Missing carbon monoxide in planetary disks was hiding in the ice

In planetary disks, carbon monoxide is lurking in large chunks of ice, solving the decade-old question, ‘Where is the CO?’

Artist’s illustration of a planetary disk, a region of dust and gas where planets form. The zoom-in insert displays carbon monoxide molecules in the ice phase. (Credit: M.Weiss/Center for Astrophysics | Harvard & Smithsonian)

Astronomers frequently observe carbon monoxide in planetary nurseries. The compound is ultra-bright and extremely common in protoplanetary disks—regions of dust and gas where planets form around young stars—making it a prime target for scientists.

But for the last decade or so, something hasn’t been adding up when it comes to carbon monoxide observations. A huge chunk of carbon monoxide is missing in all observations of disks, if astronomers’ current predictions of its abundance are correct.

A solution to the mystery has now emerged from an interdisciplinary collaboration at UC Santa Cruz led by Diana Powell, who earned her Ph.D. in astrophysics at UCSC in 2021 and is now a NASA Hubble Fellow at the Center for Astrophysics | Harvard & Smithsonian.

Working with Ruth Murray-Clay, the Gunderson Professor of Theoretical Astrophysics at UCSC, and Xi Zhang, associate professor of Earth and planetary sciences, Powell developed a new model indicating that carbon monoxide has been hiding in ice formations within the planet-forming disks. The model’s predictions were validated by observations from the ALMA radio observatory, and the team has reported its findings in a paper published August 22 in Nature Astronomy.

“This may be one of the biggest unsolved problems in planet-forming disks,” said Powell. “Depending on the system observed, carbon monoxide is three to 100 times less than it should be; it’s off by a really huge amount.”

And carbon monoxide inaccuracies could have huge implications for the field of astrochemistry.

“Carbon monoxide is essentially used to trace everything we know about disks—like mass, composition, and temperature,” Powell explained. “This could mean many of our results for disks have been biased and uncertain because we don’t understand the compound well enough.”

As a graduate student at UCSC, Powell was studying planet formation in protoplanetary disks with Murray-Clay, and in a separate project with Zhang she was studying cloud physics in planetary atmospheres. Her work in these two areas inspired her to apply a modeling approach used in cloud physics to understand the formation of ice particles in planet-forming disks.

“Ices are very important building blocks of the planets,” Zhang explained.

Powell made alterations to an astrophysical model used to study clouds on exoplanets, or planets beyond our solar system.

“What’s really special about this model is that it has detailed physics for how ice forms on particles,” she said. “So how ice nucleates onto small particles and then how it condenses. The model carefully tracks where ice is, what particle it’s located on, how big the particles are, how small they are, and then how they move around.”

Powell applied the adapted model to planetary disks, hoping to generate an in-depth understanding of how carbon monoxide evolves over time in planetary nurseries. To test the model’s validity, Powell then compared its output to real ALMA observations of carbon monoxide in four well-studied disks—TW Hya, HD 163296, DM Tau and IM Lup.

The new model lined up with each of the observations, showing that the four disks weren’t actually missing carbon monoxide at all — it had just morphed into ice, which is currently undetectable with a telescope.

Murray-Clay said the new results are an inspiring example of the interdisciplinary approach fostered by UCSC’s Astrobiology Initiative. “This was the culmination of Diana bringing insights from cloud physics together with our work on protoplanetary disks, and it came together in this beautiful and unexpected outcome,” she said.

“To me it was a surprise that the very small-scale microphysics of these ice particles has such a large-scale impact that its effects can be detected from light-years away in observations of protoplanetary disks,” Zhang added.

Radio observatories like ALMA allow astronomers to view carbon monoxide in space in its gas phase, but ice is much harder to detect with current technology, especially large formations of ice, Powell said.

The model shows that unlike previous thinking, carbon monoxide is forming on large particles of ice—especially after one million years. Prior to a million years, gaseous carbon monoxide is abundant and detectable in disks.

“This changes how we thought ice and gas was distributed in disks,” Powell says. “It also shows that detailed modeling like this is important to understand the fundamentals of these environments.”

Powell hopes her model can be further validated using observations with NASA’s Webb Telescope—which may be powerful enough to finally detect ice in disks, but that remains to be seen.

Powell, who loves phase changes and the complicated processes behind them, says she is in awe of their influence. “Small-scale ice formation physics influences disk formation and evolution—now that’s really cool.”