Can Sulfur Be a Signature of Life on Exoplanets?

Script: Humanity has long been driven by a single question, “are we alone in this universe?” I’m taking the first steps towards proving that sulfur might be the key to finding life beyond Earth. On K2-18b, an exoplanet with a hydrogen rich atmosphere, hints of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) were detected. On Earth, these molecules only come from life, but origins elsewhere are unclear. Major potential origins include atmospheric chemical reactions, biological, volcanic, and geological processes. Looking into chemical reactions, I emulated the atmospheric spectra for the infrared, Raman scattering, and UV-visible light and tested stability. The creation of DMS and DMDS must have hydrogen sulfide in the atmosphere. Through comparisons with my simulated spectra, I found that there was an insufficient amount of hydrogen sulfide to photochemically produce these molecules. This eliminates one of the major possible origins, making this exoplanet a likely host of life in the form of sulfate-reducing bacteria.

Motivation

Inspired by the discovery of elemental sulfur on Mars by NASA’s Curiosity Rover (Figure 2), I investigate reduced sulfur molecules as an indicator for life. Sulfur is an essential component of all life forms on Earth, like in humans, animals, microorganisms, and plants. Observational data from Kepler 2-18b, a hydrogen-rich exoplanet 120 light-years away from and 2.6x larger than Earth (Figure 1), detected hints of dimethyl sulfide (DMS/CH3SCH3) and dimethyl disulfide (DMDS/CH3SSCH3) (Figure 4). On Earth, these molecules only originate from life (Smith, 2023). It's unclear whether these molecules come from biological origins on the exoplanet. My project investigates one of the major origins: atmospheric chemical reactions, which requires the presence of H2S (Figure 3).

Why Reduced Sulfur?

Reduced sulfur was chosen because of its excess electrons, which are prominent in marine phytoplankton or sulfur-reducing bacteria. This life releases sulfur gas into the atmosphere. The electrons stored in reduced sulfur serve as reducing agents for enzymatic processes, biological tools which accelerate chemical reactions. Furthermore, in low oxygen environments (the most common state in space), the oxidized form of sulfur bears little significance.

Objectives

Determine if the origins of DMS and DMDS found on exoplanet K2-18b is derived through atmospheric chemical reactions.

Provide researchers with the tools to select exoplanets to further investigate for habitability, reducing resources used like money and time.

Applications

Determining the source of the detection of molecules provides valuable insight into the possibility of life on the exoplanet, specifically in sulfate-reducing bacteria.

Optimization

Optimizing a molecule brings its quantum chemical energy to its lowest and most stable form, which makes it most likely in atmospheric chemical reactions. Molecular optimizations were carried out on UBC’s Advanced Research Computing resource with Gaussian16 software using Minnesota 15 (MN15) density functional theory (DFT), a quantum chemical approach to calculating the electronic structure of molecules, as seen in Figure 5. I used the aug-cc-pVQZ basis set for its large database, allowing greater accuracy for molecules.

I calculated the monomer, dimer, and trimer structures of each molecule, to further understand how the molecules would act when placed in a low temperature environment. All calculations were done at 273 Kelvin and 1 atm.

MN15 was used for its high accuracy for small molecules, especially ones containing sulfur. The initial structure of the molecules were manually built based on known information for molecules.

Infrared, Raman Scattering, Ultraviolet-visible spectra

Infrared (IR) and Raman Scattering spectra were calculated using Gaussian16 software and the key command, "freq(raman)", which measures frequencies based on the optimized structure given (Figure 6).

Ultraviolet-visible (UV-vis) spectra were created using time-dependent calculations in the singlet phase. I looked at a total of 25 states (Figure 7).

Testing spectra

Figures 8 and 9 shown in the Results Section were generated in ChemCraft using Lorentzian broadening (5000 cm-1) with wavenumbers (cm-1) to intensity.

To ensure accuracy with observational data, my spectra was tested against results from the National Institute for Standards and Technology. Small molecule spectra (CH4, CO2, etc.) were tested against NASA atmosphere composition.

Gibbs Free Energy

The reference sheet was created from the zero point energy in the optimization calculations (Table 1). To calculate the Gibbs Free Energy, I found the difference between the calculated energy and the energy from the reference sheet (kJ/mol) to determine stability.

Spectral Fingerprints

Unique spectral characteristics were graphed to simulate the infrared (red), Raman (green), ultra violet-visible (blue) spectra and identify the most intense features (Figure 8). The H2S molecule has a large peak at 1,250 cm-1.

