Humans have had such a dramatic impact on Earth that some scientists say we've kickstarted a new geological era known as the Anthropocene. A fascinating new paper theorises that alien civilizations could do the same thing, reshaping their homeworlds in predictable and potentially detectable ways. The authors are proposing a new classification scheme that measures the degree to which planets been modified by intelligent hosts.
Whenever a distant exoplanet is discovered, astronomers categorise it according to its most obvious physical features and orbital characteristics. Examples include hot-Jupiters, Earth-like terrestrial planets, and brown dwarfs. With ongoing advances in telescope technology, the day is coming when astronomers will be able to expand on these simple characterizations, classifying a planet according to other features, including atmospheric or chemical composition.
But as a new study led by University of Rochester astrophysicist Adam Frank points out, we may eventually be able to place exoplanets within an astrobiological context, too. In addition to taking the usual physical measures into account, Frank and his colleagues are proposing that astronomers take the influence of a hypothetical planet's biosphere into account -- including the impacts of an advanced extraterrestrial civilisation. Frank's hypothetical planets, ranked from Class I through to Class V, range from dead, rocky worlds through to planets in which a host intelligence has solved the problems caused by its own existence, like excessive use of resources and climate change. Moreover, as Frank explained to Gizmodo, this paper presents more than just a planetary classification scheme -- it's a potential roadmap to an environmentally viable future. If we discover signs of an advanced alien civilisation -- and that's a big if -- we may learn a thing or two about how we might be able to survive into the far future.
"We're currently trying to understand the next steps we need to take to become a sustainable civilisation, and that's really why we're working on this," said Frank. "We need to develop a sustainable version of the civilisation that we currently have -- but how do we know this is even possible? Does the Universe even create civilizations that do that?"
Indeed, we're at a critical juncture in our history, one in which we're crafting the planet according to our will -- and so far, we're not doing a very good job of it. There's ongoing debate as to whether or not our planet has crossed into the Anthropocene epoch, a new geologic chapter in which we've become the primary driver of planetary change. Some scientists point to the fact that half of the planet's land surface has been claimed for human use, or that Earth's biogeochemical cycles of nitrogen and phosphorus have been radically altered on account of agriculture and fertiliser use, as evidence that we have.
The US Eastern seaboard at night. (Image: NASA)
While the technical debate over what constitutes evidence of a geologic shift continues, it's clear humanity is altering Earth in some rather profound ways. So much so, says Frank, that we need to place our planet, and the Anthropocene itself, within an astrobiological context. What's happening here on Earth, says Frank, is likely happening elsewhere in the Galaxy. Though we may be inclined to think that our situation is somehow special or unique, we have no good reason to believe that's really the case.
"We need to look at our presence here on Earth not as a one-time story, but as something that has happened a lot," Frank told Gizmodo. "We already know a huge amount about how planets work, and how planetary systems respond to [chemical and biological] inputs. With any civilisation, therefore, the results are entirely predictable. Sure, there will be different sociological aspects, but all civilizations [for example] need to harvest energy and use it." Frank goes to far as to say that virtually every advanced civilisation like ours, should they exist, has to tackle the issue of climate change at some point in its development.
As the new paper points out, humanity, or any alien intelligence for that matter, is simply another expression of the biosphere:
Any world hosting a long-lived energy-intensive civilisation must share at least some similarities in terms of the thermodynamic properties of the planetary system. Understanding these properties, even in the broadest outlines, can help us understand which direction we must aim our efforts in developing a sustainable human civilisation.
"We have to place ourselves within that context," says Frank. "But we currently don't know how."
Which is where the new planetary classifications come in. In their new paper, Frank and his colleagues consider the history of coupled Earth systems, that is, the tightly bound relationship between the planet's atmosphere, ocean, ice, lithosphere (the ground), and biosphere. All these systems affect each other, changing the overall constitution of the planet. Without the biosphere, for example, we wouldn't have oxygen. And today, with our copious greenhouse gas emissions, we're affecting the atmosphere even further. With this in mind, the authors present a planetary classification scheme based on "the magnitude by which different planetary processes -- abiotic, biotic and technologic -- generate free energy, i.e. energy that can perform work within the system."
Under the proposed scheme, a Class I planet is a world with little or no atmosphere at all, similar to Mercury. A Class II planet has a thin atmosphere containing greenhouse gases, but no life (think Mars), while Class III planets have a thin biosphere and some basic forms of life; we have no examples of this planet in our Solar System, but early Earth may have passed through this phase. A Class IV planet is where we find ourselves at the current moment -- an orb with a thick biosphere that's sustained by photosynthetic activity and where life (including intelligent life) is strongly affecting the planetary flow of energy. Finally, there's the hypothetical Class V planet -- a world in which a technological civilisation has found a way to live in harmony with its home planet.
