Authors: Abel Méndez, Edgard G. Rivera-Valentín, Dirk Schulze-Makuch, Justin Filiberto, Ramses M. Ramírez, Tana E. Wood, Alfonso Dávila, Chris McKay, Kevin N. Ortiz Ceballos, Marcos Jusino-Maldonado, Nicole J. Torres-Santiago, Guillermo Nery, René Heller, Paul K. Byrne, Michael J. Malaska, Erica Nathan, Marta Filipa Simões, André Antunes, Jesús Martínez-Frías, Ludmila Carone, Noam R. Izenberg, Dimitra Atri, Humberto Itic Carvajal Chitty, Priscilla Nowajewski-Barra, Frances Rivera-Hernández, Corine Y. Brown, Kennda L. Lynch, David Catling, Jorge I. Zuluaga, Juan F. Salazar, Howard Chen, Grizelle González, Madhu Kashyap Jagadeesh, and Jacob Haqq-Misra
First Author’s Institution: University of Puerto Rico at Arecibo
Status: Published in Astrobiology [open access]
How to determine the potential habitability – the ability of an area to host life – of exoplanets is one major open question in the field of astrobiology. Often, habitability is discussed purely in terms of whether a planet resides in its host star’s habitable zone. However, the habitable zone only describes whether the planet is at the right temperature to have liquid water. It does not account for the planet’s atmospheric composition or other factors that determine if life can truly be sustained in an area. Today’s authors propose using modeling methods already used by Earth ecologists to develop more detailed habitability models for exoplanets.
Modeling in Ecology
Not all areas of Earth are equally habitable for all organisms – you wouldn’t expect to find whales in the Sahara Desert, for example. Ecologists have well-established models for determining the habitability of these different terrestrial environments, called Habitat Suitability Models (HSMs, also known as species distribution models). To establish these HSMs, ecologists consider factors such as temperature, available nutrients, and disturbances to the environmental systems, then prepare and calibrate a model from there. This leads to a Habitat Suitability Index (HSI) for specific types of life in specific habitats relative to the factors that went into the model, which gives an idea of how habitable an area is for a certain species or group of species.
Though the authors recommend astrobiologists adapt these models, they acknowledge that there are challenges, primarily the fact that the HSMs can only be validated for other planetary environments if they happen to align with an environment on Earth. The authors recommend four main action items to address these challenges:
- Involve more ecologists in astrobiology to help with the standardization, since they can use their knowledge of terrestrial habitability to extrapolate to other environments.
- Create more connections with scientists studying lesser-understood Earth environments, such as the deep ocean and stratosphere.
- Identify knowledge gaps in existing habitability models, so they can be addressed in this process.
- Develop a standard measurement of habitability based on observables from lander and orbiter missions.
Establishing the Model for Astrobiology
To guide the development of similar habitability models for astrobiologists, the authors recommend that scientists keep in mind the following questions:
- What variables do you have for your model? Temperature, nutrient concentrations, and precipitation amounts, for example, are all good places to start, although any known information can help.
- What area on Earth is comparable to the area being studied (if any)? Determining this area gives a more tangible
- Once you’ve determined the limiting factors and the Earth analog for your target, how habitable does the target seem to be? What information do you have that suggests it would be habitable or non-habitable? This assessment would require establishing a habitability scale, and a threshold for the target being habitable.
- How much biomass — in this case the total mass of organisms in a given area or volume, not fuel — could the planet support? Likely only an upper limit could be determined without being able to visit the planet, but it would be a start.
- How well do we expect habitability to be correlated with biosignatures such as atmospheric methane? On Earth they are well-correlated, but that isn’t necessarily the case for other planets or for “life as we don’t know it” (i.e., non-carbon based life).
The authors suggest that the astrobiology community should use these questions to guide creating consistent models, and should not reinvent the wheel: ecologists already have tools in place for Earth (many of them available freely through GitHub or as R packages), and astrobiologists can just build from there, accounting for the different knowns and unknowns. By creating standardized models, scientists can more easily compare and contrast the habitability of different bodies, and more information can likely be learned overall!
Astrobite edited by Sumeet Kulkarni
Cover image credit: Kristian Peters