A recent study utilizing the James Webb Space Telescope's MIRI instrument has detected mid-infrared spectral features in the atmosphere of exoplanet K2-18 b that are consistent with the presence of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS), both considered potential biosignature gases. The observed abundances of these molecules are significantly higher than what would be expected from abiotic sources, suggesting a possible biological origin. However, further observations are necessary to confirm these findings and to rule out non-biological explanations.​

Src: https://arxiv.org/abs/2504.12267

How Scientists Detect Potential Biosignatures on Distant Planets?

The search for life beyond Earth has been one of the most profound scientific pursuits of the modern era. While telescopes have peered deep into the universe for signs of intelligent civilizations, a more fundamental and promising line of investigation focuses on chemical precursors of life—molecules that are essential to or strongly associated with biological processes. Among these, Dimethyl Sulfide (DMS) and Dimethyl Disulfide (DMDS) have emerged as notable candidates.

These and other complex organic molecules could serve as potential biosignatures, indicating the presence or past presence of life. But what exactly are DMS and DMDS, how do they relate to life, and how can we detect such molecules from light-years away?

DMS and DMDS: Biological Molecules with Astrobiological Significance

Dimethyl Sulfide (DMS):

DMS is a volatile sulfur compound most commonly produced on Earth by marine phytoplankton, tiny oceanic organisms. It plays a critical role in the Earth’s sulfur cycle and contributes to cloud formation, influencing the planet’s climate. Because it is predominantly biological in origin, DMS is considered a biosignature gas—a molecule that may only exist in significant quantities if produced by life.

Dimethyl Disulfide (DMDS):

DMDS is chemically related to DMS and is also associated with biological activity. It is found in decaying plant matter and is released by certain bacteria. Though less volatile than DMS, it is still relevant in biogeochemical cycles and is of interest in astrobiology due to its association with microbial metabolism.

Other Known Precursors of Life

While DMS and DMDS are sulfur-based compounds, several other molecules are widely regarded as precursors or indicators of life. These include:

• Amino Acids: Building blocks of proteins. Over 70 amino acids have been identified in meteorites, suggesting they can form in space.

• Nucleobases: Components of DNA and RNA. Some, like adenine and guanine, have been detected in carbon-rich meteorites.

• Formaldehyde (H₂CO): A simple organic compound that can polymerize to form sugars, essential in prebiotic chemistry.

• Methane (CH₄): While not definitive proof of life, its abundance and variability in an atmosphere can hint at biological processes.

• Phosphine (PH₃): A highly debated molecule recently proposed as a possible biosignature in Venus's atmosphere.

• Acetonitrile (CH₃CN): Detected in interstellar clouds, this molecule plays a role in prebiotic chemistry.

• Isoprene and other terpenes: Produced by plants and microbes, these are volatile organic compounds that could be used to infer life in oxygen-rich environments.

How Are These Molecules Detected From Earth?

Detecting biosignature gases like DMS or DMDS in the atmosphere of a distant planet is a challenging yet fascinating task. Scientists rely on spectroscopy, the study of how matter interacts with electromagnetic radiation.

1. Transit Spectroscopy

When a planet passes in front of its host star (a transit), starlight filters through its atmosphere. Molecules in the atmosphere absorb specific wavelengths of light, creating a unique pattern—an absorption spectrum. By comparing the light before, during, and after the transit, astronomers can identify the chemical fingerprints of atmospheric gases.

2. Emission and Reflection Spectroscopy

In some cases, scientists analyze the light emitted or reflected by the planet itself. Instruments onboard telescopes like the James Webb Space Telescope (JWST) are sensitive enough to detect faint infrared signatures of various molecules, including potential biosignatures.

3. Radio Astronomy

Some complex organic molecules in space emit radio waves as they rotate and vibrate. Using radio telescopes like ALMA (Atacama Large Millimeter/submillimeter Array), astronomers have detected numerous life-related molecules in interstellar clouds and protoplanetary disks.

4. Direct Imaging (Future Possibility)

Though still in its infancy, direct imaging of exoplanets aims to capture light directly from the planet, separate from the glare of the host star. Future missions like LUVOIR and HabEx aim to apply this technique to observe Earth-like planets and search for biosignatures.

Challenges in Detecting DMS and DMDS

While detecting simple molecules like methane or water vapor is now routine, detecting complex biosignature gases like DMS or DMDS remains difficult due to:

• Low Abundance: These molecules may exist in trace amounts, requiring extremely sensitive instruments.

• Spectral Overlap: Their spectral lines may overlap with those of other gases, making identification ambiguous.

• False Positives: Some molecules can be produced abiotically (without life), so context is crucial to interpretation.

• Signal-to-Noise Ratio: Atmospheric interference, instrumental limitations, and cosmic noise can distort data.

To understand and communicate the complex process of searching for life beyond Earth, we prepare a flow chart that outlines each key step in the journey of biosignature detection. The goal is to visually represent the logical sequence of scientific tasks—from selecting candidate planets to analyzing chemical data—that guide our search for extraterrestrial life.

