The diversity of life on Earth is so vast, scientists haven’t even discovered it all yet. The variety we experience ranges micro and macroscopically from differences in shapes, sizes, and colors to diets and reproduction. But when it comes to choices of habitat, we find some commonality: we either live on land or by sea. Or at least, that’s what our eyes lead us to believe. If you look close enough, you can find life thriving even in the air we breathe—a fact that has some astrobiologists looking for extraterrestrial life in high places on other celestial bodies.
New research led by Jack Yates at the University of Edinburgh found that the atmospheric habitable zone–the region of an object’s atmosphere where microbes can survive–could exist on other planets and even on larger gaseous bodies like brown dwarfs. Yates and his team found that the coldest brown dwarfs, known as Y dwarfs, have upper atmospheric temperatures similar to lower atmospheric temperatures on Earth, where biologists have previously found microbes. Yates modeled the atmosphere of the infamous Y dwarf, WISE 0855 (which made headlines for being the coldest known brown dwarf and also for having water clouds), for this research. The success of Yates’ model suggests that microbial life could exist in the upper atmospheres of giant gaseous bodies such as Jupiter or even in the atmosphere of uninhabitable terrestrial worlds like Venus.
Yates was also able to put a constraint on the size of the microbes they’re looking for. Gaseous bodies are typically partially or fully convective, where their atmospheres mix around from vertical winds moving inward and outward away from and toward the surface. If these winds are strong, Yates’ model suggests the need for more massive organisms that can counteract the strong upward convection wind, to stay in their comfortable, habitable temperature zone. If the winds are weaker, smaller organisms can survive.
But this habitability still relies on one very important ingredient: water. Water clouds must be present, like on WISE 0855, and it’s assumed that water clouds can only come from the presence of liquid water. So looking for water is still the first step to looking for life.
With new observing technology on the horizon, like the James Webb Space Telescope, looking for water is going to get a lot easier, and looking for tiny air-based life forms will become a reality. Yates predicts that there are over one billion Y dwarfs in the Milky Way, with at least a few handfuls of them close enough to Earth to observe. To look for life like this, astronomers will still search for biosignatures, the same way they search for terrestrial life because these microbes will be too tiny to spot individually from this far away.
But before we start searching for elusive microbial Venusians, there’s one more thing to consider—how life would have gotten to these seemingly inhabitable planets and brown dwarfs in the first place. Some scientists hypothesize that life could be sparked by interactions with lightning, which creates the high energy needed to force simpler molecules into the amino acids we know today as the “building blocks of life.” Another hypothesis is that life could have migrated from elsewhere in the universe and evolved to survive in their new environment, like the popular hypothesis for how life got here on Earth.
When it comes to our knowledge of the variety of life on Earth, we’re still skimming the surface in the sea of diversity. The two most common habitats–land and sea–aren’t even as simple as their binarity might suggest, and now scientists have a third place to look for life–the atmosphere. Expanding the search for life on other celestial objects to include those with atmospheric habitable zones will not only teach astronomers where to look for life but also what exactly to look for.