Alien X-Rays: Analyzing the X-Ray Emissions of Comets

When ‘Oumuamua was discovered hurtling around the sun in October of 2017, it was the first large object from interstellar space ever seen in our solar system. ‘Oumuamua promised to be of enormous scientific value; as a remnant from a far-away solar system, it seemed to hold information about the way planets develop. In the end, however, ‘Oumuamua proved quite challenging to study. When examined with visible and infrared light, it looked like a rocky asteroid, but it sped up during its journey around the sun like an icy comet shooting off a tail of gas. The mystery of ‘Oumuamua may never be solved, but thanks to a team of astronomers at Yale, we’ll be more prepared the next time an interstellar object swings by. The team, led by Yale graduate student Sam Cabot, developed a model to study and classify interstellar objects using X-rays, a novel technique that could help us learn more about their composition and origins.

Using X-rays to study an object like ‘Oumuamua might seem counterintuitive. “X-rays usually emanate from the hottest, most extreme objects and environments in the galaxy,” Cabot said. “You would never expect X-rays to come from asteroids, comets, or anything else that’s cold or rocky.” But solar wind—streams of plasma that emerge from the upper atmosphere of the sun—can light up cold objects like ‘Oumuamua in unexpected ways. Ions from the solar wind snatch electrons from certain types of cold gas, which then release x-rays as they lose energy. Importantly, this process can even visualize gasses like hydrogen or nitrogen that are invisible to astronomers using other methods. If an interstellar object emits these gasses, Cabot’s team found, x-rays might offer a critical window into its nature and composition.

For their study, Cabot and his team developed a model to predict the X-ray emissions from different types of interstellar objects, based on their unique properties. They found that if ‘Oumuamua had an invisible halo of gas—which would have explained its strange acceleration—it might have emitted x-rays. If we had been able to study these X-rays, we would have gained valuable information about ‘Oumuamua’s composition, perhaps shedding light on how it developed. “Then there’s a new challenge: figuring out where interstellar objects come from and how they form,” Cabot said. “Just encountering ‘Oumuamua tells us that our understanding of how planets and solar systems form is incomplete.” 

When the Vera C. Rubin Observatory is completed in Chile in 2025, it will begin scanning the sky for transient astronomical events, perhaps discovering new interstellar objects like ‘Oumuamua in the process. Cabot and his team hope that their technique will shed light on these interlopers, helping us learn even more about the development of distant solar systems, as well as our own. “There are lots of problems in astronomy that are very important, that people have spent a lot of time trying to make progress on, often incrementally,” Cabot said. “It’s not too often that you get a brand-new line of research, like ‘Oumuamua provided for us.” We may have missed our chance with ‘Oumuamua, but perhaps x-rays will help us solve the mysteries of our next interstellar visitors.