The quest to uncover life elsewhere in the universe has shifted from the philosophical to the practical. DMS, DMDS, and a suite of other life-related molecules are offering real, tangible clues. Their detection in extraterrestrial environments could mark a revolutionary moment in science, reshaping our understanding of biology, chemistry, and our place in the cosmos.

1. Hypothesis – The Starting Point of Science

A hypothesis is a testable prediction. It’s a starting guess, based on observation, that can be tested through experiments.

Example: If plants are given more sunlight, they will grow taller. This is a hypothesis because it predicts an outcome that can be tested.

🔹 How it differs: A hypothesis is not yet confirmed. It’s like a detective’s first lead in a mystery.

2. Theory – The Grand Explanation

A theory is a well-supported explanation of natural phenomena, based on extensive evidence from experiments and observations.

If a hypothesis is repeatedly independently demonstrated by experiment to be true, it becomes a scientific theory.

Example: The theory of evolution explains how species change over time due to natural selection. It’s supported by fossils, genetics, and real-world observations.

🔹 How it differs: Unlike a hypothesis, a theory has been rigorously tested and widely accepted. It’s not just a wild guess—it's backed by tons of evidence.

3. Law – The Universal Rule

A law is a concise statement that describes a consistently observed natural phenomenon, often using a mathematical formula. It is a descriptive account of how nature will behave under certain conditions

Example: Newton’s law of gravity states that objects attract each other with a force proportional to their masses and inversely proportional to the square of the distance between them.

🔹 How it differs: A law describes what happens but doesn’t explain why it happens. That’s the job of a theory. For example, the law of gravity tells us objects fall, but Einstein’s theory of general relativity explains why they do.

Scientific laws and theories can not be proven. It can only be verified.

4. Fact – The Unquestionable Truth

A fact is an observation that has been repeatedly confirmed.

Example: Water boils at 100°C at sea level. You can test this again and again, and it will always hold true under those conditions.

🔹 How it differs: Facts are data points. They don’t explain anything on their own—that’s what theories and hypotheses do.

5. Principle – A Fundamental Concept

A principle is a general rule that forms the foundation of scientific knowledge. This is often used to mean the same thing as "law".

Example: The principle of conservation of energy states that energy cannot be created or destroyed, only transformed.

🔹 How it differs: Principles are broader than laws. They are foundational ideas that apply across many disciplines.

6. Postulate – An Assumption for Reasoning

A postulate is something assumed to be true for the sake of argument or theory development.

Example: Euclid’s postulate that “a straight line can be drawn from any two points” forms the basis of geometry.

🔹 How it differs: A postulate is not necessarily tested—it’s an agreed-upon assumption that helps build a system of knowledge.

7. Conjecture – An Educated Guess

A conjecture is a statement believed to be true but not yet proven.

Example: The Goldbach Conjecture states that every even integer greater than 2 can be expressed as the sum of two prime numbers. Mathematicians believe it’s true, but no one has proven it for all numbers.

🔹 How it differs: A conjecture is like a hypothesis in math—it suggests something might be true but needs proof.

8. Proof – The Ultimate Confirmation

A proof is a logical demonstration that a statement must be true.

Example: In mathematics, Pythagoras’ theorem (a² + b² = c² for right triangles) is proven using logical steps, making it indisputable.

🔹 How it differs: Proof is absolute, mostly found in mathematics. In science, theories are never "proven"—they are supported by overwhelming evidence but always open to revision.

9. Verification – Checking for Consistency

Verification is the process of confirming that a hypothesis, theory, or experiment aligns with evidence.

Example: A scientist testing whether a vaccine prevents disease will verify results by running multiple trials.

🔹 How it differs: Verification checks if something holds true under specific conditions. It’s a way to support but not necessarily prove something.

10. Axiom – The Unquestioned Truth in Math

An axiom is a self-evident truth in mathematics that doesn't need proof.

Example: “For any two distinct points, there exists exactly one straight line that passes through them” (Euclidean geometry).

🔹 How it differs: Unlike hypotheses or theories, axioms are accepted without question because they are the foundation of logical reasoning.

11. Lemma – A Stepping Stone in Proofs

A lemma is a minor result used to help prove a larger theorem.

Example: In proving Fermat’s Last Theorem, many lemmas were established before reaching the final proof.

🔹 How it differs: A lemma is like a supporting paragraph in an essay—it helps build toward the main conclusion.

12. Corollary – A Bonus Conclusion

A corollary is a statement that follows directly from a proven theorem.

Example: A corollary of the Pythagorean Theorem is that a triangle with sides 3, 4, and 5 is a right triangle.

🔹 How it differs: If a theorem is a recipe, a corollary is an extra dish you can make using the same ingredients.

Final Thought

Science is precise, and understanding these terms helps us think clearly and critically. When you hear “It’s just a theory” in the news, now you know that a scientific theory is one of the strongest ideas out there, backed by loads of evidence. And when someone asks for "scientific proof," you can explain that science builds theories, not absolute proofs.

As a Liquid Bird reader, you now see science through a sharper lens. Keep questioning, keep exploring!