Astronomers Detect Carbon Dioxide on Distant Exoplanets Using Ground-Based Telescope

More than 5,000 planets have now been confirmed beyond our Solar System. Most of them are too distant and too faint to see directly. Instead, astronomers study the thin layers of gas that surround them.

These atmospheres act like chemical fingerprints. When a planet passes in front of its host star, some of the starlight filters through the planet’s atmosphere. Molecules in the gas absorb specific wavelengths of light, leaving narrow patterns in the spectrum. Each molecule leaves a distinct signature.

Carbon dioxide is one of the most important of these molecules. It plays a central role in planetary climate, chemistry, and formation history. Detecting it on distant planets can reveal how those worlds formed and what materials were present when they were born.

Until recently, carbon dioxide was most clearly observed in exoplanet atmospheres by space telescopes. Infrared instruments above Earth’s atmosphere have the advantage of avoiding interference from our own planet’s carbon dioxide.

Now, researchers have shown that it can also be detected from the ground.

Detecting a Difficult Signal

The study focused on a group of known giant exoplanets. These planets are large, hot, and orbit relatively close to their stars. Because of their size and temperature, their atmospheres produce stronger signals than smaller, cooler planets.

The observations were carried out using a high-resolution spectrograph mounted on a large ground-based telescope in Chile. High-resolution spectroscopy spreads light into extremely fine wavelength intervals. This allows astronomers to separate overlapping signals that would otherwise blend together.

Earth’s own atmosphere contains carbon dioxide, which complicates the search. Any signal from a distant planet must be distinguished from the much stronger absorption produced by our own air.

The researchers addressed this challenge by carefully modeling Earth’s atmospheric contribution and by using the Doppler shift of the exoplanets.

As a planet orbits its star, its velocity changes relative to Earth. This motion shifts the wavelengths of its spectral features slightly back and forth. Earth’s atmospheric features do not shift in the same way. By tracking these moving patterns, astronomers can isolate the planetary signal.

Clear Carbon Dioxide Signatures

Using this approach, the team identified statistically significant carbon dioxide absorption features in multiple exoplanet atmospheres.

The detections were made at infrared wavelengths where carbon dioxide has strong absorption bands. The signals matched theoretical predictions of how carbon dioxide should appear under the temperature and pressure conditions expected in hot giant planets.

The analysis showed that the detected features were consistent across independent observations, strengthening confidence that the signals originated from the planets rather than from instrumental noise or Earth’s atmosphere.

The ability to recover these features from ground-based data marks a technical milestone. It demonstrates that carbon dioxide, once thought to require space-based infrared observatories for clear detection, can be measured using Earth-based facilities under the right conditions.

Why Carbon Dioxide Matters

Carbon dioxide is more than just a familiar greenhouse gas. In planetary science, it is a tracer of elemental abundances.

The ratio of carbon to oxygen in a planet’s atmosphere provides clues about where and how the planet formed. In the early stages of a planetary system, different materials condense at different distances from the star. Ices rich in carbon or oxygen accumulate in specific regions of the protoplanetary disk.

If a giant planet forms beyond certain “snow lines,” it may incorporate large amounts of icy material rich in carbon-bearing molecules. If it forms closer to the star, it may reflect a different chemical balance.

By measuring molecules such as carbon dioxide, water vapor, carbon monoxide, and methane, astronomers can estimate atmospheric carbon-to-oxygen ratios. These values help reconstruct the planet’s formation history.

In this study, the detected carbon dioxide abundances were broadly consistent with expectations for hot giant planets. While the work did not attempt to provide precise abundance measurements for each world, it demonstrated that the molecule’s presence can be robustly identified.

That capability opens the door to more detailed chemical inventories in future observations.

The Power of High-Resolution Spectroscopy

Traditional transmission spectroscopy often uses lower spectral resolution. In those cases, molecular features appear as broader absorption bands. These observations have been highly successful, especially with space telescopes.

High-resolution spectroscopy takes a different approach. Instead of measuring broad features, it resolves thousands of individual molecular lines. Each molecule produces a forest of narrow absorption lines at precise wavelengths.