On the right, the new spectral features show the most intense spectral features for the dimers and trimers. The biggest difference in infrared intensity between monomer and dimer is present in H2S, where the greatest peak moves from 1,250 cm-1 to 119 cm-1.

When compared with experimental atmospheric models, like the ones found from the James Webb Space Telescope (JWST), I was able to distinguish and determine the molecules present in the atmosphere.

Dissociation energies are also included (kJ/mol) because changes in the energies are positive, all the dimers and trimers are found to be not spontaneous and are stable.

Stacked Representation

Figure 9 predicts what the absorption spectrum should look like if DMS and DMDS are produced by photochemical means. The JWST was able to measure in the mid-infrared region (Figure 4), but my plot extends to the near-infrared, far-infrared, and UV-visible light.

I also pinpoint the detection windows for reduced sulfur species where particles can be located, since they are isolated and the spectral lines of other molecules do not overlap. Researchers use these windows to identify molecules present in the atmosphere.

It is important to note that H2S has a significantly intense peak in the mid-infrared range (4,000 cm-1 to 400 cm-1).

Gibbs Free Energy Stability

Stability was graphed relative to the reference table (Figure 10). The more positive the dissociation energy is, the more likely it is to break apart under the harsh conditions of space. Key molecules to consider: H2S and CH4 are both less stable relative to DMS and DMDS, so it is most likely that H2S will dissociate to become more stable in the form of DMS and DMDS.

Confirming Data from Photochemical Equations

Photochemical (atmospheric reactions) origins of DMS and DMDS must include CH4 and H2S. This is confirmed in Figure 11, so identifying both molecules is critical to the atmospheric formation of DMS and DMDS.

Discoveries (Lack of Hydrogen Sulfide)

When compared with Figure 4, the combined (black) spectra represents the identified region in the stacked spectra. The H2S spike in Figure 9 at 1,300 wavenumbers cm-1, is not present in the observational data (Figure 4), showing a surprising “dip,” indicating an insufficient amount of H2S to create DMS and DMDS photochemically. This provides conclusive evidence that the DMS and DMDS are not from atmospheric chemical reactions, ruling out one of the major origins.

Eliminates the Possibility of Photochemical Origins

Because H2S is not present in the observational data, it is verifiable that the DMS and DMDS detected on K2-18b cannot be from atmospheric chemical reactions. This leaves the origins to life, geological, or biological origins.

This confirms that my spectra can eliminate one of the major origins of molecules present on many exoplanet atmospheres. By using my data to filter through planets, researchers will be able to more accurately choose planets to investigate further for habitability, reducing the amount of expensive equipment used and time.

Potential biological origins: Sulfate Reducing Bacteria

Usually when I mention my project focuses on searching for life in space, many people ask me whether I am trying to find little green aliens...like in Figure 12. Unfortunately, the life I am looking for is probably not intelligent, but in the form of bacteria.

If it is determined that the DMS and DMDS could be naturally occurring from sulfate reducing bacteria which expels H2S, the necessary molecule to create DMS and DMDS, shown in the cover image and Figure 12. Sulfate reducing bacteria is a living organism that is able to live in anoxic (low-oxygen), low temperature, low pressure environments, making it a highly plausible origin of sulfur.

Extend Databases

I plan to input my calculated spectra onto databases such as the Life Detection Forum and NIST. Currently, there is no readily-available biosignature spectra to compare to exoplanet atmospheres. I will change this through my current work, as well as expanding the number of molecules I analyze.

More Observations

Additionally, I hope that more observations for longer time periods could be viewed with the James Webb Space Telescope, the Hubble Space Telescope, and other telescopes to strengthen experimental data detections and, in turn, strengthen my methods.

Thank you to my family and friends, especially my dad, for supporting and encouraging me throughout this project.

Thank you greatly to my research supervisor, Dr. Robert Szilagyi, for welcoming me into your research group and granting me access to the lab’s resources.

Thank you to UBC Aspire and CFI-JELF for providing financial support to the Szilagyi Research group.

Thank you to UBC Advanced Research Computing (Sockeye) and the Digital Research Alliance of Canada (Nibi/Fir) for computational resources.

Thank you to my school’s science fair coordinator, Shaundra Curtis, and my regional fair coordinator, Bob Roddie!

