Planetary Entropy Production (PEP) from a Frederick study.

The increasing rate of discovery of exoplanets around nearby stars requires additional criteria useful for classifying their habitability and selecting candidates to which to devote a more challenging observational campaign. In a study published in the journal Monthly Notices of the Royal Astronomical Society, Professor Luigi Petraccone, professor of physical chemistry in the Department of Chemical Sciences at Federico II University, introduced the concept of planetary entropy production (PEP), which could prove crucial in determining a planet's suitability for complex life.

The basic idea starts from the observation that any biosphere in order to originate, evolve and grow must have the ability to produce entropy at a rate proportional to its complexity. The same is true for all those climatic processes that are intimately connected with the development and sustenance of the biosphere. With this in mind, it is proposed in the paper that the upper limit of a planet's PEP value can provide a first estimate of the thermodynamic potential of the planetary environment to sustain a complex biosphere. Through his study, Professor Petraccone provides a simple method for estimating the upper limit of the PEP value and the corresponding free energy as a function of the temperature of the host star and the orbital parameters of the planet. Application of the method to a series of Earth-like exoplanets discovered in recent years shows that for none of them is the PEP value of Earth reached suggesting that exoplanets orbiting small-mass stars may not be capable of developing life and/or sustaining an Earth-like biosphere.

The study also shows that there are significant thermodynamic differences between the inner and outer edges of the circumstellar habitable zone, with the inner edge being thermodynamically more advantageous for the development of complex biospheres. The study could offer a new perspective in the search for extraterrestrial life by introducing a useful thermodynamic index to assess an exoplanet's potential to support a complex biosphere.