When searching for potentially habitable exoplanets, scientists are forced to use the low-hanging fruit approach. Because Earth is the only planet we know that is capable of sustaining life, this search is basically reduced to the search for "Earth-like" planets. But what if the Earth is not the meter of habitation that we all tend to think it is?
This was the topic of a lecture recently given at the Goldschmidt Geochemistry Congress, held from 18 to 23 August, in Barcelona, Spain. Here, a team of NASA-backed researchers explained how a study of what goes into defining habitable zones (HZs) shows that some exoplanets may have better conditions for life to flourish than Earth itself.
The presentation is based on a study entitled "Limited Habitat for Difficult Living," which appears in the June 2019 issue of The Astrophysical Journal. The study was conducted by researchers at Caltech, NASA Goddard Space Research Institute, NASA Astrobiology Institute, NASA Postdoctoral Program, NExSS Virtual Planetary Laboratory, Blue Marble Space Institute, and many universities.
As indicated in their study, HZs are usually defined as the range of star-host distances within which liquid water may exist on the surface. However, this does not take into account the atmospheric dynamics necessary to guarantee climate stability – which include carbonate-silicate feedback to maintain surface temperatures over a range.
Because only indirect methods are available to determine what the conditions of distant exoplanets are, astronomers rely on complex models for planetary climate and evolution. In the course of presenting his synthesis of this approach during a keynote lecture, Dr. Stephanie Olson of the University of Chicago (co-author of the study) described the demand for identifying the best extraterrestrial environments:
The NASA universe is focused on the so-called habitable planets, which are worlds that have the potential for liquid water oceans. 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 is aimed at identifying the exoplanetary oceans that have the greatest capacity to live abundant and active lives globally. Life in the Earth's oceans depends on an inflow (upward flow) that returns nutrients from the dark depths of the ocean to the sunny parts of the ocean, where photosynthetic life lives. Most swelling means more nutrients, which means more biological activity. These are the conditions we must look for on exoplanets. "
For the sake of their research, Olsen and her colleagues modeled what conditions would likely be on different types of exoplanets using the ROCKE-3D software. This General Circulation Model (GCM) was developed by the NASA Godard Space Research Institute (GISS) to explore different points in Earth's history and other Earth-to-Earth planets (such as Mercury, Venus, and Mars).
This software can also be used to simulate the climate and ocean habitats of different exoplanets. After modeling various exoplanets (based on over 4,000 discovered so far), they were able to determine which exoplanets are most likely to develop and sustain thriving biospheres.
This consisted in the use of an ocean circulation model that identified which exoplanets would have the most efficient development and thus could maintain oceans with hospitable conditions. What they found was that planets with higher atmospheric density, lower rotational speeds, and the presence of continents lead to higher rotational speeds.
The main draw from this is that the Earth may not be optimally habitable, given the rather fast rotational speed. "This is a surprising conclusion," says Dr. Olson, "it shows us that the conditions of some exoplanets with favorable ocean circulation patterns may be more suited to sustaining a life that is more abundant or more active.
This is kind of good news / bad news. On the one hand, it breaks the illusion that Earth is the standard by which other potentially habitable exoplanets can be measured. On the other hand, it shows that life may be more abundant in our universe than previous conservative estimates would show.
But as Olson pointed out, there will always be a chasm between life and the one we discover because of limitations in our technology. Therefore, this study is important because it encourages astronomers to focus their efforts on the subset of exoplanets most likely to favor "large, globally active biospheres, where life will be easiest to discover – and where undetection will be most meaningful" ".
This will be possible in the next decade thanks to the deployment of next-generation telescopes such as the James Webb Space Telescope (JWST), which astronomers expect to play an important role in characterizing the atmosphere and extraterrestrial environments. Other telescopes still on the drawing board could go even further – thanks in part to studies like this.
"Ideally, this work will inform the design of the telescope to ensure that future missions," says Dr. Olson, "such as the proposed LUVOIR or HabEx telescope concepts have the right capabilities; now we know what to look for, so we need to start looking. "
When it comes to finding evidence of life beyond (or within) our solar system, knowing what to look for may be even more important than having the most sophisticated tools to deal with . In the coming years, astronomers will take advantage of state-of-the-art technology and advanced methods, using everything we've learned so far to find evidence of a life different from our own.
Additional reading: Eureka Alert !, Goldschmidt 2019 arXiv