How Spectroscopy Could Help Find Life Outside Our Solar System

By John Oncea, Editor

Spectroscopy lets us decode the atmospheres of distant exoplanets for biosignatures like oxygen, methane, and dimethyl sulfide, advancing the search for alien life.

Quick – what was the first planet discovered in our Solar System?

Earth? Can the planet a person is living on really be discovered? Let’s say … no.

What about Mercury? Or Venus? Mars, Jupiter, or Saturn? These five planets, which were identified in the 2nd millennium BCE by ancient Babylonian astronomers, always have been easy to see by the unaided human eye. As such, people have been looking at them, whether they understood what they were or not, since the first person gazed at the night sky. So, for our purposes, they weren’t discovered; rather, they were observed.

That leaves Uranus and Neptune with the winner being … Uranus, which was discovered by Sir William Herschel in 1781. Neptune was discovered 65 years later by John Couch Adams.

The discoveries of Uranus and Neptune relied on both mathematical predictions and telescopic observations. Uranus was discovered using a telescope, while Neptune’s existence was predicted mathematically based on perturbations in Uranus’ orbit before being confirmed with a telescope.

As of this writing, the most recent significant discovery in our Solar System is the interstellar object 3I/ATLAS, a comet that originated from another star system. It was first detected by the ATLAS survey on July 1, 2025 – 180 years after the discovery of Neptune – and confirmed as interstellar by the Minor Planet Center the following day. This is only the third interstellar object ever observed passing through our solar system.

Discovering Exoplanets

Let’s move beyond our Solar System and consider the discovery of extrasolar objects, or any astronomical object outside our Solar System, including exoplanets, exomoons, and other celestial bodies.

The first objects discovered outside of our Solar System were exoplanets, specifically planets orbiting the pulsar PSR B1257+12, which were discovered in 1992. The first exoplanets discovered around a Sun-like star were found in 1995 and opened up the possibility of finding planets like Earth in other solar systems.

Extrasolar objects are primarily discovered using several techniques that rely on both observational astronomy and physics: the transit method, radial velocity (or Doppler) method, gravitational microlensing, and direct imaging.

Space-based telescopes, like the Kepler Space Telescope and TESS, have been instrumental in exoplanet detection, particularly using the transit method, which observes the slight dimming of a star’s light as a planet passes in front of it. Ground-based telescopes, along with advanced instruments, also play a crucial role in confirming and characterizing these distant worlds.

Photonics, particularly laser frequency combs and photonic integrated circuits, play a key role in the search for exoplanets, enabling precise measurements of starlight to help detect the subtle wobbles in stars caused by orbiting planets or the dimming of starlight during planetary transits.

Laser frequency combs generate a precise and stable ruler of optical frequencies, allowing astronomers to measure the radial velocity of stars with extreme accuracy. By observing the slight shift in the star’s light spectrum (Doppler effect), they can infer the presence and orbital characteristics of planets.

Photonic integrated circuits, miniaturized photonic devices that can be integrated into space telescopes, enable more compact and cost-effective exoplanet detection systems. They can be used for various techniques, including transit photometry and radial velocity measurements.

Transit photometry is another photonics technique used to identify extrasolar objects and involves monitoring the brightness of a star over time. When a planet passes in front of its star (transit), it causes a slight dip in the star’s brightness, which can be detected using photonics-based instruments.

Finally, the radial velocity method detects the wobble of a star caused by the gravitational pull of an orbiting planet. Laser frequency combs play a crucial role in precisely measuring the star’s velocity changes.

In essence, photonics provides tools for making the highly sensitive and precise measurements needed to discover and characterize exoplanets, especially those that are Earth-sized and potentially habitable.

The Search For Life Beyond Earth

To date, Earth is the only planet known to harbor life within our Solar System. None of the other eight planets, five dwarf planets, hundreds of moons, or thousands of asteroids, comets, or other celestial objects orbiting the Sun have ever been found to harbor signs of life.

