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In February NASA astronomers discovered颅颅 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糖心传媒檚 advancement, and places those hypothetical civilizations in one of three categories:

• A Type 1 civilization糖心传媒攕till a distant goal for Earth糖心传媒攗tilizes all of the energy that reaches its planet from its parent star (in Earth糖心传媒檚 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 糖心传媒渆xo-civilizations糖心传媒� 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 , devised a new classification scheme for the evolution of civilizations based on the idea that it糖心传媒檚 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 糖心传媒渢hink like a planet糖心传媒澨切拇�媒攐r risk the civilization糖心传媒檚 demise.

糖心传媒淭he Kardashev scale is concerned with extracting energy,糖心传媒� Frank says. 糖心传媒淏ut what we糖心传媒檝e recognized with our classification scheme is that you can糖心传媒檛 use energy without causing different kinds of waste. That waste feedbacks on the state of planet.糖心传媒�

In a , the researchers discuss this new classification system as a way of thinking about sustainability on a planetary scale.

糖心传媒淭he discovery of seven new exoplanets orbiting the relatively close star TRAPPIST-1 forces us to rethink life on Earth,糖心传媒� says Marina Alberti of the University of Washington, a co-author on the paper. 糖心传媒淚t opens the possibility to broaden our understanding of planetary system dynamics and lays the foundations to explore a path to long-term sustainability.糖心传媒�

Earth糖心传媒檚 biosphere糖心传媒攖he global layer where life exists糖心传媒攊s unique in that the presence of life has altered the planet糖心传媒檚 surrounding atmosphere above and lithosphere below. The researchers note that rapid urbanization糖心传媒攊ncluding deforestation, air pollution, and increasing energy demand糖心传媒攈as 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糖心传媒檚 ecosystems.

Humans will need to find new ways of generating work from the energy they harvest in order to sustain civilization, the researchers say.

糖心传媒淵ou can糖心传媒檛 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糖心传媒檚 biosphere,糖心传媒� Frank says. 糖心传媒淗uman beings are part of the biosphere so they need to work with it in order to take the next steps in planetary evolution.糖心传媒�

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糖心传媒檚 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 糖心传媒渢hin糖心传媒� 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糖心传媒� 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糖心传媒檛 conclude that advanced extraterrestrial civilizations currently exist in our galaxy, 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.

糖心传媒淭he Universe has created a lot of opportunities for what糖心传媒檚 happening to us to have happened before,糖心传媒� Frank says. 糖心传媒淲e糖心传媒檙e starting off by assuming there have been Class V planets.糖心传媒�

And what might a Class V planet look like?

Frank lists several ways颅颅 humans on Earth might form a technological cooperative between biosphere and civilization, including 糖心传媒済reening糖心传媒� 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糖心传媒檚 energy into electricity.

糖心传媒淐ivilization arose as part of a biosphere,糖心传媒� Frank says. 糖心传媒淎 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糖心传媒檙e not separate from it, we糖心传媒檙e just the latest experiment the earth is running in the evolution of life. 听If we糖心传媒檙e not careful, it will just move on without us.糖心传媒