Cosmic roadkill: when planets collide, what happens to their atmospheres?

Around 200,000 years ago there was an epic crash on the cosmic highway. In the solar system orbiting the lyrically named star HD 172555, an Earth-size exoplanet collided with another planet with such force it sheared the larger planet of its atmosphere.

This space bingle has been recorded in a study, out today in the journal Nature, by an international team of astronomers from the Massachusetts Institute of Technology (MIT), US, the National University of Ireland at Galway and Cambridge University, UK, among others.

The researchers say it’s the first time this kind of atmosphere-robbing collision has been documented ‘in the wild’. So how can they tell?

Astronomers have always been interested in the HD 172555 system, because its star is surrounded by a cloud of gas and dust of unusual composition. In recent years, scientists have found that the star’s dust cloud contains uncommon minerals in grains that are finer than astronomers would expect for a typical stellar disk.

“Because of these two factors, HD 172555 has been thought to be this weird system,” says lead author Tajana Schneiderman, a graduate student in MIT’s department of Earth, Atmospheric and Planetary Sciences.

To solve the puzzle, Schneiderman and her colleagues obtained data from the Atacama Large Millimeter Array (ALMA) in Chile, which comprises 66 radio telescopes.

The team looked through data from the ALMA public archive, seeking signs of carbon monoxide around nearby stars.

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

Analysing the ALMA data, the team was able to identify an abundance of carbon monoxide around the star – around 20% of the carbon monoxide found in Venus’ atmosphere. It also observed that the gas was orbiting the star in large quantities and surprisingly close – about 10 astronomical units, or 10 times the distance between Earth and the sun.

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

This is because carbon monoxide is vulnerable to photodissociation, a process in which a star’s photons break down and destroy the molecule. The team tested various hypotheses about why so much of the gas might be orbiting close to the star despite this process, and found that the only scenario that fit the data was a massive planetary collision in recent (cosmically speaking) history.

“Of all the scenarios, it’s the only one that can explain all the features of the data,” says Schneiderman. “In systems of this age, we expect there to be giant impacts, and we expect giant impacts to be really quite common. The timescales work out, the age works out, and the morphological and compositional constraints work out. The only plausible process that could produce carbon monoxide in this system in this context is a giant impact.”

The team believes this bust-up occurred around 200,000 years ago – recent enough that the star hasn’t completely blitzed through the carbon monoxide produced by the collision. Based on the abundance of the gas, the team believes the impact must have been epic, involving two planets potentially comparable in size to Earth.

“This is the first time we’ve detected this phenomenon of a stripped protoplanetary atmosphere in a giant impact,” says Schneiderman. “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. Now we have additional insight into these dynamics.

“Now there’s a possibility for future work beyond this system. 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.”

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