References

Images

Amen T., Eljamal O., Khalil A., & Matsunaga N. (2018) [Evaluation of sulfate-containing slide stabilization and the alleviation of methanogenesis inhibition at mesophilic temperature] [Diagram demonstrating sulfate-reducing bacteria] Journal of Water Process Engineering.

https://www.sciencedirect.com/science/article/abs/pii/S2214714418303283

Boersma, A. (2017). [Researchers find exciting potential for little-known exoplanet - and discover another planet in the process (D. Campbell, Ed.)] [Artistic Interpretation]. University of Toronto.

https://utsc.utoronto.ca/news-events/breaking-research/researchers-find-exciting-potential-little-known-exoplanet-and-discover-another

ESA/Hubble, Kornmesser M. (2019) [Artist’s Impression of Exoplanet K2-18 b] [Artistic Interpretation]. NASA.

https://science.nasa.gov/asset/hubble/artists-impression-of-exoplanet-k2-18b/

Kushkevych I. & Moroz O. (2012) [Growth of various strains of sulfate-reducing bacteria of human large intestine] [Image of sulfate-reducing bacteria]. Studia Biologica.

https://www.researchgate.net/publication/234038217_Growth_of_various_strains_of_sulfate-reducing_bacteria_of_human_large_intestine

Life Detection Forum [Select Potential Biosignatures] [Potential Biosignature Query] Life Detection Forum.

https://lifedetectionforum.com/

Madhusudhan N., Constantinou S., Holmberg M., Sarkar S., Piette, A., & Moses J. (2025) [New Constraints on DMS DMDS in the Atmosphere of K2-18 b from JWST MIRI] [Mid-infrared spectra of exoplanet K2-18 b captured by the James Webb Space Telescope]. The Astrophysical Journal Letters.

https://iopscience.iop.org/article/10.3847/2041-8213/adc1c8

NASA/JPL-Caltech/Lizbeth B. De La Torre (2023) [Spectroscopy Infographic] [Drawing detailing how to read exoplanet atmospheres]. NASA.

https://science.nasa.gov/resource/spectroscopy-infographic/

NASA/JPL-Caltech/MSSS (2024) [Curiosity Views Sulfur Crystals Within a Crushed Rock] [Photo captured by Curiosity Rover]. NASA.

https://science.nasa.gov/resource/curiosity-views-sulfur-crystals-within-a-crushed-rock/

Reed N., Shearer R., McGlynn S., Wing B., Tolbert M., & Browne E. (2024) [Abiotic Production of Dimethyl Sulfide, Carbonyl Sulfide, and Other Organosulfur Gases via Photochemistry: Implications for Biosignatures and Metabolic Potential] [Figure 2, example chromatogram]. The Astrophysical Journal Letters.

https://iopscience.iop.org/article/10.3847/2041-8213/ad74da

Smith A. (2020) [This rainy exoplanet can be rich for life (D. Clery, Ed.)] [Artistic Interpretation]. Science.

https://www.science.org/content/article/rainy-exoplanet-could-be-ripe-life

Smith, A. & Madhusudhan, N. (2023). [Methane and carbon dioxide found in atmosphere of habitable-zone exoplanet (S. Collins, Ed.) [Artistic Interpretation]. University of Cambridge.

https://www.cam.ac.uk/stories/carbon-found-in-habitable-zone-exoplanet

Swayne, M. (2025) [Quantum Chemistry Gets Error-Corrected Boost from Quantinuum’s Trapped-Ion Computer] [Drawing]. Quantum Insider.

https://thequantuminsider.com/2025/05/22/quantum-chemistry-gets-error-corrected-boost-from-quantinuums-trapped-ion-computer/

USA Today (2025) [Have astronomers found extraterrestrial life on a distant planet? Here’s the discovery (Petras G. & Loehrke J.)] [Artistic Interpretation]. USA Today.

https://www.usatoday.com/story/graphics/2025/04/18/extraterrestrial-alien-life-found-planet-k2-18b/83135300007/

For greater personal understanding

NASA/JPL-Caltech/MSSS (2024) Curiosity Views Sulfur Crystals Within a Crushed Rock. NASA. Retrieved from: https://science.nasa.gov/resource/curiosity-views-sulfur-crystals-within-a-crushed-rock/

NASA, ESA, CSA, STScl, Olmsted J. (STScl) (2024). Reading an Earth-like Exoplanet’s Transmission Spectrum. NASA. Retrieved from:

https://science.nasa.gov/asset/webb/reading-an-earth-like-exoplanets-transmission-spectrum/

