This is How You Get Moons. An Earth-Sized World Just got Pummeled by Something Huge.

Titanic collisions are the norm in young solar systems. Earth’s Moon was the result of one of those collisions when the protoplanet Theia collided with Earth some 4.5 billion years ago. The collision, or series of collisions, created a swirling mass of ejecta that eventually coalesced into the Moon. It’s called the Giant Impact Hypothesis.

Astronomers think that collisions of this sort are a common part of planet formation in young solar systems, where things haven’t settled down into predictability. But seeing any of these collisions around other stars has proved difficult.

A team of astronomers has found one of these young systems still in its chaotic youth. They’ve found evidence of a collision between a roughly Earth-sized planet and a smaller impactor that stripped away atmosphere from the larger planet. The collision occurred about 200,000 years ago. While previous research has shown that a collision likely took place, the discovery of atmospheric stripping is new.

The team has published a paper in the journal Nature that outlines their observations. The paper is titled “Carbon monoxide gas produced by a giant impact in the inner region of a young system.” The lead author is Tajana Schneiderman, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

The discovery is centred around a star that caught the attention of astronomers back in the 1980s. It’s called HD 172555 and it’s about 95 light-years away and about 23 million years old. The star was notable for its brightness in the mid-infrared. At its young age, astronomers think that this solar system is in the early stages of forming terrestrial planets.

When a solar system is forming terrestrial planets astronomers expect to see things like pyroxene and olivine in the star’s protoplanetary disk. But instead, HD 172555 is surrounded by unexpected materials like amorphous silica and SiO gas. But SiO gas is basically vaporized rock, and its presence meant something extremely energetic took place to vaporize it. Not only that but the star is also surrounded by a lot of dust. And in this case, the dust grains are unusually fine.

Only a high-speed impact could’ve vaporized the rock into SiO. It takes a lot of energy to vaporize rock, and only speeds in the range of 10 km per second, or over 22,000 mph could have done it.

This spectrum, or plot of infrared data, from NASA’s Spitzer Space telescope reveals the presence of vaporized and melted rock, along with rubble, around the young, hot star HD 172555. Image Credit: NASA/JPL-Caltech/C. Lisse (Johns Hopkins University.)

But there’s more. Not only is there fine-grained dust and vaporized rock orbiting HD 172555, but the star also has a ring of carbon monoxide that co-orbits with the SiO and the dusty debris. The group of researchers thinks that the collision is responsible for that, too. They think that the CO is a portion of the larger planet’s atmosphere that was torn away by the collision. This dual-detection of debris and CO got the team excited.

“Because of these two factors, HD 172555 has been thought to be this weird system,” Schneiderman said in a press release.

“This is the first time we’ve detected this phenomenon, of a stripped protoplanetary atmosphere in a giant impact,” says lead author Tajana 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.”

The stripped-away carbon monoxide orbiting the star played a critical role in this research. Astronomers look for CO because of its brightness. “When people want to study gas in debris disks, carbon monoxide is typically the brightest, and thus the easiest to find,” said Schneiderman. “So, we looked at the carbon monoxide data for HD 172555 again because it was an interesting system.”

The team pored over data from ALMA (Atacama Large Millimeter/sub-millimeter Array), a powerful array of radio dishes that work as an interferometer. They looked for evidence of CO in the data and found it. They were able to measure its abundance and the team says that they found CO equal to about 10 times the mass of Earth’s entire atmosphere.

But beyond the significance of finding that much CO, its location was even more intriguing. The gas was only 10 AU from the star, which is surprisingly close. Typically, gas and dust in a protoplanetary disk would extend out to tens or hundreds of AU, according to the paper.

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

And it’s not just the proximity of CO that requires an explanation. It’s the fact that it’s still there. Young stars are born with primordial disks of gas and dust, but very few last as long as the age of HD 172555. At 23 million years old, that gas would have to have been shielded somehow to survive this long. “Young A-type stars are born surrounded by protoplanetary disks of primordial gas and dust, but only 2–3% survive beyond the first 3 Myr of a star’s lifetime,” the authors write. “Even if the CO observed around HD 172555 were primordial, with its lifetime extended through shielding, the system would remain a remarkable outlier not only in age (at 23 Myr old) but also in dust mass…”

Altogether, the type of materials around the star, the fine-grained dust, and the CO add up to a very unusual system. Could it have formed this way without any impact to explain it? Possibly, but unlikely, according to the authors.

Explaining it all without a high-speed impact would be difficult. It’s possible that there are unseen planetary companions around the star which has shaped the disk and kept the CO close. It’s possible that shocks in the solar nebula vaporized the rock into SiO gas, similar to the shocks that formed chondrules in our own Solar System. And it’s possible that an ongoing cascade of collisions between asteroids could’ve created the sheer mass of dust detected around HD 172555.

But that’s not likely, according to the authors. Neither are any of the other possible explanations, like an inward scattering of comets from something like the Kuiper Belt here in our own Solar System.

This is an artist’s illustration of HD 17255. In 2017 astronomers using the Hubble Space Telescope discovered carbon monoxide and silicon gas around HD 172555 and attributed it to in-falling comets from the distant reaches of the star’s solar system. But this new research shows that only a titanic collisions between planets can account for it. Image Credit: NASA, ESA, A. Feild and G. Bacon (STScI)

There’s only one conclusion that accounts for all the observations according to the authors.

“The detection and morphology of CO gas, combined with previous evidence from dust imaging and spectroscopy, supports a picture where a giant impact took place at least 0.2?Myr?ago in the outer terrestrial planet-forming region of the 23-Myr-old HD?172555 system,” they write. These types of planetary impacts are expected to be common in systems of this age.

“Of all the scenarios, it’s the only one that can explain all the features of the data,” Schneiderman says. “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.”

Finding CO around HD 172555 could be a real boon to the study of young solar systems.

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


Evan Gough

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