By cross-correlating observed spectra with theoretical templates, astronomers can combine the information from many lines at once. Even if each line is weak, their collective signal becomes detectable.

This method is particularly powerful for molecules like carbon dioxide that have complex spectral patterns.

It also allows researchers to measure atmospheric dynamics. Because high-resolution data preserve Doppler information, wind speeds and rotational velocities can sometimes be inferred from subtle shifts and broadenings in the lines.

In the present study, the emphasis was on detecting carbon dioxide itself. But the same technique can be extended to probe atmospheric circulation and temperature structure in future work.

Working Around Earth’s Atmosphere

One of the main challenges of ground-based infrared astronomy is contamination from Earth’s atmosphere.

Water vapor, carbon dioxide, methane, and other molecules in the air absorb light at many of the same wavelengths that astronomers wish to study in exoplanets.

To overcome this, the researchers relied on careful calibration. Observations were timed and processed to account for variations in Earth’s atmospheric conditions. Sophisticated software models were used to subtract the telluric absorption, which refers to absorption by Earth’s atmosphere.

Even after subtraction, residual noise can remain. The team addressed this by using the orbital motion of the planets to separate planetary signals from static or slowly varying terrestrial features.

Because the exoplanets move rapidly in their orbits, their spectral lines shift noticeably during an observing sequence. This motion provides a distinguishing signature.

The study demonstrates that with sufficient spectral resolution and careful data analysis, ground-based telescopes can overcome much of the interference from Earth’s atmosphere.

Complementing Space Telescopes

Space telescopes such as the James Webb Space Telescope have recently delivered detailed atmospheric measurements of exoplanets, including carbon dioxide detections.

However, space observatories are limited in number and observing time. They cannot monitor every known exoplanet extensively.

Ground-based facilities offer flexibility and long-term access. Large telescopes equipped with advanced spectrographs can revisit targets repeatedly, improving signal quality over time.

The new results show that ground-based observations can complement space-based data. While space telescopes provide broad wavelength coverage and high sensitivity, ground-based high-resolution spectroscopy can offer detailed line-by-line information.

Together, these approaches create a more complete picture of exoplanet atmospheres.

Expanding the Toolkit for Planetary Science

The study’s findings have implications beyond the specific planets observed.

As new extremely large telescopes come online in the coming years, their increased collecting area will allow even fainter signals to be detected. Instruments on these facilities will be capable of even higher spectral resolution and improved stability.

This means that molecules once thought to be out of reach from the ground may become routinely observable.

For planetary scientists, this expands the available toolkit. Carbon dioxide joins water vapor, carbon monoxide, and other molecules that can now be probed with ground-based spectroscopy.

Each additional molecule adds another constraint on atmospheric composition and planetary formation pathways.

Remaining Questions

While the detections are robust, several questions remain.

Precise abundance measurements require detailed modeling of temperature profiles, cloud properties, and atmospheric mixing. Carbon dioxide absorption strength depends not only on how much of the molecule is present but also on the surrounding atmospheric conditions.

Clouds and hazes can mute or alter spectral signatures. Vertical temperature gradients can change how absorption lines form.

Future studies will aim to combine high-resolution ground-based data with lower-resolution but broader wavelength coverage from space telescopes. This combination can help break degeneracies, which are situations where different atmospheric models produce similar spectra.

By integrating multiple observing methods, astronomers can refine their estimates of atmospheric composition.

A Step Toward Deeper Atmospheric Insights

The detection of carbon dioxide from the ground marks a technical and methodological advance.

It shows that Earth-based observatories can identify key atmospheric molecules on distant worlds despite the challenge of our own atmosphere.

This capability strengthens efforts to map the chemical diversity of exoplanets. It also provides an additional pathway for studying how giant planets assemble from the disks of gas and dust that surround young stars.

Each atmospheric detection adds a piece to the broader story of planetary formation.

With improved instruments and continued refinement of data analysis techniques, the chemical fingerprints of distant worlds are becoming clearer.

The research was published in The Planetary Science Journal on December 24, 2025.