This exoplanet got hit so hard it literally had the wind knocked out of it

A young solar system is a particularly violent place as protoplanets and other material bump, bang, and crash into each other, but a new study reveals just how incredibly violent such collisions can be.

While exploring a nearby star system, HF 172555 – which is in its early stage of formation – astronomers at the Massachusetts Institute of Technology, the National University of Ireland at Galway, Cambridge University, and other institutions discovered evidence of an early planetary collision so violent, it tore away part of a planet's atmosphere.

The star the astronomers were looking at when they made the discovery is about 23 million years old – really just an infant in star-years – making it an excellent candidate to study planetary formation, since it should still likely have a disc of material around it full of dust and debris that eventually gives rise to larger planets.

When studying this system, the scientists caught wind of a giant collision among the debris that occurred between an Earth-sized planet and a smaller body with an impact velocity of about 10km a second, or about 22,000 miles an hour. Among the signs of this cataclysmic event was a large amount of gas and dust within the debris.

“This is the first time we’ve detected this phenomenon, of a stripped protoplanetary atmosphere in a giant impact,” says Tajana Schneiderman, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences and lead author of the new study, which was published this week in the journal Nature. “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.”

A solar system in the process of forming with a moon-forming disc around an exoplanet — A young solar system in the process of forming, similar to HD 172555 (Image credit: ESO)

HD 172555 has been a particularly interesting system to study for astronomers, both because it is so close, but also because it has a number of interesting characteristics astronomers were not expecting, particularly its unusual composition, but also the fine grain of the dust within the star's accretion disk.

"Because of these two factors, HD 172555 has been thought to be this weird system,” said Schneiderman.

Schneiderman and her colleagues decided to examine the composition of the gases within the star's accretion disk to see what it could tell them about the star system's evolution.

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

What the research team found was unexpectedly high levels of CO circling the star in large amounts, a volume about equal to 20% of Venus' atmosphere. It was also closer to the star than is typical, at around 10 AU from HD 172555, which is 10 times the distance between the Earth and the sun.

At this distance, the CO molecules would be steadily torn apart by the star's photons, so the CO gas couldn't have been there for very long. Given the age of the HD 172555, a gigantic protoplanetary impact is the most likely cause, the researchers believe.

“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 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.”

Based on the available evidence, the researchers believe that the impact occurred about 200,000 years ago, which wouldn't be enough time for the star to break down the gas into its component elements. Given the amount of gas we're talking about, one of the planets had to be about the size of Earth, and possibly even both.

“Now there’s a possibility for future work beyond this system,” said Schneiderman. “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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Analysis: The universe as spectacle

One of the greatest things about studying and reading about space is that the scale of the forces we are examining are simply beyond our understanding, so much so that we can't help but be left with a sense of awe about it all.

The forces at work here on Earth have long been the subject of some of humanity's greatest art, from literature like Melville's Moby Dick to paintings like Hokusai The Great Wave off Kanagawa, but even the tsunami or most violent volcanic eruption has nothing on the force of a supernova or a collision between two planets at 22,000 miles an hour.

You probably wouldn't want to be on one of those planets when it happens (or maybe you would; it'd be one heck of a way to leave this mortal coil with a bang, after all). Still, there's no question that getting a bird's eye view of the stellar demolition derby of an early solar system would be a show worth seeing if we're ever so fortunate to catch a bang-up in action some day – from a safe distance of a few dozen light-years, of course.