This isn’t for lack of trying with the search for life beyond Earth as a formal scientific endeavor starting in the mid-20th century, specifically in the late 1950s and early 1960s. This era saw the emergence of astrobiology, a field that combines biology, astronomy, and other sciences to study the possibility of life beyond Earth.

The event marking the start of this search occurred in 1959 when NASA funded its first exobiology project and established an Exobiology Program. A year later, Frank Drake conducted the first modern SETI (Search for Extraterrestrial Intelligence) experiment, known as Project Ozma, at the Green Bank National Radio Astronomy Observatory.

In 1961, the Drake equation, a framework for estimating the number of detectable alien civilizations, was formulated, and in the years that followed, the field of astrobiology began to take shape, with scientists exploring the possibility of life in the universe through various avenues, including radio astronomy and space exploration.

The concept of searching for extraterrestrial intelligence using radio telescopes gained traction throughout the 60s, fueled by theoretical work on interstellar communication. The 1970s witnessed NASA's Viking program exploration of Mars, searching for signs of microbial life, though the results were inconclusive. Since then, the search for life beyond Earth has continued to evolve, with ongoing research in astrobiology, exoplanet exploration, and SETI, with NASA, the SETI Institute, and other organizations actively involved.

The search for life in our Solar System, Everything Everywhere points out, is within the realm of possibility. Searching for life outside our Solar System, however, is a much more difficult challenge, given our current technology, because we can’t send probes to other star systems.

“The furthest we have ever sent a probe is Voyager 1,” writes Everything Everywhere. “It was launched in 1977 and is approximately 167 astronomical units from the Sun, or fifteen billion, five hundred twenty-three million, six hundred twenty thousand miles, or about 25 billion miles, as of the time of this recording. Assuming that Voyager 1 was pointed at the nearest star to the Sun, Proxima Centauri, it would take another 75,000 years to get there.”

Given this limitation, astronomers have turned to various other techniques to search for life beyond our Solar System, including spectroscopy.

Spectroscopy And The Search For Life

Spectroscopy has revolutionized the search for extraterrestrial life. Whereas the early years of exoplanet discovery relied on measuring minute dips in starlight as planets transited their host stars, advances in photonic instrumentation now allow researchers to parse not only the presence but the composition of exoplanetary atmospheres, writes Everything Everywhere. Using the interplay between photons and matter, spectroscopy has emerged as the principal tool to extract molecular fingerprints from light traveling across astronomical distances.

High-resolution transit spectroscopy is currently the most productive mode of atmospheric analysis. When an exoplanet passes in front of its star, the star's photons sweep through the planet’s atmospheric limb, interacting with any molecules encountered along the way. Each atmospheric gas absorbs distinct wavelengths, imprinting spectral lines that encode the composition, structure, and even physical conditions of the planet’s atmosphere. Transmission spectroscopy is especially powerful at this task, focusing on the telltale absorption features that only manifest during planetary transits, according to Penn State University.

Eclipse measurements add further nuance. During secondary eclipse events, when the planet passes behind its star, astronomers capture the combined light of both star and planet just before, and then the star alone during, the event. By contrasting these fluxes, they deduce the planet’s thermal emission or reflected light spectrum, offering direct insight into surface and atmospheric temperature, chemical mixing, and energy balance.

Direct imaging spectroscopy, though more challenging, is another frontier. Using starshades or coronagraphs, observers physically block the overwhelming starlight, isolating the dim signal from planets that are typically both far from their star and have atmospheres thick enough to remain optically visible in infrared or visible wavelengths. This methodology, while still limited in the number of applicable targets, showcases the technological ambition driving future projects, according to Nature.

All these methods depend on resolving signals that are often less than one part in ten thousand of the host star’s emission and disentangling extraterrestrial features from terrestrial contaminants. According to arXiv, astronomers routinely calibrate for atmospheric, instrumental, and cosmic noise, making their work a tour de force in extreme sensitivity and precision.

Discerning The Presence Of Life: What Counts As A Biosignature?