Image: Frank et al., 2017
"A Class V is where the energy harvesting civilisation has come to some kind of cooperative arrangement with the biosphere," says Frank. "It's figured out how to harvest energy from the coupled planetary systems in a way that doesn't drive the planet into a detrimental situation, such that the intelligent civilisation doesn't kill itself."
At this stage, says Frank, a civilisation has learned how to think like a planet, and it's figured out the rules of the game. It's able to modify and adjust its own activities, and likely the biosphere itself, such that the civilisation -- and planet -- can be sustainable over the longterm. Frank suspects that we'll eventually be able to detect Class V planets, picking up on various technosignatures, such as those produced by massive planetary solar farms, or signs of energy being converted into useful work, leading to an unusual increase in energy dissipation not indicative of natural sources.
Not coincidentally, this scheme is reminiscent of the Kardashev scale -- an attempt to classify the amount of energy available to hypothetical extraterrestrial civilizations. According to this scheme, Kardashev I civilizations have harnessed the power of their host star, KII civilizations can capture all the energy produced by their Solar System, and KIII civs have galactic-scale energy at their disposal. Frank says that his proposed planetary scale can work alongside the Kardashev schema, but that in order for a civilisation to reach KII status it must first inhabit a Class V planet.
"You've gotta get to a Class V planet and be there for a while before you get to become a KII civilisation," he told Gizmodo. "Only then can you start worrying about building Dyson spheres [and other megaprojects]. A civilisation has got to make it through the bottleneck, or it won't make it any further. Only then can we talk about how realistic it is to move forward. The Kardashev scheme makes no sense if you can't even get to a KI status -- planets have their own rules."
Frank says his team's proposed planetary classifications are just a start, and that their team is merely pointing towards a potential new area of research. "I'm hoping this is the beginning of a new field -- the astrobiology of the Anthropocene," he said. "This study is the beginning, not the end."
Avi Loeb, an astronomer at the Harvard-Smithsonian Center for Astrophysics who wasn't involved in the new study, says the new framework has practical connections to future observations of planetary atmospheres, while also encouraging us to reflect on the effect we have on Earth.
"For example, by observing other Earth-like planets, we might detect evidence for 'environmental graveyards' of civilizations that destroyed their natural habitat," Loeb told Gizmodo. "Perhaps this will convince politicians to take better care of our own planet in order to avoid a similar fate. This is one interesting way by which astronomy could have an effect on our society."
Anders Sandberg, a researcher at Oxford's Future of Humanity Institute, likes the "thermodynamic" approach, saying that civilizations are physical systems, too.
"Analysing how the energy, matter and information flows is useful to get some bounds on what they can do and how they look," Sandberg told Gizmodo. "Just like we may want to classify planets by their geochemistry and maybe their type of biosphere, it makes sense to add the civilisation type, too. Of course, to actually use it we need to observe a few planets like this."
Sandberg, who wasn't involved in the new study, wonders if the classification system can include higher levels, such as Class VI or VII planets, and include something beyond a purely technological civilisation. But the problem as he sees it is that it may be difficult to tell the levels apart.
"To us, life is very dissimilar from non-life, but I wonder if there is always an obvious separation," he says. "Similarly, should we consider anthills and beaver dams as the earliest traces of a technosphere? The useful thing is that it helps analyse the whole issue in terms of physics -- but that may also be a problem: what would this scheme miss? As [the researchers] point out about the Kardashev scale, it just describes the amount of energy flowing, not how it is flowing. I might say that it also matters how the energy is controlled and coordinated -- are the flows organised on large scales, and how?"
No doubt, Frank's new classification system is far from perfect and certainly not complete. But like in any burgeoning field, there are more mysteries than there are answers. Hopefully, with the next generation of space-based telescopes we'll start to detect these technosignatures in the atmospheres of distant exoplanets and take inspiration from civilizations that have somehow figured out a way to live in harmony with their planet.
It's also entirely possible that our searches will yield nothing. Our technologies may never get so advanced that we can detect biosignatures of microbial life on other planets, let alone the subtle differences between a Class IV and V planet. What's more, we have no reason to believe that a Class V planet even exists. Our current technological stage may be as advanced as it gets, with resource depletion, pollution, climate change, and apocalyptic technologies serving as impenetrable filters.
That doesn't mean we shouldn't look. If we find nothing, we'll remain in the same bubble of ignorance we currently find ourselves in now. But if we find biosignatures or alien radio emissions, we'll finally know life is possible elsewhere. We may even find a batch of Class I-IV planets, but not a single Class V planet, which would be alarming. But as is often said, absence of evidence isn't evidence of absence.