In search of potentially habitable exoplanets, scientists are forced to choose the low-hanging fruit approach. Since Earth is the only planet that we know can sustain life, this quest is essentially about finding planets that are "Earth-like." But what if Earth is not the measure of habitability that we all believe?
This was the subject of a keynote address recently held at the Goldschmidt Geochemistry Congress from 18 to 23 August in Barcelona, Spain. Here, a team of NASA-supported researchers explained how exploring the definition of habitable zones (HZs) shows that some exoplanets have better living conditions than Earth.
The presentation was based on a study entitled "A Limited Habitable Zone for Complex Life" published in the June 201
As indicated in their study, HZs are commonly defined as the range of removal of a host star in which liquid water can exist on the surface. However, this does not take into account the atmospheric dynamics required to ensure climate stability – including a carbonate-silicate feedback to keep the surface temperatures within a certain range.
With Only Indirect Methods Available to Determine Conditions On distant exoplanets, astronomers rely on sophisticated models of planetary climate and evolution. Dr. Stephanie Olson of the University of Chicago (co-author of the study) described the search for the best living environments on exoplanets to present the synthesis of this approach during the keynote address:
"NASA's search for life in the universe focuses on so-called habitable Zone planets, worlds that have the potential for oceans with liquid water. But not all oceans are equally hospitable – and some oceans will be better habitats than others because of their global distribution patterns.
"Our work aims to identify the exoplanet oceans that have the largest intake capacity for a variety of and active life worldwide. Life in the oceans of the earth depends on the upward movement, which returns the nutrients from the dark depths of the ocean to the sunlit parts of the ocean where the photosynthetic life lives. More upswing means more nutrient intake, which means more biological activity. These are the conditions we need to look for exoplanets. "
For their study, Olsen and her colleagues used the ROCKE 3D software to model what conditions are likely on various species of exoplanets. This general circulation model (GCM) was developed by NASA's Goddard Institute for Space Studies (GISS) to investigate various points in Earth's history and other terrestrial planets of the Solar System (such as Mercury, Venus, and Mars).
This software can also simulate how the oceans' climate and habitats would look on different types of exoplanets. After modeling a variety of possible exoplanets (based on the more than 4,000 previously discovered), they were able to identify which species of exoplanets are most likely to develop and maintain thriving biospheres.
This was to use an ocean circulation model that identified which exoplanets had the most efficient upward movement and could thus obtain oceans with hospitable conditions. What they found was that planets with higher atmospheric density, slower rotation rates and the presence of continents yield higher rates of ascent.
An important finding from this is that the earth may not be optimally habitable due to its relatively fast rotation speed. "This is a surprising conclusion," Dr. Olson, "It shows us that the conditions on some exoplanets with favorable ocean circulation patterns are better suited to support life more or more active than life on Earth."
This is a kind of good / bad news situation. On the one hand, it shatters the illusion that Earth is the benchmark against which other potentially habitable exoplanets can be measured. On the other hand, this suggests that life in our universe may be more numerous than earlier conservative estimates suggest.
But as Olsen has suggested, there will always be a gap between life and what can be perceived by us in our technology because of limitations. This study is significant in that it encourages astronomers to focus their efforts on the subgroup of exoplanets, who most likely "favor large, globally active biospheres where life is easiest to identify – and where non-recognition makes the most sense are".
This will be possible in the coming decade through the use of next-generation telescopes such as the James Webb Space Telescope (JWST), which astronomers expect to be crucial in characterizing the atmosphere and surface environments exoplanets. Other telescopes still on the drawing board could go even further, thanks to such studies.
"Ideally, this work will influence telescope design to ensure future missions," Dr. Olson, "how the proposed LUVOIR or HabEx telescope concepts have the right capabilities; Now we know what to look for.
When it comes to looking for evidence for life beyond (or within) the solar system, it may be even more important to know what to look for than the most sophisticated tools with which to do so. In the coming years, astronomers will benefit from cutting-edge technology and improved methods, using everything we have learned so far to find evidence of a life other than our own.
Reference: Eureka Alert !, Goldschmidt 2019 arXiv