The second challenge lies in interpretation—specifically, distinguishing biosignatures, or molecular evidence of life, from non-biological phenomena. Most of our knowledge stems from terrestrial biology, where life is carbon-based, DNA-driven, and overwhelmingly responsible for the gaseous makeup of Earth’s atmosphere. Yet the search for biosignatures in exoplanet atmospheres must navigate the ambiguity inherent in extrapolating Earth-centric models across the galaxy.

On Earth, free oxygen is an extraordinary signpost for life. Photosynthetic organisms produce O₂ by harnessing sunlight to split water and carbon dioxide, creating sugars for energy and releasing oxygen as a waste product. This process has sustained a 21% oxygen concentration in our atmosphere, a level maintained only because continual biological replenishment outpaces oxidation and other sinks. If every biosphere vanished, writes the National Center for Biotechnology Information, nearly all atmospheric oxygen would be consumed by chemical processes within a geologically insignificant window, underscoring its role as a dynamic biomarker.

Nevertheless, oxygen’s presence need not be biogenic. Several abiotic routes – photodissociation of water by ultraviolet light, for example – can build up oxygen, especially on small, hot planets where hydrogen escapes efficiently into space. Planetary surface chemistry or unusual atmospheric dynamics may also mimic the signal of biosphere-driven oxygen, meaning confirmation of life demands context beyond O₂ detection alone.

Methane (CH₄) is likewise an important, but double-edged, biosignature. On Earth, methanogenic microbes thrive in anoxic environments, converting organic molecules into methane, a gas quickly destroyed by atmospheric oxidation. Its persistence in chemical disequilibrium with oxygen, as observed in rare terrestrial or Martian cases, is compelling because it requires constant replenishment from opposing, and usually biological, sources. But geological processes such as serpentinization or cryovolcanism are known to generate methane as well, so its mere appearance demands complementary lines of evidence, according to Astronomy.

Of particular note is the case of K2-18b, a super-Earth orbiting in the habitable zone of the star K2-18. Recent observations using the James Webb Space Telescope (JWST) revealed spectral traces of carbon dioxide and methane in its atmosphere, a first for an exoplanet of this size and location, NASA writes. In 2019, water vapor was confirmed, and in 2023, a further discovery added methane and CO₂ to the list. But it was the tentative detection of dimethyl sulfide (DMS) that set the scientific community abuzz, adds Sky at Night Magazine.

DMS on Earth is primarily produced by marine phytoplankton. Its chemical structure – a sulfur atom bound to two methyl groups – has no robust abiotic creation pathway at the planetary scale, making it a prime biosignature candidate. On Earth, DMS is quickly broken down by photochemistry; sustained atmospheric concentrations require continual biological generation, which is why confirmed detection elsewhere would be extraordinary. However, the potential for unknown geochemical processes and the uncertainties inherent in interpreting weak, noisy spectral signals demand cautious optimism. Some analysts question whether the observed spectral features can be robustly attributed to DMS without ruling out alternative explanations.

The excitement over K2-18b, according to The Planetary Society, hinges not only on the individual detection of oxygen, methane, water vapor, or DMS, but on their co-presence, especially in apparent chemical disequilibrium. Such a combination ticks multiple “bingo boxes” for life, increasing the odds that active, perhaps even microbial, processes are shaping its atmosphere. Critics caution that atmospheric modeling is still in its infancy, that planetary conditions may be unearthly in unknown ways, and that future observations will be essential to resolve the debate. This cautious approach ensures that the scientific process prioritizes reliability and reproducibility, setting a high bar for any claim of life beyond Earth.

The Photonic Frontier

As photonic instrumentation matures and telescopes grow ever more sensitive and discriminating, our toolkit for biosignature detection expands. Simulations and laboratory experiments are essential for understanding the robustness of spectroscopic signals, helping to differentiate genuine signatures of life from mimics created by planet-specific chemistry or exotic environmental conditions. Future missions promise to extend the reach of transmission and direct imaging spectroscopy into the regime of temperate, rocky exoplanets where Earth-like conditions might prevail.

Thus, the search for life is an interplay of photons and molecules, context, and caution. Every new detection challenges our assumptions, refines our models, and brings the tantalizing possibility of another living world a little closer.