In February NASA astronomers discovered seven Earth-like planets, potentially harboring life, orbiting the star TRAPPIST-1, not too far from Earth.
Scientists have yet to discover life, or evidence of civilizations, on these or other planets. But in the search for extra-terrestrial intelligence, they often categorize hypothetical worlds according to the amount of energy their inhabitants could potentially harness.
They do this using what is known as the Kardashev scale. Named in 1964 for Soviet astronomer Nikolai Kardashev, the scale takes energy use as the key indicator of a civilization's advancement, and places those hypothetical civilizations in one of three categories:
A Type 1 civilization--still a distant goal for Earth--utilizes all of the energy that reaches its planet from its parent star (in Earth's case, the Sun).
A Type 2 civilization is capable of using all the energy put out by its star and planetary system.
A super-advanced Type 3 civilization harnesses all the energy of its home galaxy.
The Kardashev scale has been a gold standard classification system for thinking about "exo-civilizations" for decades. It does not, however, take into account how a civilization in turn affects its planet when it gathers and uses energy.
That omission is increasingly significant as, in the half-century since Kardashev proposed his classification scheme, evidence is accumulating that our energy-intensive, industrial civilization is affecting our planet.
Given those effects, can planets and civilizations co-exist for the long haul? And if so, how?
To answer these questions, a team of researchers led by Adam Frank, a professor of physics and astronomy at the University of Rochester, devised a new classification scheme for the evolution of civilizations based on the idea that it's not just how much energy you use, but how you use it that matters.
With this new scale, the researchers determined that in order to survive long-term, a civilization must learn to "think like a planet"--or risk the civilization's demise.
"The Kardashev scale is concerned with extracting energy," Frank says. "But what we've recognized with our classification scheme is that you can't use energy without causing different kinds of waste. That waste feedbacks on the state of planet."
In a paper in the journal Anthropocene, the researchers discuss this new classification system as a way of thinking about sustainability on a planetary scale.
"The discovery of seven new exoplanets orbiting the relatively close star TRAPPIST-1 forces us to rethink life on Earth," says Marina Alberti of the University of Washington, a co-author on the paper. "It opens the possibility to broaden our understanding of planetary system dynamics and lays the foundations to explore a path to long-term sustainability."
Earth's biosphere--the global layer where life exists--is unique in that the presence of life has altered the planet's surrounding atmosphere above and lithosphere below. The researchers note that rapid urbanization--including deforestation, air pollution, and increasing energy demand--has had damaging effects on the planet. Currently most of the energy on Earth comes from fossil fuels, a limited resource that puts pressure on the earth's ecosystems.
Humans will need to find new ways of generating work from the energy they harvest in order to sustain civilization, the researchers say.
"You can't just bring a planet to heel, you need to bring it a plan and figure out how to extract energy while also maintaining the health of the planet's biosphere," Frank says. "Human beings are part of the biosphere so they need to work with it in order to take the next steps in planetary evolution."
The new classification system for planetary evolution is composed of five levels:
Class I: Planets without an atmosphere. The ability of the planet to change and evolve is severely limited. (Mercury or Earth's moon)
Class II: Planets with atmospheres but no life forms. The flow of gases and fluids leads to change and evolution in the form of climate and weathering. (Venus and Mars)
Class III: Planets with a "thin" biosphere that might sustain some biological activity, but this does not affect the planet as a whole. There are no current examples of Class III planets. However, Earth 2.5 billion years ago, before life created the oxygen atmosphere, would have been a Class III world. If early Mars hosted life when it had liquid water on its surface then it too might have been a Class III world. Once life appears, new forms of change, evolution, and innovation become possible.
Class IV: Planets with a thick biosphere strongly affecting the flow of energy and work through the rest of the planetary systems. Planets co-evolve with their biospheres as life dominates many of the processes happening between the surface and the upper atmosphere. (Earth today)
Class V: Planets in which an energy-intensive technological species establishes a sustainable form of cooperation with the biosphere that increases the productivity of both. On these planets the civilization enhances the ability of the biosphere to innovate and evolve.
According to the researchers' findings, Earth might reach Class V in the future if humanity successfully advances to harvest energy in forms like solar that do not harm the biosphere.
Although researchers can't conclude that advanced extraterrestrial civilizations currently exist in our galaxy, previous work by Frank demonstrates that unless the laws of the Universe are highly biased against them, other technologically advanced civilizations are likely to have existed at some point in cosmic history.
"The Universe has created a lot of opportunities for what's happening to us to have happened before," Frank says. "We're starting off by assuming there have been Class V planets."
And what might a Class V planet look like?
Frank lists several ways humans on Earth might form a technological cooperative between biosphere and civilization, including "greening" large desert land masses such as the Sahara by finding ways to plant trees that will absorb carbon and release oxygen; or creating genetically modified trees with photovoltaic leaves that covert the sun's energy into electricity.
"Civilization arose as part of a biosphere," Frank says. "A Type 2 civilization on the Kardashev scale that is super space-baring could live without a biosphere. But a young civilization, like ours, has to see itself as a part of the biosphere. We're not separate from it, we're just the latest experiment the earth is running in the evolution of life. If we're not careful, it will just move on without us."