The Key to Habitable Alien Worlds Might Lie in Entropy

We are all aware that in order for life to exist on a planet, there are three essential requirements: water, warmth, and food. However, there is another factor known as “entropy” that also plays a crucial role in determining whether a planet can sustain and foster complex life.

Luigi Petraccone, a chemistry researcher at the University of Naples in Italy, has taken a keen interest in planetary entropy. His focus lies in understanding how scientists identify planets that have the potential to support life. In a recently published paper, he delves into the concept of “planetary entropy production” (PEP) and its significance.

A habitable world necessitates the presence of a biosphere teeming with living organisms. All forms of life thrive and expand by utilizing the available resources of water, warmth, and food. Interestingly, entropy manifests within a planet’s biosphere and a relatively high PEP is required.

This high PEP indicates a greater likelihood of complex life systems existing and makes the planet an ideal candidate for exploration. According to Petraccone’s research, the chemical composition of the life forms, whether carbon-based, silicon-based, or composed of another element, is irrelevant. What truly matters is the progression of life towards increased complexity.

What’s Entropy?

Let’s begin by discussing entropy before delving into Petraccone’s paper. In physics, entropy is defined as a thermodynamic quantity that represents the unavailability of a system’s thermal energy for conversion into mechanical work.

According to the second law of thermodynamics, the universe moves in a direction where entropy increases. This concept may seem intricate, so let’s simplify it by considering entropy as a measure of randomness or disorder within a system. An orderly system possesses just enough energy to fulfill its necessary functions.

When a system produces or acquires more energy, it results in a higher level of entropy. Living organisms, for instance, are highly organized and require a continuous input of energy to maintain a state of low entropy. As part of the life process, they generate waste, by-products, and inevitably lose energy. The greater the energy influx and subsequent loss to the surroundings, the more disorderly and random the system becomes. Essentially, its entropy state increases.

In biology, entropy plays a role when examining the systems that contribute to life on a planet. Petraccone explains that the extent of entropy production is directly related to the ability of these systems to dissipate free energy, allowing them to “live,” evolve, and grow in complexity. Generally, a certain threshold of entropy production must be surpassed for the emergence of intricate self-organizing structures. Therefore, entropy production can be seen as the thermodynamic force that propels the emergence and evolution of life.

That brings us to the “planetary entropy production” (PEP) value that can help scientists target likely life-friendly planets.

The most habitable ones will be where life can generate the most entropy. The more complex and dynamic the life forms are, the more entropy they’ll produce and the higher PEP value they maintain. Petraccone proposes that different planets will have more or less of an energy potential, predicting which planets are most likely to be habitable.

Applying Planetary Entropy Production to a Life Search

Determining the location and potential existence of life on a planet is of utmost importance. Initially, it must be situated within the circumstellar habitable zone (CHZ) surrounding its star, where liquid water can be present on the planet’s surface.

The positioning of the planet within the CHZ also plays a significant role. If it is too close to the inner edge, the planet may lose its water due to excessive heating from the star, resulting in a runaway greenhouse effect. On the other hand, if it is closer to the outer edge, it may not be as suitable for supporting life compared to a planet located in the central region of the CHZ. Furthermore, even if a planet is in the ideal part of the zone, it may still face other obstacles in sustaining a biosphere.

Why not explore planets throughout the entire CHZ? This is because there are thermodynamic disparities between the inner and outer edges of the CHZ. The inner edge provides more favorable conditions for the development of complex biospheres.

Both the Planetary Energy Potential (PEP) and the available free energy for Earth-like planets increase with the temperature of the star. Taking this into account, Petraccone and his team utilized their calculations to assess the PEP and free energy for a selected group of proposed habitable planets.

Scientists also need to determine the upper limit of a planet’s PEP value and the corresponding free energy it receives based on the temperature of the star and the planet’s orbital parameters.

For instance, Petraccone explains that only Earth-like planets within the CHZ of G and F stars can possess a PEP value higher than that of Earth (which serves as a reference). This implies that they are more likely to support life compared to planets located in other regions of the habitable zone.

Why Use PEP as a Rationale for Planetary Habitability?