Born to Be Habitable: How the First Moments of Planet Formation Shape Worlds Like Ours

Benjamin Farcy and collaborators argue that a planet’s ability to host life depends largely on the conditions during its earliest stages of formation. Instead of focusing only on a planet’s current environment, they suggest that the very chemistry and structure a planet inherits from its birth within a protoplanetary disk determines much of its future habitability. The paper proposes a “comparative planetology” approach, studying planets as part of their larger system, including their host stars, to better understand the origins of habitable worlds.

Setting the Stage for Habitability

The authors begin by outlining how the field of astrobiology has expanded from searching for life on Mars to investigating habitable worlds across the galaxy. They emphasize that the chemistry of a planet’s bulk material, its rocks, metals, and volatile compounds, sets the foundation for key features such as its atmosphere, magnetic field, and surface conditions. Since these features emerge during the planet’s earliest formation stages, understanding those initial environments is essential for uncovering how planets become suitable for life.

What Planets Are Made Of

Farcy and colleagues highlight the importance of a planet’s bulk composition, dominated by elements like magnesium, iron, silicon, and oxygen. Even small differences in their ratios can influence a planet’s structure, such as the size of its core and the type of crust it develops. These differences are tied to the metallicity of the host star, the amount of heavy elements it contains, which determines the materials available in the surrounding protoplanetary disk. Observations from missions like the upcoming Habitable Worlds Observatory could use spectroscopy to link a planet’s observed atmosphere with its underlying silicate composition. Comparing these results across planets and their stars, the authors suggest, can reveal which combinations of chemical ingredients are most favorable for life.

The Role of Volatiles: Life’s Building Blocks

The next section focuses on volatile elements such as carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. These substances, which easily evaporate, are essential for prebiotic chemistry and the formation of atmospheres and oceans. The authors explain that volatile abundances vary with distance from the host star; for example, Mars is richer in volatiles than Earth, while Mercury is poor in them. They also note that a property called oxygen fugacity, the availability of oxygen for chemical reactions, shapes whether a planet’s mantle is rich in metal or silicates, affecting its core size and magnetic field potential. Farcy and coauthors emphasize that both early disk conditions and later impacts from comets or asteroids can deliver these key volatiles to planets, seeding them with the ingredients for life.

Cores and Magnetic Shields

Planetary cores are central to habitability because they generate magnetic fields that shield atmospheres from stellar winds. Figure 2 in the paper (page 3) shows how the ratio of metallic to oxidized iron controls core size: planets closer to the Sun, like Mercury, have larger metallic cores, while more distant planets, like Mars, have smaller ones. The composition of a planet’s core, set by its oxygen conditions and light element content, affects its ability to sustain a geodynamo, the movement of molten metal that powers magnetic fields. The loss of Mars’s magnetic field about 3.8 billion years ago, for instance, may have allowed the solar wind to strip away its atmosphere and water. Observations of exoplanets with thick atmospheres may therefore signal the presence of strong magnetic protection.

The Heat That Keeps Worlds Alive

The planetary heat engine, a combination of residual heat from formation, radioactive decay, and tidal heating, drives vital processes like mantle convection, volcanism, and tectonics. These, in turn, regulate a planet’s atmosphere and surface water. The authors explain that radioactive elements such as potassium, thorium, and uranium contribute to this heat, and their relative abundances depend on stellar composition. Additionally, gravitational forces from neighboring planets can cause tidal heating, as seen on Jupiter’s moon Io. Understanding these heat sources, Farcy notes, is key to predicting whether an exoplanet can sustain geological activity long enough to support life.

A Cosmic Perspective on Habitability

In conclusion, the paper argues that to understand why some planets are habitable, scientists must study the birth conditions of planetary systems. Comparing Earth and its neighbors, alongside exoplanets observed by telescopes like JWST and Gaia, can reveal how initial chemistry, heat, and structure shape the potential for life. The authors call for the astrobiology community to focus on these formative processes, emphasizing that habitability may not just evolve over time, it may be written into a planet’s origins from the very start.

Source: Farcy