Under Pressure: How the Density of Exoplanets' Atmospheres Weighs on the Odds for Alien Life

At this early stage in the search for extraterrestrial life in our solar system and beyond, the emphasis is on liquid water. Where it can exist on a planet’s or moon's surface, so the thinking goes, life as we know it has a chance. Much of the observational and theoretical work in astrobiology therefore concerns the "habitable zone," the orbital band around stars where a rocky world's water neither freezes away nor boils off.

In a new contribution to this effort, a recent study has looked at a little-explored influencer on the ability of water to remain liquid on a world's surface: atmospheric pressure.

"Atmospheric pressure affects the liquid water temperature range that is commonly used to define planetary habitability," said Giovanni Vladilo of the Trieste Astronomical Observatory in Italy and lead author of the paper published in the Astrophysical Journal. "So, if you wish to estimate habitability, you should explicitly take into account pressure in your problem."

On Earth, the space around us is filled with air molecules that collectively weigh on our bodies. Although you cannot feel it, Earth's atmosphere presses down with the force of one kilogram per square centimeter, or 14.7 pounds per square inch. That pressure works out to about a ton per square foot. Our terrestrial biology evolved to operate in this pressure, which, while startling-sounding, pales when compared to underwater creatures' bodies in the deep sea that can withstand dozens of tons per square foot.