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When they search for potentially habitable exoplanets, scientists are forced to adopt a pragmatic approach. Since Earth is the only planet we know that is able to support life, this search is essentially a search for planets resembling the Earth. But what if the Earth is not the way to measure the habitability we all tend to think?
This was the subject of a recent conference at the Goldschmidt Geochemistry Congress, which took place from 18 to 23 August in Barcelona, Spain. A team of researchers supported by NASA has explained here that a review of what comes into the definition of habitable zones (ZS) shows that some exoplanets can offer better living conditions than the Earth itself.
The presentation was based on a study entitled "A Limited Living Area for a Complex Life", published in the June 2019 issue of The astrophysical journal. The study was conducted by researchers at Caltech, NASA's Goddard Institute for Space Studies, NASA Astrobiology, NASA Postdoctoral Program, NExSS Virtual Global Lab, Institute Science of Blue Marble and many universities.
As they indicate in their study, HZs are generally defined as the distance that separates a host star from the presence of liquid water on the surface. However, this does not take into account the atmospheric dynamics necessary to ensure climate stability, which includes carbonate-silicate feedback to maintain surface temperatures within a certain range.
As only indirect methods are available to evaluate conditions on distant exoplanets, astronomers depend on sophisticated models for climate and the evolution of the planet. In presenting the synthesis of this approach at the main conference, Dr. Stephanie Olson of the University of Chicago (co-author of the study) described the research to identify the best living environments for exoplanets :
"The search for life in the NASA universe is centered on so-called habitable planets, which are potentially water-filled oceans worlds. But not all oceans are equally hospitable – and some oceans will be better places to live than others because of their global circulation patterns.
"Our work was aimed at identifying the exoplanet oceans that have the greatest ability to host an active and abundant world life. Life in the Earth's oceans depends on upwelling, which sends nutrients back from the dark depths of the ocean to the sunny parts of the ocean where photosynthetic life lives. More upwelling means more replenishment of nutrients, which means more biological activity. These are the conditions we must look for on the exoplanets ".
As part of their study, Olsen and his colleagues modeled the conditions that could prevail on different types of exoplanets using the ROCKE-3D software. The NASA Goddard Institute for Space Studies (GISS) has designed this model of general circulation to study different points in the history of the Earth and other terrestrial planets of the solar system (such as Mercury, Venus and Mars) .
This software can also be used to simulate ocean climates and habitats on different types of exoplanets. After modeling various possible exoplanets (out of more than 4,000 discoveries to date), they have been able to determine which types of exoplanets are most likely to develop and maintain burgeoning biospheres.
This involved using an ocean circulation model identifying exoplanets with the most efficient upwelling and therefore able to maintain the oceans in hospitable conditions. What they found is that planets with higher atmospheric density, lower rotational speeds and the presence of continents all generate higher rates of lift.
We can deduce that the Earth may not be optimally habitable, given its speed of rotation quite fast. "It's a surprising conclusion," said Dr. Olson, "that shows us that conditions on some exoplanets with favorable ocean circulation patterns might be better suited to a life more abundant or more active than life on Earth . "
It's kind of good / bad news. On one side, it breaks somehow the illusion that the Earth is the standard by which other potentially habitable exoplanets can be measured. On the other hand, this indicates that life may be more abundant in our universe than previous conservative estimates suggested.
But as Olsen said, there will always be a gap between life and what we detect, because of the limitations of our technology. This study is therefore important because it encourages astronomers to focus their efforts on the subset of exoplanets that will probably favor "large active biospheres at the global level where life will be easier to detect and where non-detections will be more meaningful. ".
This will be possible in the next decade through the deployment of new generation telescopes such as James Webb Space Telescope (JWST), that astronomers expect to play in the characterization of atmospheres and surface environments of exoplanets. Other telescopes, which are still on the drawing board, could go even further – thanks in part to such studies.
"Ideally, this work will guide the design of the telescope to ensure that future missions," said Dr. Olson, "such as the concepts offered by the LUVOIR or HabEx telescope, have adequate capabilities; now we know what to look for, so we have to start looking. "
When it comes to looking for evidence of life beyond our solar system (or within it), knowing what to look for may be even more important than have the most sophisticated tools to do it. In the coming years, astronomers will benefit from advanced technology and improved methods, using everything we have learned so far to find evidence of life other than ours.
Further reading: Eureka Alert!, Goldschmidt 2019, arXiv
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