Astronomers discover signs of an atmosphere stripped from a planet in a giant collision

Younger planetary systems generally experience severe growing pains, as infants’ bodies collide and fuse to form progressively larger planets. In our solar system, the Earth and the Moon are thought to be products of this type of giant impact. Astronomers believe such collisions should have been common in early systems, but they were difficult to observe around other stars.

Now astronomers at MIT, the National University of Ireland in Galway, the University of Cambridge, and elsewhere have found evidence of a giant impact that occurred in a nearby star system, just 95 light-years from Earth. The star, called HD 172555, is about 23 million years old, and scientists suspect that its dust bears traces of a recent impact.

The team led by the Massachusetts Institute of Technology observed further evidence of a giant impact around the star. They determined that the collision most likely occurred between a planet roughly the size of Earth and a smaller one at least 200,000 years ago, at 10 kilometers per second, or more than 22,000 miles per hour.

Crucially, they detected gas indicating that such a high-speed collision would likely blow off part of the larger planet’s atmosphere — an exciting event that would explain the observed gas and dust around the star. The results, which appear today in temper nature, represents the first detection of its kind.

“This is the first time we’ve detected this phenomenon, of stripping the atmospheres of protoplanets in a giant impact,” says lead author Tajana Schneiderman, a graduate student in the Department of Earth, Atmospheric and Planetary Sciences at MIT. “Everyone is interested in observing a giant effect because we expect it to be common, but we don’t have evidence in a lot of systems for that. Now we have additional insight into these dynamics.”

clear signal

The star HD 172555 has been the subject of intrigue among astronomers due to the unusual composition of its dust. Observations in recent years have shown that star dust contains large amounts of unusual minerals, in grains much finer than astronomers would expect for a typical stellar debris disk.

“Because of these two factors, HD 172555 is thought to be this peculiar system,” Schneiderman says.

She and her colleagues wondered what the gas might reveal about the system’s impact history. They looked at data captured by ALMA, Chile’s Large Millimeter Atacama Array, which consists of 66 radio telescopes, the distances between which can be adjusted to increase or decrease the resolution of their images. The team looked at data from the ALMA public archives, looking for signs of carbon monoxide around nearby stars.

“When people want to study the gas in debris disks, carbon monoxide is the brightest, and therefore easier to find,” Schneiderman says. “So, we looked at the carbon monoxide data for HD 172555 again because it was an interesting system.”

in the aftermath

By careful reanalysis, the team was able to detect carbon monoxide around the star. When they measured its abundance, they found that the gas made up 20 percent of the carbon monoxide in Venus’s atmosphere. They also noted that the gas was orbiting in large quantities, surprisingly close to the star, at about 10 AU, or 10 times the distance between the Earth and the Sun.

“The presence of carbon monoxide in this close proximity requires some explanation,” Schneiderman says.

That’s because carbon monoxide is typically subject to photodissociation, a process in which a star’s photons disintegrate and destroy the molecule. At close range, there is usually very little carbon monoxide near the star. Therefore, the group tested different scenarios to explain the abundant appearance of the gas.

They quickly ruled out a scenario in which the gas originated from the debris of a newly formed star, as well as a scenario in which the gas was produced by a belt close to icy asteroids. They also considered a scenario in which gas was emitted from many icy comets streaming from a distant asteroid belt, similar to our own Kuiper belt. But the data didn’t quite fit that scenario either. The final scenario the team considered was that the gas was left over from a giant impact.

“Of all the scenarios, only he can explain all the features of the data,” Schneiderman says. “In systems of this age, we expect that there will be giant effects, and we expect that giant impacts are really very common. Time scales work, age works, morphological and compositional constraints work. The only plausible process that carbon monoxide can be produced in this system is in this context. A gigantic effect.”

The team estimates that the gas was released from a giant collision that occurred at least 200,000 years ago — recent enough that the star hasn’t had time to completely destroy the gas. Depending on the abundance of the gas, the impact is likely to be enormous, involving two protoplanets, likely the size of Earth. The effect was so great that it likely caused part of a planet’s atmosphere to explode, in the form of gas the team observed today.

“There is now potential for future work outside of this system,” Schneiderman says. “We show that if you find carbon monoxide in a location and morphology compatible with a giant impact, it provides a new way to search for giant impacts and understand how debris behaves in the aftermath.”