When searching for potentially habitable exoplanets, scientists are forced to take the low-hanging fruit approach. Since Earth is the only planet we know of that is capable of supporting life, this search basically comes down to looking for planets that are “Earth-like”. But what if Earth is not the meter stick for habitability that we all tend to think it is?
That was the subject of a keynote lecture that was recently made at the Goldschmidt Geochemistry Congress, which took place from Aug. 18th to 23rd, in Barcelona, Spain. Here, a team of NASA-supported researchers explained how an examination of what goes into defining habitable zones (HZs) shows that some exoplanets may have better conditions for life to thrive than Earth itself has.
The presentation was based on a study titled “A Limited Habitable Zone for Complex Life“, which appeared in the June 2019 issue of The Astrophysical Journal. The study was conducted by researchers from Caltech, the NASA Goddard Institute for Space Studies, the NASA Astrobiology Institute, the NASA Postdoctoral Program, the NExSS Virtual Planetary Laboratory, the Blue Marble Space Institute of Science, and multiple universities.
As they indicate in their study, HZs are commonly defined as the range of distances from a host star within which liquid water can exist on the surface. However, this does not take into account the atmospheric dynamics that are needed to ensure climate stability – which include a carbonate-silicate feedback to maintain surface temperatures within a certain range.
Since only indirect methods are available to gauge what conditions are like on distant exoplanets, astronomers are reliant on sophisticated models for planetary climate and evolution. In the course of presenting their synthesis of this approach during the keynote lecture, Dr. Stephanie Olson of the University of Chicago (a co-author on the study) described the search to identify the best environments for life on exoplanets:
“NASA’s search for life in the Universe is focused on so-called Habitable Zone 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 due to their global circulation patterns.
“Our work has been aimed at identifying the exoplanet oceans which have the greatest capacity to host globally abundant and active life. Life in Earth’s oceans depends on upwelling (upward flow) which returns nutrients from the dark depths of the ocean to the sunlit portions of the ocean where photosynthetic life lives. More upwelling means more nutrient resupply, which means more biological activity. These are the conditions we need to look for on exoplanets”.
For the sake of their study, Olsen and her colleagues modeled what conditions would likely be on various kinds of exoplanets using ROCKE-3D software. This general circulation model (GCM) was developed by NASA’s Goddard Institute for Space Studies (GISS) to study different points in the history of Earth and other Solar System terrestrial planets (like Mercury, Venus, and Mars).
This software can also be used to simulate what climates and ocean habitats would be like on different types of exoplanets. After modeling a variety of possible exoplanets (based on the over 4000 that have been discovered to date), they were able to determine which kinds of exoplanets are the most likely to develop and sustain thriving biospheres.
This consisted of using an ocean circulation model that identified which exoplanets would have the most efficient upwelling and thus be able to maintain oceans with hospitable conditions. What they found was that planets with higher atmospheric density, slower rotation rates, and the presence of continents all yield higher upwelling rates.
A major takeaway from this is that Earth might not be optimally habitable, given its rather rapid rotation rate. “This is a surprising conclusion”, said Dr. Olson, “it shows us that conditions on some exoplanets with favorable ocean circulation patterns could be better suited to support life that is more abundant or more active than life on Earth.”
This is sort of a good news/bad news situation. On the one hand, it does kind of shatter the illusion that Earth is the standard by which other potentially-habitable exoplanets can be measured. On the other hand, it indicates that life may be more plentiful in our Universe than previous conservative estimates would indicate.
But as Olsen indicated, there will always be a gap between life and that which is detectable by us, owing to limitations in our technology. This study is therefore significant in that it encourages astronomers to direct their efforts towards the subset of exoplanets that will most likely favor “large, globally active biospheres where life will be easiest to detect–and where non-detections will be most meaningful”.
This will be possible in the coming decade thanks to the deployment of next-generation telescopes like the James Webb Space Telescope (JWST), which astronomers expect to be instrumental in characterizing the atmospheres and surface environments of exoplanets. Other telescopes, which are still on the drawing board, could go even further – thanks in part to studies like this.
“Ideally this work this will inform telescope design to ensure that future missions,” said 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 looking for evidence of life beyond our Solar System (or within it) knowing what to look for might be even more important than having the most sophisticated tools to do it with. In the coming years, astronomers will have the benefit of cutting-edge technology and improved methods, using everything we’ve learned so far to find evidence of life other than our own.