Achi news desk-
The search for extrasolar planets is currently undergoing a seismic shift. With the use of the Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS), scientists discovered thousands of exoplanets, most of which were detected and confirmed using indirect methods. But in more recent years, and with the launch of the The James Webb Space Telescope (JWST), the field has been transitioning towards one of characterization. In this process, scientists rely on emission spectra from exoplanet atmospheres to search for the chemical signatures we associate with life (biosignatures).
However, there is some debate about the types of signatures scientists should look for. Essentially, astrobiology uses life on Earth as a template when looking for signs of extraterrestrial life, much like how extraterrestrial hunters use Earth as a standard for measuring “accommodation.” But as many scientists have pointed out, life on Earth and its natural environment has evolved significantly over time. In a recent paper, an international team showed how astrobiologists could look for life on TRAPPIST-1e based on what existed on Earth billions of years ago.
The team included astronomers and astrobiologists from the Global Systems Institute, and the Departments of Physics and Astronomy, Mathematics and Statistics, and Natural Sciences at the University of Exeter. They were joined by researchers from the School of Earth and Marine Sciences at the University of Victoria and the Natural History Museum in London. The paper describing their findings, “Biosignatures of pre-oxygen photosynthetic life on TRAPPIST-1e,” will be published in the Monthly Notices of the Royal Astronomical Societyand (MNRAS).
The TRAPPIST-1 system has been the center of attention since astronomers confirmed the presence of three exoplanets in 2016, which grew to seven by the following year. As one of many systems with a low-mass, cooler, M-type (red dwarf) parent star, there are unresolved questions about whether any of its planets could be habitable. Much of this has to do with the variable and unstable nature of red dwarfs, which tend to flare and may not produce enough photons to power photosynthesis.
With so many rocky planets found orbiting the dwarf sun, including the closest exoplanet to our Solar System (Proxima b), many astronomers feel that these systems would be the ideal place to search about extraterrestrial life. At the same time, they have also emphasized that these planets would need to have thick atmospheres, intrinsic magnetic fields, adequate heat transfer mechanisms, or all of the above. Determining whether exoplanets have these prerequisites for life is something that JWST and other next-generation telescopes – such as ESO’s proposed Extremely Large Telescope (ELT) – are expected to enable.
But even with these and other next-generation instruments, there is still the question of what biosignatures we should be looking for. As noted, our planet, its atmosphere, and all life as we know it have evolved significantly over the last four billion years. During the Archean Eon (about 4 to 2.5 billion years ago), the Earth’s atmosphere consisted mainly of carbon dioxide, methane and volcanic gases, and little more than anaerobic microorganisms existed. The first multicellular life appeared and evolved to its present complexity only within the last 1.62 billion years.
Furthermore, the number of evolutionary steps (and their potential difficulty) required to reach higher levels of complexity mean that many planets may never develop complex life. This is consistent with the Grand Filter Theory, which states that while life may be common in the Universe, higher life may not. As a result, simple microbial biospheres similar to those that existed during the Archean may have been the most common. The key, then, is to carry out searches that would isolate biosignatures consistent with primitive life and the conditions common to Earth billions of years ago.
As explained by Dr. Jake Eager-Nash, a postdoctoral research fellow at the University of Victoria and lead author of the study, told Universe Today via email:
“I think Earth’s history provides many examples of what habitable exoplanets might look like, and it’s important to understand biosignatures in the context of Earth’s history as we have no other examples of what life would look like on other planets. During the Archean, when life is thought to have first emerged, there was a period of up to about a billion years before oxygen-producing photosynthesis evolved and became the main primary producer, oxygen concentrations were very low. So if habitable planets follow an Earth-like trajectory, they could spend a long time in an era like this without oxygen and ozone biosignatures, so it’s important to understand what Archean biosignatures look like.”
For their study, the team created a model that considered Archean-like conditions and how the presence of early life forms would consume some elements while adding others. This led to a model where simple bacteria living in oceans consume molecules such as hydrogen (H) or carbon monoxide (CO), creating carbohydrates as an energy source and methane (CH).4) as waste. They then considered how gases would be exchanged between the ocean and the atmosphere, resulting in lower concentrations of H and CO and higher concentrations of CH4. Eager-Nash said:
“Archaeal biosignatures are thought to require the presence of methane, carbon dioxide and water vapor as well as the absence of carbon monoxide. This is because water vapor gives you an indication that there is water, while an atmosphere with methane and carbon monoxide shows that the atmosphere is in imbalance, which means that the two species should not coexist in the atmosphere because that atmospheric chemistry would convert it all. from one to the other, unless something, similar to life, supports this injustice. The absence of carbon monoxide is important because it is believed that life would evolve rapidly as a means of using this energy source.”
When the concentration of the gases is higher in the atmosphere, the gas will dissolve into the ocean, replenishing the hydrogen and carbon monoxide used by the simple life forms. As biologically produced methane levels increase in the ocean, it will be released into the atmosphere, where additional chemistry takes place, and different gases are transported around the planet. From this, the team derived a general composition of the atmosphere to predict which biosignatures might be detected.
“What we’re finding is that carbon monoxide is likely present in the atmosphere of an Archean-like planet orbiting an M-Dwarf,” Eager-Nash said. “This is because the host star drives chemistry that results in higher concentrations of carbon monoxide compared to a planet orbiting the Sun, even when you eat this. [compound].”
For years, scientists have considered how the circumsolar habitable zone (CHZ) could be extended to include Earth-like conditions from previous geological periods. Likewise, astrobiologists have been working to cast a wider net on the types of biosignatures associated with more ancient life forms (such as retina-photosynthetic organisms). In this latest study, Eager-Nash and his colleagues have established a series of biosignatures (water, carbon monoxide, and methane) that could lead to the discovery of life on rocky Archean-era planets orbiting similar suns to the Sun and the dwarf.
Further Reading: arXiv
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