Domagal-Goldman D., Meadows V., Claire M., & Kasting J. (2021) Using Biogenic Sulfur Gases as Remotely Detectable Biosignatures on Anoxic Planets. National Center for Biotechnology Information. Retrieved from:

https://pmc.ncbi.nlm.nih.gov/articles/PMC3133782/

Pica-Ciamarra L., Madhusudhan N., Cooke G., Constantinou S., & Binet M. (2025) A Systematic Search for Trace Molecules in Exoplanet K2-18 b. Cornell University. Retrieved from:

https://arxiv.org/abs/2505.10539

Welbanks L., Nixon M., McGill P., Tilke L., Wiser L., Rotman Y., Mukherjee S., Feinstein A., Line M., Benneke B., Seager S., Beatty T., Seligman D., Parmentier V., & Sing D. (2025) Challenges in the detection of gases in exoplanet atmospheres. Cornell University. Retrieved from:

https://arxiv.org/abs/2504.21788

Collins S. (2025) Strongest hints yet of biological activity outside the solar system. University of Cambridge. Retrieved from:

https://www.cam.ac.uk/stories/strongest-hints-of-biological-activity#:~:text=On%20Earth%2C%20DMS%20and%20DMDS,life%20such%20as%20marine%20phytoplankton.

Serena (2025) The Next Big Thing is Quantum Chemistry. The Quantum Notebook. Retrieved from:

https://serenakrejcipapa.substack.com/p/the-next-big-thing-is-quantum-chemistry

Witze A. & Nature Magazine (2023). How Would We Know There’s Life on Earth? This Bold Experiment Found Out. Scientific America. Retrieved from:

https://www.scientificamerican.com/article/how-would-we-know-theres-life-on-earth-this-bold-experiment-found-out/

O’Callagan J. (2023) JWST Heralds a New Dawn for Exoplanet Science. Scientific America. Retrieved from:

https://www.scientificamerican.com/article/jwst-heralds-a-new-dawn-for-exoplanet-science/

Witze A. & Nature Magazine (2025). Why Astronomers Are Not Sold on New Alien Life Claims. Scientific America. Retrieved from:

https://www.scientificamerican.com/article/why-astronomers-doubt-claims-that-planet-k2-18-b-finding-means-alien-life/

For experimental data

Madhusudhan N., Constantinou S., Holmberg M., Sarkar S., Piette, A., & Moses J. (2025) New Constraints on DMS DMDS in the Atmosphere of K2-18 b from JWST MIRI. The Astrophysical Journal Letters. Retrieved from:

https://iopscience.iop.org/article/10.3847/2041-8213/adc1c8

Reed N., Shearer R., McGlynn S., Wing B., Tolbert M., & Browne E. (2024) Abiotic Production of Dimethyl Sulfide, Carbonyl Sulfide, and Other Organosulfur Gases via Photochemistry: Implications for Biosignatures and Metabolic Potential. The Astrophysical Journal Letters. Retrieved from:

https://iopscience.iop.org/article/10.3847/2041-8213/ad74da

For validation of spectroscopic characteristics

Canadian Astronomy Data Centre Government of Canada. Retrieved from:

https://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/search/

ExoMast. Retrieved from:

https://exo.mast.stsci.edu/

NASA, ESA, CSA, Ralf Crawford (STScI), Joseph Olmsted (STScI); Science; Nikku Madhusudhan (2023) Atmosphere Composition of Exoplanet K2-18 b (NIRISS & NIRSpec). NASA. Retrieved from:

https://science.nasa.gov/asset/webb/atmosphere-composition-of-exoplanet-k2-18-b-niriss-and-nirspec/

For future applications!

Araki M., Sanz-Novo M., Endres C., Caselli P., Rivilla V., Jiménez-Serra I., Colzi L., Zeng S., Megías A., López-Gallifa Á., Martínez-Henares A., San Andrés D., Martín S., Requena-Torres M., García de la Concepción J., & Lattanzi V. (2026) A detection of sulfur-bearing cyclic hydrocarbons in space. Nature Astronomy. Retrieved from:

https://www.nature.com/articles/s41550-025-02749-7

Muyzer G. & Stams A. (2008) The ecology and biotechnology of sulphate-reducing bacteria. Nature Reviews Microbiology.  Retrieved from:

https://www.nature.com/articles/nrmicro1892

Prisco J. (2026) Scientists discover molecule in space that hints at origin of life. CNN. Retrieved from:

https://www.cnn.com/2026/01/30/science/sulfur-molecule-space-discovery