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Earth at night
Imagery from the NASA-NOAA Suomi National Polar-Orbiting Partnership satellite shows Earth’s lights at night. In a newly published study, researchers argue that planets could be classified based on the effects of energy on the environment. (NASA Photo / Joshua Stevens / Miguel Roman)

A trio of scientists has just laid out a new classification scheme for planets that would put Earth into a hybrid category, making the transition from a diverse, photosynthetic-based biosphere to a world dominated by an energy-intensive civilization.

The researchers’ analysis meshes with the view that humanity’s influence has spawned a new geological epoch known as the Anthropocene Age.

“Our premise is that Earth’s entry into the Anthropocene represents what might, from an astrobiological perspective, be a predictable planetary transition. … In our perspective, the beginning of the Anthropocene can be seen as the onset of the hybridization of the planet,” they say in a study published by the journal Anthropocene.

This isn’t the first time scientists have tried to classify planetary realms based on theoretical scales: Perhaps the best-known example is the Kardashev Scale, set up in 1964

That scale proposed three categories: Type I for civilizations that could manipulate all the energy resources reaching their home planet, Type II for civilizations capable of harnessing all the energy from their home star, and Type III for civilizations that can control the energy of their home galaxy.

By that measure, humanity is a zero, with some hope of reaching Type I someday.

The authors of the Anthropocene study – University of Rochester astrophysicist Adam Frank, University of Washington urban ecologist Marina Alberti and Axel Kleidon, an Earth scientist at the Max Planck Institute of Biogeochemistry – propose a scale that would take in a wider range of environments, with or without life:

  • Class I: Worlds with no atmosphere, such as Mercury or Earth’s moon.
  • Class II: Worlds with a thin atmosphere but no current life, such as Venus.
  • Class III: Worlds with a thin biosphere and some biotic activity, but not enough to affect the environment as a whole. Early Mars might serve as an example, if life was present there billions of years ago.
  • Class IV: Worlds with a thick biosphere, sustained by photosynthetic activity, with a strong effect on planetary energy flow.
  • Class V: Worlds that are profoundly affected by the activity of an advanced, energy-intensive species.

The researchers contend that Earth is in transition between Class IV and V.

So what difference does a classification system make? The authors say the shift in perspective could help policymakers focus on managing the co-evolution of humanity and its host planet.

In a news release, Alberti argued that the search for extrasolar planets is one of the factors behind the paradigm shift.

“The discovery of seven new exoplanets orbiting the relatively close star TRAPPIST-1 forces us to rethink life on Earth,” she said. “It opens the possibility to broaden our understanding of coupled system dynamics and lay the foundations to explore a path to long-term sustainability by entering into a cooperative ecological-evolutionary dynamic with the coupled planetary systems.”

She and her colleagues say focusing on energy disequilibrium as a measuring stick could help guide the search for extraterrestrial life – and plot a course for the future of life on Earth.

“Any world hosting a long-lived energy-intensive civilization must share at least some similarities in terms of the thermodynamic properties of the planetary system,” they write. “Understanding these properties, even in the broadest outlines, can help us understand which direction we must aim our efforts in developing a sustainable human civilization.”

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