Young star's planet nursery bubbles with carbon dioxide, challenging planet formation models

Experts have discovered a young star whose planet-forming disk is packed with carbon dioxide and barely a trace of water. That mix flips a common expectation about what the inner zones of these disks usually hold.

The star is located in NGC 6357, a busy nursery roughly 32 quadrillion miles from Earth. Its disk does not just look unusual, it behaves in ways that push scientists to reconsider how chemistry unfolds where rocky worlds may arise.

Water is barely detectable

Jenny Frediani at Stockholm University led the analysis within the eXtreme Ultraviolet Environments program. The study was focused on XUE 10, a bright F-type star surrounded by a warm, compact gas and dust disk.

“Unlike most nearby planet-forming disks, where water vapor dominates the inner regions, this disk is surprisingly rich in carbon dioxide. In fact, water is so scarce in this system that it’s barely detectable,” said Frediani. 

Other disks often show strong water vapor near the star, as recent observations demonstrate. That makes XUE 10’s chemistry stand out.

Webb reveals carbon dioxide

Webb’s MIRI instrument splits the mid-infrared light to reveal fingerprints of molecules in warm gas.

In XUE 10, those fingerprints show four distinct forms of carbon dioxide called isotopologues that contain rare versions of oxygen or carbon atoms.

The team measured gas temperatures around 300 to 370 Kelvin in the CO2-emitting region and set a strict upper limit on water vapor. Carbon monoxide shows up faintly, while a hydrogen line called Pfund alpha is clearly present.

Ultraviolet light destroys water

The young star XUE 10 lives in a harsh light bath where the far ultraviolet flux is about a thousand times stronger than what we experience near the Sun.

Ultraviolet light can break apart molecules such as water and carbon dioxide, and that destruction frees fragments that feed other chemical reactions.

The process can push the chemistry toward more CO2 if water is split faster than it can reform. In disks around hotter stars, water lines sometimes look weak for this reason, so the environment matters.

Carbon dioxide isotopes shift

The team also sees signs that oxygen isotopes in CO2 do not match the usual interstellar ratios.

This effect, called isotope selective photodissociation, happens when rare isotopic molecules absorb different ultraviolet lines less effectively than the common ones, so they get split more often.

The liberated oxygen atoms can end up inside new CO2 molecules. If transport moves that material inward fast enough, the inner disk inherits an isotope twist that could echo what we find in some meteorites.

Why the water is missing

The physics inside disks is not just about chemistry – it’s also about traffic. Dust pebbles drift inward, releasing ices as they cross the water and carbon dioxide snow lines.

At times, dust can pile up inside the water snow line, masking the water signatures beneath a bright continuum.

In other systems, Webb has already detected CO2-rich spectra and shifts in the water-to-CO2 balance, as seen previously in the GW Lup disk.

XUE 10 raises the stakes. Here, CO2 lines are not just present, they dominate, while water remains barely detectable.

This hints at either very efficient water removal, very efficient CO2 production, stronger ultraviolet processing, or a combination of these.

What it means for planet building

Planets inherit gas and solids from their birthplaces. If a disk starts inner life rich in CO2 and poor in water vapor, the first atmospheres of rocky worlds could begin with different carbon to oxygen balances than we expect from other systems.

“It reveals how extreme radiation environments can alter the building blocks of planets,” said Maria Claudia Ramirez Tannus, lead of the XUE collaboration at the Max Planck Institute for Astronomy (MPIA).

Since most stars and many planets form in such regions, that shift may be common rather than rare.

Disk chemistry creates more carbon dioxide

The carbon dioxide detected includes both the common form and some rarer versions that contain slightly different kinds of carbon and oxygen atoms.

These signals are clear enough for scientists to figure out the temperature and amount of gas, showing that the inner part of the disk is warm and filled with carbon dioxide while water is almost absent.

The process likely unfolds as follows: strong ultraviolet light from nearby stars destroys water faster than it destroys carbon dioxide.

The leftover fragments then react with carbon monoxide to create even more carbon dioxide, while the way dust moves in the disk limits how much fresh water ice can drift inward.

Many questions remain

There are still open questions. Scientists want to know how quickly water and carbon dioxide can rebuild themselves compared to the pace of mixing and movement inside a bright young disk like this one.

Experts must also determine how much dust lies above the gas layers and blocks or changes the signals.

To answer these questions, researchers will need to compare data across different wavelengths and use models that connect chemistry with the way material flows.

Future observations that measure the full size and mass of the outer disk will also show how much material is still available to feed the inner regions.

How Webb detected the molecules

Webb’s MIRI instrument looks at light between 5 and 28 microns, a range where many molecules leave sharp, recognizable features. These features rise above the glow of warm dust, making them easier to detect.

That sensitivity is why astronomers were able to see four different versions of carbon dioxide in this disk. It also confirms that the weak signal for water is likely real, not just a limit of the telescope’s sensitivity.

Significance beyond a single star

Comparisons are important. In some young disks around T Tauri stars, water dominates the inner regions. In others, carbon dioxide is stronger, and in a few cases both show up together.

The case of XUE 10 shows that in a crowded and high-radiation environment, the balance can shift heavily toward carbon dioxide.

Taking that diversity seriously helps us predict exoplanet atmospheres with fewer wrong assumptions. It also connects disk chemistry with the odd isotope signatures scientists have puzzled over in meteorites for decades.

The study is published in the journal Astronomy & Astrophysics.

Image Credit: NASA/CXC/PSU/L. Townsley et al; UKIRT; JPL-Caltech

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