Sulfur may be a new tool for finding alien life

Astronomers have identified sulfur as a potentially crucial indicator in narrowing the search for life on other planets. While sulfur itself is not necessarily an indication of habitability, significant concentrations of sulfur dioxide in a planet’s atmosphere can suggest that the planet is likely uninhabitable, allowing researchers to eliminate it from further consideration.

The discovery of extraterrestrial life remains one of the most sought-after objectives in modern astronomy. However, this is a formidable challenge. The James Webb Space Telescope is unlikely to detect biosignatures—atmospheric gases produced by living organisms— on nearby planets. Similarly, the upcoming Habitable Worlds Observatory will only be able to assess a limited number of potentially habitable exoplanets.

One of the primary obstacles astronomers face is the typically faint nature of biosignature spectra. To address this, they focus on the potential for planets to host life, particularly through the presence of water vapor in their atmospheres. A planet with substantial water vapor may be more likely to support life.

This concept is encapsulated in the habitable zone, the region around a star where a planet receives just the right amount of radiation: not too little to freeze all water, and not too much to boil it away. In our solar system, Venus lies near the inner edge of the habitable zone with surface temperatures exceeding 800 degrees Fahrenheit beneath a dense atmosphere, while Mars resides primarily outside the zone, its water largely trapped in polar ice caps and subsurface reservoirs.

However, detecting water alone poses challenges. For instance, distinguishing between Earth and Venus based solely on atmospheric spectra is difficult due to their similarities when only searching for water vapor.

Recently, in a study published in the journal Science Advances, a team of astronomers has identified another potentially useful indicator gas for differentiating uninhabitable from possibly habitable worlds: sulfur dioxide. Warm, wet planets like Earth contain minimal sulfur dioxide because it is washed out of the atmosphere by rain. Conversely, Venus also has little detectable sulfur dioxide, as ultraviolet radiation from the sun converts it into hydrogen sulfide in the upper atmosphere, driving it downwards.

Planets orbiting red dwarf stars present another scenario. These stars emit minimal ultraviolet radiation, allowing sulfur dioxide to persist in the upper atmospheres of dry, uninhabitable planets. Red dwarfs are of particular interest because they are the most common type of star in the galaxy, and many nearby systems, such as Proxima Centauri and TRAPPIST-1, host planets around red dwarfs, making them prime targets for future searches for life.

This new approach involving sulfur dioxide does not identify planets that might harbor life but helps exclude those that likely do not. If significant sulfur dioxide is detected in the atmosphere of a rocky planet orbiting a red dwarf, it suggests a dry, hot world with a thick atmosphere and little to no water, akin to Venus. Such planets can be deprioritized in the search for life.

Conversely, the absence of significant sulfur dioxide may indicate a planet worth further observation for signs of water vapor and potential life.

The quest to find life on other planets will require extensive investigative efforts and unwavering determination. Any method, including the analysis of sulfur dioxide levels to streamline candidate lists, is highly valuable in this endeavor.