Astronomers have collected evidence of a violent collision between two planets in a distant star system. The first clues to this catastrophic event came when a rather dull star began to behave very strangely. The collision appears to be similar to the event in our history where a planetary body slammed into Earth and created the Moon.
The star in question is Gaia20ehk, an ordinarily stable main sequence star the sun is around 11,000 light-years away with a steady and predictable light effect. Until 2016, that is, when something very strange started happening.”
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Tzanidakis and colleagues discovered that the flickering of Gaia20ehk was not intrinsic to the star itself, but was the result of large amounts of rock and dust passing in front of it as it orbited the star.
The source of this debris? The collision of two planets orbiting Gaia20ehk.
“It’s amazing that different telescopes captured this feature in real time,” explained Tzanidakis. “There are only a few other planetary collisions of any kind on record, and none that have 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 a lot about the formation of our world.”
The right kind of planetary smash-up
Planets form from collisions and mergers between increasingly large clumps of material called planetesimals around young stars. During the chaos that represents the infancy of planetary systems, such impacts are common. But over hundreds of millions of years, these turbulent conditions will settle down, resulting in a stable solar system like ours.
Although planetary collisions are probably quite common, seeing them in distant planetary systems is no easy feat, requiring a lot of patience and an enormous amount of luck. The colliding planets must also orbit their star directly between it and our view for debris from a collision to cause dimming events, which can take years to unfold.
“Andy’s unique work leverages decades of data to find things that happen slowly — astronomy stories that unfold over the course of a decade,” said team member James Davenport, a researcher at the University of Washington. “There aren’t many researchers looking for phenomena in this way, which means all kinds of discoveries are potentially on the way.”
As such, discovering such an event is extraordinary to say the least. In fact, such events are so rare to see that when Tzanidakis and his team first saw the fluctuations in Gaia20ehk’s brightness, they couldn’t account for the brief periods of dimness, followed by chaotic fluctuations. It was something that had never been seen before.
The researchers could only unravel this mystery when they examined Gaia20ehk with different telescopes using infrared light.
“The infrared light curve was the complete opposite of visible light,” Tzanidakis said. “As the visible light began to flicker and dim, the infrared light increased. Which could mean that the material blocking the star is hot—so hot that it glows in the infrared.” Two planets slamming together can generate this heat, and the right kind of collision can create enough material to cause a drop in luminosity.
“It could be caused by the two planets spiraling closer and closer together,” Tzanidakis explained. “At first they had a series of grazing impacts, which wouldn’t produce much infrared energy. Then they had their big cataclysmic collision, and the infrared really increased.”
There are some clues that this collision is similar to the impact our planet experienced about 4.5 billion years ago that created debris that would coalesce to form the Moon. In fact, this obscuring dust cloud orbits the star Gaia20ehk at a distance of about 93 million miles, which is about the distance between the Earth-Moon system and the Sun. Thus, there is a possibility that as this matter around Gaia20ehk cools, it may form an exomoon and a planet-moon system similar to ours.
But it may take a few million years for this to happen. While astronomers may not get the chance to study this process to its conclusion, the hunt for other similar collisions is on. This could help reveal how common the events that created the moon are. And because Earth’s primary natural satellite is thought to have been 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 Milky Way.
“How rare is the event that created the Earth and the Moon? That question is fundamental to astrobiology,” Davenport said. “It seems like the Moon is one of the magic ingredients that makes Earth a good place for life. It can help shield Earth from some asteroids, it produces ocean water and weather that allows chemistry and biology to mix globally, and it can even play a role in driving plate tectonic activity.
“Right now, we don’t know how common these dynamics are. But if we catch more of these collisions, we’ll start to find out.”
The team’s research was published on Wednesday (March 11) in the The Astrophysical Journal Letters.
