The mystery of missing carbon monoxide in protoplanetary disks has been solved by astronomers


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
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.


Diana Powell, a NASA Hubble Fellow at the Center for Astrophysics, Harvard & Smithsonian said, 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. Now, a new model validated by observations with ALMA has solved the mystery: carbon monoxide has been hiding in ice formations within the disks. The findings are described today in the journal Nature Astronomy.


Powell, who led the study said, this may be one of the biggest unsolved problems in planet-forming disks. 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.


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


Intrigued by the mystery, Powell put on her detective hat and leaned on her expertise in the physics behind phase changes when matter morphs from one state to another, like a gas changing into a solid. On a hunch, Powell made alterations to an astrophysical model that's currently used to study clouds on exoplanets or planets beyond our solar system.

She said, what's really special about this model is that it has detailed physics for how ice forms on particles. So how does ice nucleates onto small particles and then how does it condense? The model carefully tracks where ice is, on what particle it's located, 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.


Powell said, the results and models worked really well.


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.


Powell said, 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.


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.


Powell said, this changes how we thought ice and gas were distributed in disks. 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.

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