It is a common concept -- that when humanity finally encounters extraterrestrial life, that life will have developed on a world with water, a world possibly similar to Earth in many ways. But researchers at the Massachusetts Institute of Technology (MIT) have postulated that life may arise on planets high in sulfurous molecule content, where alien life forms have evolved to "breathe" sulfurous molecules instead of oxygen. The presence of sulfurous molecules in an exoplanet's atmosphere, their findings show, could point to a biosignature that could broaden the search parameters for alien life on extrasolar worlds.
MIT planetary science doctoral student Renya Hu and his colleagues, Sara Seager and William Bains, detailed their findings May 26 at the American Astronomical Society in Boston. Researchers worked with models of exoplanets existing in what are known as the "habitable zone," that area of space where a planet can orbit and maintain liquid water on their surfaces. Working on the idea that alien life might develop where the metabolizing of sulfurous molecules occurs, much like microorganisms on Earth found near volcanoes that metabolize sulfurous molecules, the team surmised that the atmospheres of the worlds of such sulfur-dependant creatures would most likely have atmospheres that contain increased amounts of hydrogen sulfide, which is a waste product emitted from similar microbes on Earth.
Because of the vast distances of interstellar space, detecting a measurable difference between hydrogen sulfide and hydrogen oxide (water) molecules is a difficult task. However, the MIT researchers calculated that a higher atmospheric content of sulfur dioxide would in turn allow for the creation of greater amounts of pure sulfur aerosols, something that astronomers could detect due to their distinctive spectra (i.e., their "fingerprint") in the visible and infrared wavelengths.
Hu notes that hydrogen sulfide being emitted from the planet's surface would have an "immense impact on the atmospheric composition of the planet."
As no Earth-sized exoplanets have as yet been discovered in the "habitable zone" (also referred to as the "Goldilocks Zone" because it is just right in relative spatial placement from a star to sustain the formation of liquid water molecules) of Sun-like stars, Hu notes that characterizations of atmospheres of planets have only been conducted on close-in planets.
Hu also sounds a word of caution about hydrogen sulfide being used as a biosignature, or sign of life. "We need to test our assumptions thoroughly," he said. "It may be, for instance, that volcanism could produce tremendous amounts of that gas."
He says that further studies on other gases are to be considered as well. He notes that the MIT researchers have already embarked on such studies.
The MIT research isn't the first serious clinical suggestion that alien life may have developed along lines that differ radically from that of life on Earth. In December 2010, NASA scientists announced an "astrobiology finding that will impact the search for evidence of extraterrestrial life." They had discovered a bacteria at Mono Lake, California, that had developed a DNA based partially on arsenic compounds (all life on Earth has DNA based on phosphorus compounds).
With the number of exoplanets being discovered by NASA's various telescopes and space sensory equipment increasing rapidly, it might be simpler to search for planets where life could be defined by limited parameters, such as worlds that may develop life along similar lines to that of Earth. But the Drake Equation, which is a mathematical calculation of the number of planets that might produce highly evolved civilized life forms, suggests that the number of intelligent civilizations will exist on only one in a million worlds. But the Equation presupposes intelligence developing on Earth-like planets. Research findings such as that suggesting life might develop breathing various gases or gas mixes, not to mention life forms developing with alternate DNA building blocks, could make the search for alien or extraterrestrial life easier simply by increasing exponentially the number of worlds that could possibly sustain life.
And instead of worrying about whether or not humankind is alone in the universe, humans might soon begin to wonder just how crowded with life the universe might be.
(photo credit: Lucianomendez, Creative Commons)