Humanity's indelible impact on our home planet has marked the dawn of a whole new geological epoch, a sweeping change that could in fact signal an important step in the evolution of alien civilisations.
Earth's emergence into the Anthropocene – the proposed geological era dominated by human-made materials and emissions – amounts to a predictable milestone in planetary development, a new study suggests.
According to urban ecologist Marina Alberti from the University of Washington, Earth could currently be categorised as a kind of hybrid planet, according to a new astrobiological classification scheme that ranks planets based on how advanced they are in terms of the energy-harvesting ecology that thrives upon them.
Of course, the most famous example of a classification system like this is the influential Kardashev scale, devised by Soviet astrophysicist Nikolai Kardashev in 1964.
Kardashev proposed there were three stages of a civilisation's level of technological advancement.
Type I civilisations have learned to harvest every ounce of energy from their home planet, which would include maximising the yield of all possible power on the planet, including all solar, thermal, and oceanic energy, and more besides.
Human beings are still a long, long way from qualifying as a Type I then, but we're even further from being a Type II, in which alien peoples can harness the total energy of their planet's host star – something that would require a Dyson sphere (or similar) to achieve.
Type III civilisations in Kardashev's model have progressed so far along their techno-evolutionary path that they've actually figured out how to squeeze all the available energy from their entire galaxies. Neat.
In the opinion of Alberti and her fellow researchers, though, the Kardashev scale, as awesome as it is, reflects a belief in a virtually limitless Star Trek level of technological possibilities – a likely byproduct of its '60s heritage.
"The Kardashev scale originated from a particular historical moment in thinking about exo-civilisations, in which technology would be unconstrained, hence its focus on energy consumption alone," the authors write in their paper.
"In the years since Kardashev proposed the classification system, we have learned (the hard way perhaps) that biospheres are not so easily ignored."
In their new ranking scheme, the team suggests five stages in the progressive evolutionary state of planets, based on how sustainable exo-civilisations are in terms of realising "agency-dominated biospheres".
It comes down to how successfully life co-evolves with the planet it lives on – something you can see in how human life now has an enduring impact on Earth, via the behaviours that ushered in the Anthropocene.
"Our thesis is that the development of long-term sustainable versions of an energy-intensive civilisation must occur on a continuum of interactions between life and its host planet," the researchers write.
In the team's vision, Class I represents worlds with no atmosphere, like Mercury. Class II are planets with thin atmospheres, such as Mars, but no life forms.
Class III is where things get interesting – world's with a thin biosphere and organic life forms, but life that hasn't learned how to alter the evolutionary state of the planet.
According to the team, the Anthropocene is evidence that Earth has just left Class IV: worlds with a thriving biosphere where life forms have started to effect changes on a planetary scale.
So where are we headed? Into Class V, in which a future Earth has the potential to be profoundly affected by an advanced, energy-intensive species of human beings.
"Even with highly pessimistic assumptions about the probabilities for the evolution of technological civilisations over the history of the Universe, there likely have been many Class V planets across cosmic history (though we note that our galaxy could still be sterile now)," the team explains in their paper.
That said, if humanity is ever going to realise this far-off dream before wrecking the planet, the researchers think we'll have to master and fully exploit renewable energies, and particularly solar power.
Not that the researchers are presenting this as some kind of blueprint for exoplanet observations. Instead, they say, it's more about providing a framework that can explain how the Anthropocene makes sense in an astrobiological context.
But if we want to treat it like a map, that might not hurt either.
After all, the team thinks the system "might aid in understanding what sustainable outcomes must look like… if one does not know where one is going, it will be hard to get there".
The findings are reported in Anthropocene.