Some exoplanets may have a wider variety of life than Earth
The discovery of planets orbiting stars other than our own has accelerated the search for life elsewhere considerably in the last two decades. But could some of these exoplanets actually enjoy conditions that are more conducive to supporting life than our own?
A new study presented in a keynote speech at Goldschmidt Geochemistry Congress in Barcelona suggests that this could indeed be the case. The study directs astronomers towards the exoplanets best suited to the search for life and suggests ideas that could be implemented in telescope design.
In addition to this, the study provides a shock for us Earth-dwellers — there may be planets in the cosmos far more hospitable to life than our own.
Dr Stephanie Olsen from the University of Chicago and lead researcher on the study, says: “This is a surprising conclusion.
“It shows us that conditions on some exoplanets with favourable ocean circulation patterns could be better suited to support life that is more abundant or more active than life on Earth.”
Whilst the distance to these exoplanets makes even sending an unmanned probe to investigate conditions there virtually impossible. Thankfully advances in remote sensing tools and telescopes provide scientists with a method by which they can investigate the conditions on distant exoplanets.
In order to properly understand these remote observations, the development of sophisticated models for planetary climate and evolution is vital in order to allow scientists to recognize which of these distant planets that might host life.
Olsen’s study presents a new synthesis of just such a model.
She explains: “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.”
The first exoplanet — a planet outside our own solar system or ‘extra solar planet’— was definitively identified in 1992 by radio astronomers Aleksander Wolszczan and Dale Frail. Currently, much of the search for life on exoplanets focuses on those in the habitable zone around stars — the range of distances from a star where a planet’s temperature allows liquid water oceans to exist.
In most models of abiogenesis — the development of the first simple life-forms — see Earth’s primordial oceans as intrinsic to the development of life, it is of little surprise that the search for life elsewhere in the universe should focus on planets that share this feature.
Using software developed by NASA’s Goddard Institute for Space Studies (GISS), Olsen and her team modelled the conditions on several different types of exoplanets. In the process simulating climates and ocean habitats and other important characteristics.
Olsen continues: “Our work has been aimed at identifying the exoplanet oceans which have the greatest capacity to host globally abundant and active life.”
She points out that life in Earth’s oceans depends on upward flow returning nutrients from the dark depths of the ocean to areas bombarded with sunlight — where photosynthetic life lives.
Olsen adds: “More upwelling means more nutrient resupply, which means more biological activity. These are the conditions we need to look for on exoplanets”.
As the team modelled different kinds of exoplanets with varied characteristics, types which were more well-suited to support life, and even ones with burgeoning biospheres where life-thrived, began to emerge.
The team used an ocean circulation model to identify which planets would have the most efficient upwelling — and therefore, the most habitable oceans.
Olsen elaborates: “We found that higher atmospheric density, slower rotation rates, and the presence of continents all yield higher upwelling rates.”
One of the team’s more surprising findings has the discovery that Earth may not be optimally habitable — other planets may enjoy far more favourable conditions for life and thus be far more hospitable.
Dr Olsen also points out, that due to limitations with our technology, life is almost certainly more common than ‘detectable life’ is.
She continues: “ This means that in our search for life in the Universe, we should target the subset of habitable planets that will be most favourable to large, globally active biospheres because those are the planets where life will be easiest to detect — and where non-detections will be most meaningful.”
Olsen points out, that as of yet, telescopes are not sophisticated enough to adequately identify exoplanets and thus test her hypothesis. Despite this, future missions may provide such opportunities.
Olsen notes: “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; now we know what to look for, so we need to start looking”.
Commenting on the research presented by Dr Olsen, Professor Chris Reinhard of the Georgia Institute of Technology, who was not involved in the study, remarks: “We expect oceans to be important in regulating some of the most compelling remotely detectable signs of life on habitable worlds, but our understanding of oceans beyond our solar system is currently very rudimentary.
“Dr Olson’s work represents a significant and exciting step forward in our understanding of exoplanet oceanography”.
Original research discussed at Goldschmidt 2019.