Earth is the only place we know of in the entire Universe that sustains any kind of life.
That doesn't make it the only place per se, and it certainly doesn't necessarily make it the perfect place to support life. That's according to a new study that suggests that ocean currents may be critical in the search for life beyond Earth.
The authors even suggest that Earth could be considered sub-optimal, with distant exoplanets much better suited to supporting life that's both more abundant and more active.
"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," Dr. Stephanie Olson, T.C. Chamberlin Postdoctoral Fellow at the University of Chicago, is expected to say on August 23 at keynote lecture at the Goldschmidt Geochemistry Congress in Barcelona, Spain.
"But not all oceans are equally hospitable - and some oceans will be better places to live than others due to their global circulation patterns."
What do we know about distant ocean-worlds?
Astronomers' understanding of oceanography beyond our solar system is presently rudimentary.
During Exo-oceanography and the search for life in uncharted waters, Stephanie Olson - who studies Earth history, ocean-atmosphere evolution and astrobiology - will describe the search to identify the best environments for life on exoplanets, with the study demonstrating that some exoplanets may have greater variety of life than exists on Earth.
"This is a surprising conclusion", says 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."
The key is the sea
Olson's team modeled likely conditions on different types of exoplanets using ROCKE-3D software developed by NASA's Goddard Institute for Space Studies (GISS).
They simulated the climates and ocean habitats on different types of exoplanets, and were able to define which exoplanet types stand the best chance of developing and sustaining thriving biospheres.
"Our work has been aimed at identifying the exoplanet oceans which have the greatest capacity to host globally abundant and active life", says Olson.
"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."
Why is ocean circulation so important?
The process is pretty simple.
More upwelling means more nutrient resupply, which means more biological activity.
"These are the conditions we need to look for on exoplanets", says Olson.
"We have used an ocean circulation model to identify which planets will have the most efficient upwelling and thus offer particularly hospitable oceans."
The study found that higher atmospheric density, slower rotation rates, and the presence of continents all yield higher upwelling rates.
However, perhaps the most startling implication of the study is that Earth might not be optimally habitable.
Life elsewhere may be present on a planet that is even more hospitable than our own.
Why are 'exo-oceans' important?
We can't visit exoplanets and search for life. Not us (for now...), not robotic probes. They're just too far away.
So we point telescopes at them in an effort to understand what conditions prevail. To compare exoplanets - and to recognize which of them may host life - scientists need sophisticated models of their climates and evolution.
Though life may come in many forms and in many environments, we only know for sure that life exists where a planet's temperature allows liquid water oceans.
So in their search for life on the 4,000+ exoplanets astronomers must target those that will be most favorable to large, globally active biospheres.
On these planets life will be easiest to detect, and if it's not, then we'll learn more about where to look next (and where not to).
How can we check?
Future telescopes will use models like this one to search for bio signs in the atmospheres of exoplanets.
"Ideally this work this will inform telescope design to ensure that future missions, such as the proposed LUVOIR or HabEx telescope concepts, have the right capabilities", says Olson.
"Now we know what to look for, so we need to start looking".
What is LUVOIR?
Due to launch in 2039, the Large UV/Optical/IR Surveyor (LUVOIR) will be a bit of an all-rounder, much like the Hubble Space Telescope.
Using a 15-meter mirror, it will likely be capable of studying exoplanet atmospheres and searching for bio-signatures such as oxygen and methane. It will even be able to directly photograph exoplanets.
What is HabEx?
Now on the drawing board, the Habitable Exoplanet Observatory (HabEx) space telescope will directly photograph exoplanets despite them being 10 million times dimmer than their host star.
That will be thanks to a novel star-shade that will align itself many thousands of miles away from the telescope to suppress the light from faint stars of the exoplanets.
HabEx will also study exoplanet atmospheres and look for signs of life.