Since 1992, NASA has confirmed the existence of over 6,000 planets outside of our solar system, and over 7,000 suspected planets still await confirmation. These distant planets, dubbed “exoplanets,” orbit around roughly 4,700 of the more than 100 billion stars estimated to populate our Milky Way galaxy. Current estimates predict about one exoplanet for every star in the galaxy.

In just a few decades, scientists have gone from wondering if exoplanets exist to making a mass effort to characterize thousands of them. Within this intragalactic exploration is also a search for life, or at least a planet with conditions that could sustain life as we know it.

“The only way that we’re going to ever find out if there are signatures of life out there is by observing the atmosphere of these planets,” said Michelle Hill, a postdoctoral researcher with Laura Schaefer’s Planetary Modeling Group in the Stanford Doerr School of Sustainability.

Hill developed the Smaller Than Earth Habitability Model (STEHM) to explore factors that influence a planet’s ability to create and maintain an atmosphere within the context of its size. Published in The Planetary Science Journal on June 4, this research helps narrow down the vast, expensive search for potentially habitable planets to those that meet minimum size requirements.

Sifting for size, then surface conditions

A planet’s atmosphere is a shield of gases that protects its surface from the harsh conditions of space. Having enough bulk to generate surface gravity is critical for keeping this atmosphere in place, but mass is not the only characteristic that affects a planet’s atmospheric retention. Stellar radiation and particles constantly try to strip a planet’s atmosphere away, while volcanic activity at the surface and concentrations of carbon and other elements all play a role in a planet’s ability to create and replenish an atmosphere.

To produce STEHM, Hill employed ExoPlex, a Python-based code that uses a planet’s radius and internal pressures to calculate its mass and internal characteristics. Hill created six different planet profiles that ranged from half the size of Earth’s radius to the same size as Earth (in paper: 0.5 R⊕ to 1.0 R⊕) and included features like the density and thickness of the mantle, as well as the planet’s overall density. All planet profiles were based on CO₂ atmospheres on rocky, “stagnant lid” planets that, unlike Earth’s ever-shifting crust, have rigid surfaces.

Results from STEHM indicate that 0.8 R⊕ planets, or planets with a radius that’s at least 80% of the length of Earth’s radius, can maintain their atmospheres for 10 billion years or more if they’re a comfortable distance from a star similar to our sun. Smaller planets tend to lose their atmosphere within 1 billion years, but planets with 0.7 R⊕ may have a chance at maintaining their atmospheres if other factors work in their favor.

STEHM revealed that one of the most important traits for retaining an atmosphere is the planet’s initial carbon content. Because planets form from collisions of gas and dust particles that circulate around a star, each planet’s composition depends largely on the chance elemental makeup of that solar system. Higher levels of CO₂ – a greenhouse gas that envelops the planet – help keep life-sustaining heat contained. Volcanic activity on a planet’s surface releases CO₂. Concentrations of heat-producing elements, such as the radiogenic materials thorium, uranium, and potassium, in the planet’s mantle also play a role. These are known as heat-producing elements because they release heat as they decay, which extends the cycle of heating and replenishment. As they become depleted, the mantle starts to cool. This mantle cooling extinguishes volcanic activity, which halts CO₂ production and eventually leads to atmospheric loss.

Planets with a smaller core and thicker mantle can potentially hold higher concentrations of carbon and heat-producing elements, allowing them to replenish their atmospheres for a longer time.

STEHM also demonstrated that too much heat too early can reduce the longevity of an atmosphere. Hill described “hot-start” planets as ones that have higher internal temperatures at their formation that melt the mantle and its heat-regulating features early on. This can make the planet’s atmosphere more exposed during peak levels of stellar radiation, which initially declines with time as the star ages.

A planet’s risk of stellar radiation depends on its position within the star’s habitable zone, or being just far away enough to avoid burning up but not too far into the frigid outer reaches of the system. Extreme heat from radiation can split heavy CO₂ molecules into separate oxygen and carbon molecules that are lighter and more easily swept away. Sometimes these “escaping” molecules have enough momentum to drag others with them on their exit path, which further adds to atmospheric loss.

Taking all of these dynamics into account, Hill refined a simple search-by-size filter that can help scientists make more targeted investigations of potentially habitable planets.

“Maybe there’s life on other planets under the ground, but we are never going to be able to see it because we can’t send something to those exoplanets,” said Hill. “The best chance we’ve got is looking for signs of life by analyzing atmospheres from afar.”

Past, present, and future atmospheres

Hill validated STEHM’s ability to predict a planet’s atmospheric fate by inputting measurements of Earth’s closest neighbors, Venus and Mars, which sit at opposite ends of the atmosphere spectrum. The model correctly predicted that Venus has a thick, enduring CO₂ atmosphere and Mars has an extremely thin atmosphere that has dissipated over time.

Knowledge of the weak atmosphere on Mars was actually the original inspiration for STEHM. With all the public hype around “terraforming” Mars for human habitation, Hill decided to investigate if Mars ever stood a chance at maintaining an atmosphere in the past or present. STEHM indicates that even with the most favorable starting conditions, Mars’s small size and lack of plate tectonics mean the odds have always been stacked against it.

Timelines of planet formation and atmosphere retention spanning billions of years remain a mystifying hurdle in the search for extraterrestrial life, raising the question of when life can occur in addition to where it can occur.

“Maybe the answer to why we haven’t found any life yet is that we’re so early in the grand scheme of what has been created through the lives and deaths of stars,” said Hill. “Maybe we’re one of the first.”

Hill is already excited about the next phase of her research, which will create profiles of “mobile lid” rocky planets with tectonic activity similar to Earth and compare them to the “stagnant lid” planets in the current model.

Such research on exoplanets offers continual rediscovery of how truly unique Earth is in comparison to the thousands of other planets identified so far.