This question is fundamental to astrobiology,” Davenport said. “The moon appears to be one of the magic ingredients that makes Earth a habitat for life. It helps protect the planet from some asteroids, it generates ocean tides and weather, allows chemistry and biology to mix around the globe, and may even play a role in driving plate activity.
“Currently, we don’t know how common these dynamics are. But if we find more of these collisions, we’ll start to figure it out.”
The team’s findings were published on Wednesday (March 11) in Astrophysical Journal Letters.
