Blueprints for habitability

Researchers can also tweak their planetary models to create an infinite number of blueprints for possibly habitable exoplanets. For example, they can use models that can speed up the planet’s rotation, adjust the tilt of its axis, put all of the continents in one hemisphere (or remove them completely), or allow one side of the planet to face its star continuously. Continents are an integral component of oceans’ habitability. Through the weathering of land surfaces, nutrients enter the oceans to nourish the life within them, and the positions and elevations of these landmasses alter how these nutrients move to and through the oceans.

“These factors also influence the communication between the ocean and the atmosphere, and thus the detectability of life in the ocean,” Olson says. “Understanding how planetary parameters influence biological activity and ocean–atmosphere connectivity can help identify the most promising targets for exoplanet life detection that will be least vulnerable to biosignature false negatives.”

The possibility of false negatives – when there is actually life on an exoplanet but the signatures of that life escape detection – fascinates the Alternative Earths team.

In a 2017 paper led by Chris Reinhard at Georgia Tech, the Alternative Earths team flagged the danger of false negatives in the hunt for habitable planets. The presence of both methane and oxygen in an atmosphere has been viewed as a gold standard in the search for distant life. These two gases should not coexist in appreciable amounts, as they react rapidly with each other, but living organisms can constantly replenish them in the atmosphere, allowing this disequilibrium to persist.

However, if researchers were looking at early Earth over most, if not all, of its history, they may not have been able to detect both methane and oxygen in the ancient atmosphere, despite life being present for much of that time.

“[Detecting] atmospheric methane would have been problematic for most of the last ~2.5 billion years of Earth’s history,” Reinhard and colleagues write. For rocky worlds with oceans, such as Earth, these gases could be recycled within the oceans, rather than being detectable in the atmosphere. This possibility implies that “planets most conducive to the development and maintenance of a pervasive biosphere, such as those with weathering continents and vast oceans, will often be challenging to characterize using conventional atmospheric biosignatures”, they write.

Additionally, even if both oxygen and methane are present, they are not necessarily products of life.

Oxygen can be the result of photosynthesis, and microbes produce methane, but they can also form through photochemical and geological processes. In fact, the NASA Astrobiology Institute has a team investigating methane production via geological rather than biological reactions.

“The products of those reactions could sustain life on an ocean world, but the gases themselves may have nothing to do with life,” Lyons says. “You cannot evaluate what the gases mean without a rigorous context.”

“We typically view habitability as binary: a planet can either support life or it cannot, but there likely exists a spectrum of habitability,” adds Olson.