How Mars Helps Us Decipher the Secrets of Distant Exoplanets

Mars occupies a distinct and pivotal position in the history of our Solar System. It stands as a planet situated on the very edge of habitability. Billions of years ago, Mars was warm and wet, creating conditions that may have been hospitable to life as we know it. Today, however, it is a cold, dry, and inhospitable desert. This dramatic transition from a welcoming world to a barren wasteland is crucial for understanding planets beyond our Solar System. Scientists are closely studying this shift to learn how other rocky planets evolve and whether they can support life over long periods.

New research published in the Planetary Science Journal explores this question in depth. The article, titled "Mars as an Exoplanet: Lessons from a Planet at the Edge of Habitability," is led by Stephen Kane, a professor of planetary astrophysics at the University of California, Riverside. The study is also available as a preprint on arXiv, a platform where scientists share findings before formal peer review is complete. This publication offers a fresh perspective on how our neighbor can serve as a proxy for distant worlds.

The authors describe Mars as the "canonical small, rocky planet" in our Solar System. In scientific terms, "canonical" means it serves as a standard example or model. Mars transitioned from early geologic activity and surface liquid water to the cold, arid planet we see today, with a thin atmosphere dominated by carbon dioxide. By studying this transformation, scientists can learn important details about how habitable conditions develop and survive, or fail to do so. The study examines critical factors such as the planet’s size, mass, atmosphere, sunlight exposure, magnetic field, and history of collisions with space rocks.

Our knowledge of exoplanets, which are planets orbiting stars other than our Sun, has grown enormously in recent decades. Surveys of these distant worlds show that small, rocky planets are very common, often outnumbering the larger gas giant planets. However, while we know these small worlds exist in large numbers, we do not yet have a detailed understanding of their climates. We lack information on how much water or gas they hold, nor do we know their long-term potential for habitability. According to the authors of the new study, Mars can serve as a key to understanding these distant cousins.

The researchers point out that while size is a basic property of rocky planets, it does not dictate how a planet evolves. They explain that Venus, Earth, Mars, and even the Moon each underwent distinct changes regarding their volatiles, such as water and gases, tectonic activity, and atmospheres. Despite sharing the same stellar environment, meaning they were influenced by similar types of stars, they evolved differently. This illustrates that planet size alone does not uniquely determine a planet’s future or its ability to sustain life.

In this research, the authors bring together various studies on Mars. They look at how volatiles were delivered to and lost from the planet, the chemistry caused by sunlight, climate evolution, magnetism, and other factors. They argue that these aspects of Mars can help improve our overall understanding of exoplanets and their complex processes.

The authors note that exoplanet studies often use Earth’s properties as standard units of measurement. This is especially true for describing the capabilities of exoplanet detection methods. Mars has many similar properties to Earth, but its differences are what make it important for this work. By studying how Mars differs from Earth, scientists can better identify what allows a planet to remain habitable for a long time versus what causes it to become uninhabitable.

First, Mars formed differently from Earth. Its formation was rapid at first, but then it stalled. It ended up with a mass smaller than Earth’s. The authors describe it as a "stranded planetary embryo" rather than the result of later giant impacts that shaped Earth. The planet’s mass is crucial to its evolution. The researchers state that Mars occupies an important position in comparative planetology. It is a geologically rich world with a documented history of surface habitability. It also serves as a representative example of how small rocky planets can evolve toward atmospheric loss and climatic decline.

Mars can serve as a framework for understanding rocky exoplanets found elsewhere. One of the main conclusions is that planetary habitability is not a static condition. It is not fixed. Instead, the authors describe it as "a time-dependent outcome governed by competing processes."

For example, early Mars was highly volcanic. These eruptions released volatiles that built up a thick atmosphere. This thick atmosphere trapped heat, keeping the planet warm enough for liquid water. But as the planet’s interior cooled, its magnetic dynamo stopped working. Without a protective magnetic field, the solar wind stripped away the atmosphere. This atmospheric escape led to cooling and the eventual loss of habitability. These coupled processes define a pathway that may be common for planets the size of Mars.

According to our current understanding, habitability on Mars-like planets is likely to be fleeting. It is temporary rather than permanent. Earth shines as a rare example of a planet with long-term habitability. The authors write that Mars represents the edge of the habitable regime. It is large enough to host transiently clement conditions, meaning periods of mild weather. However, it is small enough that atmospheric retention, replenishment, and long-term climate regulation are not guaranteed.

While planets with a mass similar to Mars are widely detected in exoplanet surveys, there are shortcomings in those observations. The authors note that while terrestrial-size planets are abundant, confirmed Mars-mass planets with well-constrained masses and radii remain relatively rare. This is largely due to detection shortcomings. Current technology makes it difficult to measure the precise mass and size of smaller planets. This situation will change when the Nancy Grace Roman Space Telescope goes live. Its microlensing survey will allow scientists to find more of these small planets.

As we discover more Mars-mass planets with well-measured constraints, we are also developing future telescopes that are better at observing exoplanets. The researchers explain that direct imaging and thermal emission studies, particularly with next-generation facilities, will ultimately determine whether such planets commonly retain thin carbon dioxide atmospheres. These future observations will also show if these planets undergo desiccation, or drying out, or exhibit transient volatile cycles where water and gases come and go.

The key idea is that scientists can use what they learn about Mars to understand these distant observations. Mars missions will continue to measure atmospheric escape rates, volatile inventories, and climate feedbacks. They do this with a level of detail that is unattainable for exoplanets. Meanwhile, exoplanet surveys contextualize Mars within a broader statistical population of worlds. This allows scientists to see if Mars is an outlier or a typical example.

The researchers explain that as Mars exploration and exoplanet characterization converge, it will deliver an effective new way to better understand the large numbers of small rocky worlds. Scientists will better understand key properties of exoplanets. For instance, they will determine the mass necessary to sustain geological activity, such as plate tectonics. They will also develop a better understanding of the stellar environment and how it shapes atmospheric survival. Other planetary characteristics that shape habitability will also become clearer.

The authors conclude that within this framework, Mars provides a fundamental benchmark. It helps scientists evaluate the diversity, evolution, and potential habitability of rocky planets throughout the Galaxy. By treating Mars as an exoplanet, we gain a powerful tool for decoding the secrets of distant worlds. This approach turns our nearest neighbor into a guide for exploring the unknown cosmos.