Astronomers have found some pretty wild exoplanets.
Some are balls of lava the temperature of hell, one is partially made of diamond, and another may rain molten iron.
However, not all exoplanets are this extreme. Some are rocky, roughly Earth-sized worlds in the habitable zones of their stars.
Could simple Earth life survive on some of these less extreme worlds?
We currently describe a solar system’s habitable zone by liquid water.
If a planet is at the right distance range from its star to host stable surface water, we consider it to be in the habitable zone. However, new research is taking a different approach by emphasizing the role a planet’s atmosphere plays in habitability.
The scientists behind this research tested their idea by seeing if microbes could survive on simulated worlds.
The new research is “The Role of Atmospheric Composition in Defining the Habitable Zone Limits and Supporting E. coli Growth.”
It’s available on the pre-print site arxiv.org. The lead author is Asena Kuzucan, a post-doctoral researcher in the Department of Astronomy at the University of Geneva in Switzerland.
We’ve discovered close to 6,000 exoplanets in about 4,300 planetary systems. Our burgeoning catalogue of exoplanets makes us wonder about life. Is there life elsewhere, and are any of these thousands of exoplanets habitable?
Some have teased the possibility. TRAPPIST1-e and Proxima Centauri b are both rocky planets in the habitable zones of their stars. TOI-700 d orbits a small, cool star and may be in its habitable zone. There are many others.
The simple definition of the habitable zone is restricted to a planet’s distance from its star and if liquid water can persist on its surface at that distance.
However, scientists know that a planet’s atmosphere plays a large role in habitability. A thick atmosphere on a planet outside the habitable zone could help it maintain liquid water.
“Each atmosphere uniquely influences the likelihood of surface liquid water, defining the habitable zone (HZ), the region around a star where liquid water can exist,” the authors write. Liquid water doesn’t guarantee that a world is habitable, however.
In order to understand exoplanet habitability better, the researchers followed a two-pronged approach.
They started by estimating exoplanet surface conditions near the inner edge of a star’s HZ with different atmospheric compositions.
Next, they considered if Earth microbes could survive in these environments. They did lab experiments on E. coli to see how or if they could grow and survive. They focused on the different compositions of gas in these atmospheres. The atmospheric compositions were standard (Earth) air, pure CO2, N2-rich, CH4-rich, and pure molecular hydrogen.
Their experiments used 15 separate bottles, 3 for each of the 5 atmospheric compositions. Each bottle was inoculated with E. coli K-12, a laboratory strain of E. coli that is a cornerstone of molecular biology studies.
“This innovative combination of climate modelling and biological experiments bridges theoretical climate predictions with biological outcomes,” they write in their research.
Along with their laboratory experiments, the team performed a series of simulations with different atmospheric compositions and planetary characteristics. “For each atmospheric composition we simulate, water is a variable component that can condense or evaporate as a function of the pressure/temperature conditions,” they write. For each atmospheric composition, they simulated planets at different orbital distances in order to define the inner edge of the HZ. They also varied the atmospheric pressure.
“Using 3D GCM (General Circulation Model) simulations, this study provides a first look at how these atmospheric compositions influence the inner edge of the habitable zone, offering valuable insights into the theoretical limits of habitability under these extreme conditions,” the authors explain.
“Our findings indicate that atmospheric composition significantly affects bacterial growth patterns, highlighting the importance of considering diverse atmospheres in evaluating exoplanet habitability and advancing the search for life beyond Earth,” they write.
- coli did surprisingly well in varied atmospheric compositions. Though there was a lag following inoculation as the E. coli adapted, cell density increased in some of the tests. The hydrogen-rich atmosphere did surprisingly well.
“By the first day after inoculation, cell densities had increased in standard air, CH4-rich, N2-rich, and pure H2 atmospheres,” the authors write. “While cell densities increased similarly in standard air, CH4-rich, and N2-rich atmospheres, a slightly stronger increase was observed in the pure H2 atmosphere. The rapid adaptation of E. coli to pure H2 suggests that hydrogen-rich atmospheres can support anaerobic microbial life once acclimatization occurs.”
Conversely to the H2 results, the CO2 results lagged. “Pure CO2, however, consistently presented the most challenging environment, with significantly slower growth,” the paper states.
Their results suggest that planets with anaerobic atmospheres that are dominated by H2, CH4, or
N2 could still support microbial life, even if the initial growth is slower than it is in Earth’s air. “The ability to adapt to less favourable conditions implies that life could persist on such planets, given sufficient time for acclimatization,” the authors write.
The CO2-rich atmosphere is the outlier in this work. “The consistently poor growth in pure CO2 highlights the limitations of this gas in supporting life, particularly for heterotrophic organisms like E. coli,” Kuzucan and her co-researchers write. However, the authors point out that some life forms can make use of CO2 as a carbon source in some environments. They explain that planets with these types of atmospheres could still host organisms adapted to them, like chemotrophs or extremophiles.
This work combines atmospheric and biological factors to understand exoplanet HZs. “One of our key objectives was to define the limits of the HZ for planets dominated by H2 and CO2 using 3D climate modelling, specifically the Generic PCM model,” the authors explain.
They found that H2 atmospheres have a warming effect, “pushing the inner edge of the HZ to further orbital distances than CO2-dominated atmospheres.” It could extend out to 1.4 AU at 5 bar, while the CO2 atmospheres at the same pressure were limited to 1.2 AU. “This demonstrates the profound impact of atmospheric composition on planetary climate and highlights how H2 atmospheres can extend the
habitable zone further from their host stars,” the researchers write.
Some of the atmospheres they tested are not likely to persist in nature, but the results are still scientifically valuable.
“Although some of the atmospheric scenarios presented here (1-bar H2 and CO2) are simplified, and
may not persist over geological timescales due to processes like hydrogen escape and carbonate-silicate cycling, they nonetheless provide valuable insights into the radiative effects of these gases on habitability,” write the authors.
We know atmospheres are extremely complex, and this research supports that. It also shows how resilient Earth life can be. “Overall, these results highlight both the resilience of E. coli in adapting to diverse atmospheric conditions and the critical role atmospheric composition plays in determining
microbial survival,” the authors explain in their conclusion. Though the authors acknowledge that their findings are rooted in an Earth-centric framework, the results have broader implications. Life could likely thrive in wildly different atmospheric compositions and conditions, according to these results.
“Thus, our study highlights the importance of atmospheric composition and pressure for habitability while acknowledging the limitations of our Earth-centric perspective,” they write.
“By exploring both atmospheric conditions and microbial survival, we gain a deeper understanding of the complex factors that influence habitability, paving the way for future research on the potential for life beyond our solar system.”
Written by Evan Gough/Universe Today.
