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Global warming may be key to finding E.T.

Exoplanets that are too hot may be a sign of alien life

A new research paper suggests that scanning for waste heat radiating off of Earth-like planets may be important in the search for extraterrestrial intelligence. Like the radio signals emitted by Earth, heat may serve as an interstellar smoke signal indicating energy production and consumption. While this is not a new idea, the study argues that heat is a viable marker of alien civilization that should be considered in plans for building future telescopes.

With nearly 2000 exoplanets discovered so far, detailed study of them may yet reveal signs of alien civilization. One signal that we have been listening for in the search for extraterrestrial intelligence is radio waves, which our rapidly industrialized Earth civilization has also been beaming into outer space for the past 100 years or so. These artificial signals traveling at the speed of light were humanity’s first “hello” to the universe. But even these signals are becoming silenced with the end of terrestrial broadcasting; if the same thing happened to E.T., what are some other large-scale signs that could indicate the presence of intelligent life?

The thermal radiation of a planet may be an unintentional calling card, a biomarker of extraterrestrial life. In particular, the way heat is distributed as islands in space and time on a planet’s surface — think of the bright spots of illuminated cities on Earth at night — may be a signature of Earth-like civilizations. Because the trend on this planet is towards ever greater energy consumption, civilizations as or more advanced than Earth’s may also be recognized by the heat (detected as infrared light) that accompanies energy production.

Kepler-16b, an exoplanet orbiting a pair of stars

Kepler-16b, an exoplanet orbiting a pair of stars. Modified from original image / Courtesy NASA/JPL-Caltech / Public Domain

The researchers modeled heat signals that might be emitted by an exoplanet with Earth-like properties of reflectance, scattering, and geography, but a heat profile 50 times larger than Earth’s. An advancing civilization will likely be accompanied by a warming planet; as the authors put it, “a civilization with growing power needs will eventually reach a point where they become uncomfortably warm”. Even if the alien civilizations found a way to engineer their planet to reflect heat out of the atmosphere, this would still leave a detectable heat signature.

The more advanced a civilization is, the argument goes, the more energy it needs, and the more it will move towards harnessing all the available energy in its solar system, perhaps culminating in a Dyson sphere. We can compare energy consumption with the theoretical maximum available (i.e. the amount of energy incident from the host star) to get a measure of how advanced (in terms of energy consumption and thus heat output) a civilization is. For Earth, the researchers estimate we are now consuming 0.04% of the flux of the Sun; more advanced civilizations will tend towards 100%.

They gave the model planet certain energy inputs (solar and planetary heat sources) and outputs (from photosynthesis and other biological activity as well as technology) and found that the brightness of the planet changed predictably with its programmed rotation and planetary orbit period. To discount parts of the observed brightness that are due to reflected radiation (e.g. from cloud cover), they removed the short-wavelength light. In this way, they were able to show that the heat profile of a civilization is distinct from the heat profile attributable purely to planetary motion.

Looking at the visible and infrared light coming off a planet, it is thus in principle possible to ascertain thermal signatures compatible with alien civilization. This method could be enhanced by coupling it with existing tools like spectroscopy, which looks at the reflected spectrum of electromagnetic radiation to determine the chemical composition of exoplanets and stars. Still, the heat signature will be confounded by planets completely covered in clouds or water.

In order to find these hot planets, really big telescopes are needed, the authors argue. The challenge is to achieve sufficient contrast sensitivity — being able to distinguish the planet’s faint thermal signatures from the heat generated by the star it orbits — and for that you need telescopes with large-diameter objectives, plus other technology to block light from other stars and focus the light you do want to see. A 70-meter telescope would be sufficient to study exoplanets in the habitable zones of stars in Earth’s near-neighorhood, within 20 parsecs. The world’s largest planned telescopes, however, are all under 40 meters. The authors therefore advocate for the 74-meter Colossus telescope that would have the contrast sensitivity required to detect the heat signatures of any advanced civilizations on near-Earth exoplanets.

Finally, the authors suggest, we can learn something important about the survival probability of advanced civilizations (very low) if no alien heat signatures are found with a survey of the habitable planets in the near-Earth neighborhood.


Kuhn, J. R., & Berdyugina, S. V. (2015). Global warming as a detectable thermodynamic marker of Earth-like extrasolar civilizations: the case for a telescope like Colossus. International Journal of Astrobiology, 14(3), 401-410. DOI: 10.1017/S1473550414000585