File CT-R09 · UFOs · Fact recordFiled 2026-09-18

The Kardashev scale: how to rank alien civilizations

The Kardashev scale ranks a civilization by how much energy it can put to use, not by its technology or population, in three steps: control of a planet's own energy (Type I), of its star's entire output (Type II), and of its home galaxy's (Type III). Soviet astronomer Nikolai Kardashev proposed it in 1964, setting Type I close to humanity's own 1960s power use; on the continuous version Carl Sagan later drew from it, the one astronomers cite now, human civilization rates only about K=0.7, short of Type I. No other civilization of Type I, II or III has ever been detected.

What the scale measures

The Kardashev scale ranks hypothetical civilizations by one measure only, how much energy they control, not by intelligence, culture or population. Soviet astrophysicist Nikolai Kardashev proposed it in a 1964 paper, "Transmission of Information by Extraterrestrial Civilizations," published in Soviet Astronomy-AJ, volume 8, pages 217 to 221, with a Russian original in Astronomicheski Zhurnal, volume 41. He described three types, separated by enormous jumps in power, as a guide to what radio signals a search for extraterrestrial intelligence might realistically expect to find.

Where the idea came from

Kardashev reached the idea while chasing a different, ultimately wrong lead. He and fellow Soviet astronomer Iosif Shklovsky had proposed that irregular radio emissions from the quasar CTA 102 might be an artificial beacon, a claim sensational enough that it prompted an international press conference and, later, a song by the Byrds, according to the SETI Institute. Scientists eventually showed CTA 102 was simply a naturally variable quasar, not a signal, but Kardashev's attempt to define what an artificial signal should look like led directly to his 1964 scale, published the same year.

The three types and their numbers

Kardashev defined Type I as a civilization able to harness the energy available from its own sun, plus geothermal and tectonic sources, consuming energy at a rate of about 4 x 10^12 watts, close to the level human civilization itself had reached by the 1960s. Type II civilizations harness their entire star's output, about 4 x 10^26 watts. Type III civilizations command a whole galaxy's output, about 4 x 10^37 watts. Kardashev expressed these figures in ergs per second (4 x 10^19, 4 x 10^33 and 4 x 10^44), the same numbers in a different unit. By those figures, Type II uses about a hundred trillion times the power of Type I, and Type III about a hundred billion times that of Type II.

How far the scale could go

Kardashev did not stop at Type III. He wrote that he was prepared to consider civilizations with resources up to 10^25 times greater than humanity's own, far beyond even the galactic scale of Type III, as a working upper bound for what an extremely advanced signal source might look like. The three named types were meant as reference points along that much longer range, not as a ceiling on how powerful a civilization could become.

Filling the gaps: Sagan's fractional scale

Because Kardashev's three types are separated by such huge jumps, astronomer Jason Wright of Penn State has described how Carl Sagan later filled in the gaps with a continuous, fractional version of the scale, reasoning from the small share of the sun's light that reaches Earth against the vast number of stars in the galaxy. On Sagan's version, a civilization using about 10 billion megawatts, roughly 5 percent of the sunlight striking Earth, would already count as K=1, and one that harnessed nearly all of a sun-like star's output, about 4 x 10^20 megawatts, would reach K=2.06.

Where humanity ranks

Wright's own figures let humanity's rating be worked out directly. Present-day human civilization draws on roughly 10 million megawatts of power, by his estimate, against the 10 billion megawatts that marks K=1 on Sagan's scale, a gap of three orders of magnitude. Since each order of magnitude in power costs 0.1 on the K scale, that gap works out to 1 minus 0.3, or K is about 0.7, matching the figure Wright and other sources give for the world today; even capturing all the sunlight that reaches Earth would only bring humanity to K=1.12.

How scientists use it today

Astronomers still treat the scale as a starting point rather than a finished tool. Scientific American, asking outright whether humanity can climb it, notes the scale has been extended and criticized since 1964, including for ignoring how a civilization might shrink its energy footprint through efficiency rather than only ever consuming more. The SETI Institute continues to credit Kardashev, who died in 2019, as one of the field's most consequential early practitioners.

Has anyone found one?

No Type I, Type II or Type III civilization has ever been detected, by Kardashev's original definition or Sagan's fractional one. Surveys have searched for the waste heat or radio signatures such civilizations might produce and found none. As Universe Today put it, that absence has only a few possible readings: searchers are looking in the wrong places, looking for the wrong signs, have not looked long enough, or some mix of all three; nothing so far distinguishes between them.

Verdict

Real or Legend stamps the Kardashev scale itself confirmed: Kardashev really did publish this system of types in 1964, and Sagan really did extend it to fractional values, both matters of public record rather than dispute. What the scale predicts about other civilizations is a separate question. No civilization at Type I, let alone II or III, has been found anywhere, and by the numbers in this article, humanity itself is still roughly three-tenths of a point short of the first rung.