Small Planet Strips Earth-Sized World's Atmosphere in 22,000mph Collision

Astronomers have spotted the evidence of a massive planetary collision in a nearby star system for the first time.

The collision is believed to have occurred between an infant planet roughly the same size as Earth and a smaller body around the star HD 172555, which is 95 light-years away from Earth.

During the impact, which occurred around 200,000 years ago, the smaller body was traveling at over 22,000 miles per hour. This resulted in a cosmic crash so powerful that the Earth-sized world's atmosphere was stripped.

While astronomers have long believed these kinds of collisions are common throughout the Universe, this is the first time that they have spotted evidence of one that stripped an atmosphere in such a way around a distant star.

The findings were revealed in a paper published in the journal Nature.

"This is the first time we've detected this phenomenon, of a stripped protoplanetary atmosphere in a giant impact," lead author and a graduate student in Massachusetts Institute of Technology's Department of Earth, Atmospheric and Planetary Sciences, Tajana Schneiderman, said in a university press release. "Everyone is interested in observing a giant impact because we expect them to be common, but we don't have evidence in a lot of systems for it."

This collision is similar to the impact our planet experienced during its infancy around 4.5 billion years ago, which is believed to have created the moon.

Such collisions have long been considered an important part of how young planetary systems evolve. As infant planetary bodies, or planetesimals, collide, they merge and form progressively larger planets.

The HD 172555 System
A stock illustration of the debris, dust and gas around the star HD 172555. This "weird" disc of debris has been revealed to be partially comprised of the atmosphere of an Earth-sized planet ripped away in a high-speed collision. NASA

The team of astronomers made this discovery by investigating the unusual composition of dust around the star HD 172555. Recent observations of this dust have revealed that it contains an unusually high concentration of minerals.

Not only have the concentrations of these minerals confused planetary scientists, but they also seem to be in grains that are far finer than scientists would usually expect to find in disks of debris around stars.

"Because of these two factors, HD 172555 has been thought to be this weird system," Schneiderman said. She and her team set about discovering what the gas might tell them about the history of planetary collisions in that system.

To do this they looked for traces of carbon monoxide in data collected by the Atacama Large Millimeter Array (ALMA), one of the world's most powerful telescopes comprised of 66 antennas distributed across the Atacama Desert, in Chile.

"When people want to study gas in debris disks, carbon monoxide is typically the brightest, and thus the easiest to find," Schneiderman added. "So, we looked at the carbon monoxide data for HD 172555 again because it was an interesting system."

The team found carbon monoxide circling HD 172555 in large amounts at surprisingly close proximity to the star. The reason this is unusual is the light from stars is expected to break down carbon monoxide molecules. That means we don't usually find this compound close to stars as it has usually been destroyed.

Investigating different scenarios that could explain these close clouds of carbon monoxide around HD172555, the team arrived at the conclusion that the gas could be the remnant of a relatively recent titanic collision.

"Of all the scenarios, it's the only one that can explain all the features of the data," Schneiderman said. "In systems of this age, we expect there to be giant impacts, and we expect giant impacts to be really quite common.

"The only plausible process that could produce carbon monoxide in this system in this context is a giant impact."

The abundance of carbon monoxide also allowed the astronomers to pinpoint when this collision had occurred, approximately 200,000 years ago, and told them that this impact had literally blown apart one of the planet's atmospheres.

Professor in Earth, planetary, and space sciences at the University of California at Los Angeles, Hilke Schlichting, was not involved in the research, but its potential impact has impressed him.

He said in the press release: "What is particularly exciting about this work, in my opinion, is that it demonstrates the importance of atmospheric loss by giant impacts.

"It also opens up the possibility to study the composition of the atmospheres of extra solar planets undergoing giant impacts, which ultimately may help shed light on the atmospheric condition of the terrestrial planets during their own giant impact stage."

Schneiderman concluded by adding that she and her team will now consider investigating this phenomenon in systems other than that of HD 172555 as a way of searching for other massive cosmic collisions.

She said: "We are showing that, if you find carbon monoxide in a place and morphology consistent with a giant impact, it provides a new avenue for looking for giant impacts and understanding how debris behaves in the aftermath."

Planet Collision
A stock illustration showing the collision between an Earth-sized planet and a smaller body around the star HD 172555. Researchers have discovered the first evidence of a planetary collision, thought to be an important step in planetary growth. JPL-Caltech/NASA

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