Exciting speculations are circulating within the field of astrobiology. The [telescope], which has been meticulously observing the universe since its launch in 2022, has recently been highly productive, leading insiders to believe that it may have finally identified [a significant discovery]. According to astrophysicist Rebecca Smethurst, as cited by [source], “I anticipate that we will soon receive a research paper presenting compelling evidence of a biosignature on an exoplanet.”
In essence, this is fantastic news! However, it is essential to maintain a sense of perspective. The term “strong evidence for a biosignature” falls short of confirming the existence of life on other planets. A biosignature essentially indicates a signal that aligns with the characteristics of life but could also originate from other sources. While intriguing, it does not constitute indisputable proof. Given the numerous uncertainties inherent in a field that is still evolving, it is wise for the public to temper their expectations. Sara Seager, an astrophysics professor at MIT, cautions, “Many are eager for this breakthrough to happen this year. There will certainly be assertions made, but there won’t be definitive findings.”
One of the challenges in pinpointing unequivocal evidence of life stems from the ambiguity surrounding the definition of life itself. While terrestrial biology involves DNA, carbohydrates, and necessitates liquid water, the chemical makeup on other planets could differ significantly. Perhaps life forms could thrive on liquid methane instead of water, or utilize silicon in lieu of carbon. Thus, at its core, what defines life, and how can we identify the signs?
A prevailing notion is that life always exists far from chemical equilibrium. For instance, Earth’s atmosphere contains both oxygen and methane, which under normal circumstances would react to produce carbon dioxide and water, leading to the depletion of oxygen and methane. Astrobiologist Marc Neveu explains, “The coexistence of these molecules is sustained only through continuous replenishment by some process, which, in our case, is attributed to life.”
Therefore, observing Earth from a distance and noting the simultaneous presence of methane and oxygen could hint at the presence of life. Nonetheless, this observation does not amount to conclusive evidence since these gases can also be generated through non-biological mechanisms.
Despite the challenge of establishing irrefutable evidence, the absence of definitive proof does not negate the possibility of the existence of alien life. In fact, there are multiple indicators that could potentially be recognized as the initial traces of extraterrestrial life.
In 2004, scientists reported the detection of a minute quantity of methane gas in the atmosphere of Mars using a spectrometer aboard the European Space Agency’s Mars Express orbiter. Concurrently, astronomers using ground-based telescopes also identified signs of methane. While on Earth, methane is predominantly produced by living organisms (including cows) but can also result from seismic activity. Neveu suggests, “It’s plausible that there are subsurface liquid water streams, potentially harboring microbial life.”
While Jupiter and Saturn may be inhospitable to life due to their extreme cold temperatures and distance from the Sun, some of their icy moons are believed to possess subsurface oceans that could support life. Europa, one of Jupiter’s moons, particularly stands out. Recently, the Webb telescope detected carbon dioxide within a geologically active region on Europa’s surface, implying the presence of an ocean beneath.
For a long time, astronomers have theorized the existence of planets orbiting stars beyond our solar system, with the first such discovery made in 1992 around neutron stars. Presently, we are aware of [a considerable number] exoplanets, yet there is no concrete evidence regarding their potential to sustain life. However, a recent claim by a team of U.S. and U.K. astronomers suggests the detection of methane, carbon dioxide, and possibly dimethyl sulfide (DMS) around a star named K2-18b, located 124 light-years away. On Earth, DMS is a byproduct of biological activity.
Nevertheless, this finding remains highly speculative. Due to the vast distances and dimness of exoplanets relative to their host stars, even the powerful Webb telescope cannot directly image them. Instead, it relies on detecting variations in light intensity when the planet transits in front of its star, particularly in the infrared spectrum. Seager remains skeptical, disputing the interpretation of the data as evidence of DMS. She asserts, “There is no definitive signal.”
While identifying biosignatures is intriguing, the ultimate breakthrough would be the confirmation of extraterrestrial life through the discovery of a technosignature—a gas that can only be produced artificially through advanced technology. For example, the presence of [a specific gas] on a distant planet could signify artificial manipulation as it does not occur naturally. Seager suggests, “This would constitute a more substantial discovery.”
Short of that, the only way to definitively prove the existence of alien life is to locate it and study it up close—observing its metabolic processes, growth, and reproduction. However, this task presents significant challenges. If life exists on Venus, it likely thrives in the planet’s cooler acidic atmosphere, while potential methane-producing microbes on Mars might reside miles beneath the surface. The most inaccessible scenario involves life around distant stars, with the closest star requiring [a considerable amount of time] to reach using current technology.
Exploring the subsurface oceans of icy moons may offer a more feasible avenue. NASA is set to launch the [probe] in October, designed to traverse through the icy plumes ejected by Europa and analyze the molecules present, providing insights into potential subsurface life. Positive findings could spur further exploration missions. Additionally, German researchers are developing a probe capable of penetrating the icy crust of a moon to explore its subterranean ocean.
Seager remains optimistic about the quest for extraterrestrial life, albeit acknowledging that progress may not unfold as rapidly as anticipated. She draws parallels to the discovery of exoplanets, which initially seemed far-fetched but eventually became commonplace. Similarly, the initial signs of alien life may be met with skepticism before evolving into widely accepted truths.
Neveu concurs, emphasizing the necessity of patience in this endeavor. He remarks, “I believe this will be a gradual process, and it may already be underway.”