Flyby

Ever notice how strange-not-strange your house looks when you walk inside after a long time away? Ever spot a loved one from a distance across a crowded room, and, in that tiny moment when you don't recognize them, you see them with new eyes? Ever come across a decades-old letter you don't remember writing? A childhood toy you'd forgotten?

Seeing the familiar with new eyes is a talent, a skill that can be cultivated because it is always valuable. And it's a gift.

If a shifted perspective is valuable, then shift. Shift even further. Imagine you are on Europa, the water-world moon in orbit around Jupiter. From there, you can look up and see Sagan's "pale blue dot."^{2, 3}

What do you discover? What could you learn about Earth?

In fact, Earth is positively awash in anomalous — and obviously artificial — radio noise. Emissions from TV, radio, satellite transmissions, and radar would use very specific frequencies. They would stand out immediately against the background noise of stars, pulsars, and interstellar gas, which is broad and what might be called "coarsely noisy."

Earth's radio emissions would be very conspicuous. They would have structure and, in some cases, periodicity. Some — like radar or radio observatory emissions — would be tightly focused and directed at specific targets. Anything coming from a satellite in orbit would change in frequency like a loud race car on a circular track: higher as it approached, then lower as it moved away, then higher again as it came towards you once more. The Doppler shift, orbitally periodic.

Unmistakable. Definitive. Planetary electromagnetic emissions would immediately suggest that ....

Because what else could cause that awful, unnatural noise?

Life would not be all that difficult to confirm: you can see substances, patterns, and objects that, taken together, would provide clear evidence:^{4, 5} biosignatures.

As sunlight bounces off the Earth, any gases it passes through will selectively absorb some of the light. With a little cleverness, you could work out a lot of detail about a planet's atmosphere by observing what is absorbed and what is reflected.

As a technique, spectrography is a pretty good trick. In the case of Earth, it would reveal unusual but — given the anomalous radio signatures — unsurprising details of the atmosphere's composition.

Atmospheric water vapour. Liquid surface water. Raging amounts of free oxygen and ozone, with plenty of free methane and carbon dioxide. Without some mechanism to sustain them, none of these substances should float around together in such quantities for any length of time.⁶ In fact, disequilibrium in a few "gas pairs" can be used as signatures for different kinds of worlds.⁷ Earth, you discover, is unique.

As you watch, you can see seasonal variations change the balances between these observable gases, and—if your instruments are good enough — you can see changes in coloration near the poles and in the large land masses. In particular, you notice⁸ solar-induced fluorescence and a peculiar rise in reflectance that changes periodically over time: this is a characteristic of oxygenic photosynthesis responding to the angle of the sun. And you can detect dimethyl sulphide and other free-floating organosulphur gases, produced by phytoplankton.⁹

Earth is not only alive: it is green.

More observing and more thinking will show further evidence — again in substances and forms — of the physical manifestations of deliberate engineering:⁵ technosignatures.

Of course, Earth's radio signature — the first thing you'd probably notice — shines bright and is clearly diagnostic of a technical civilization. Not much else could explain it.

Even using only simple sensing technologies, Earth's detectable technosignature would span 13 orders in magnitude, from tiny but observable effects to the shocking and unmistakeable thunder of planetary radar or the scintillating flickers of space-pointed laser instruments.

Huge patches of bright lights from cities on dark continents would be very conspicuous. These bright patches would match up with infrared heat signatures emitted by all the activity and by urban heat-island effects.

Using sunlight passing directly through the thin halo of Earth's atmosphere, you could detect anomalous levels of nitrogen dioxide (NO₂) and carbon dioxide from fuel combustion, chlorofluorocarbons (CFCs)¹⁰ from industrial processes, ammonia (CH₃) and laughing gas (N₂O) from agriculture products ... and dozens more substances that would clearly indicate agriculture, industry, and technology.

You could easily tell — at distances much greater than Europa in Jupiter's orbit — that a whole lot of activity is taking place on Earth. You would know the Earth hosts a civilization capable of significantly altering its environment.

In fact, you could see ...

With a little basic geometry and a simple machine (a thermal emission spectrometer) to monitor radiation, you could measure how much energy reaches the Earth from the Sun, and then measure how much energy leaves the planet and goes back into space.¹¹

You could see more energy reaches Earth than leaves it. The Earth has an energy imbalance.

The longer you watched, the greater energy imbalance would become. In fact, if you had started measurement some years ago, you'd see an obvious — and terrifying — correlation between the rise in technosignatures and the rising energy imbalance. If your instrument were good enough, you could track seasonal changes in the Earth's reflectance, largely caused changes in the polar regions. Over time, you would see a gradual diminishment of the Earth's ability to reflect heat.

You would know that Earth's industrial civilization is making its planet warmer.

Reading

1. Dawkins, Richard. Unweaving the Rainbow: Science, Delusion, and the Appetite for Wonder. Boston: Houghton Mifflin, 1998.
2. “Voyager 1’s Pale Blue Dot - NASA Science,” April 10, 2024. https://science.nasa.gov/mission/voyager/voyager-1s-pale-blue-dot/.
3. Sagan, Carl. Pale Blue Dot: A Vision of the Human Future in Space. New York: Random House, 1994.
4. Chan, Marjorie A., Nancy W. Hinman, Sally L. Potter-McIntyre, Keith E. Schubert, Richard J. Gillams, Stanley M. Awramik, Penelope J. Boston, et al. “Deciphering Biosignatures in Planetary Contexts.” Astrobiology 19, no. 9 (September 1, 2019): 1075–1102. https://doi.org/10.1089/ast.2018.1903.
5. Sheikh, Sofia Z., Macy J. Huston, Pinchen Fan, Jason T. Wright, Thomas Beatty, Connor Martini, Ravi Kopparapu, and Adam Frank. “Earth Detecting Earth: At What Distance Could Earth’s Constellation of Technosignatures Be Detected with Present-Day Technology?” The Astronomical Journal 169, no. 2 (February 2025): 118. https://doi.org/10.3847/1538-3881/ada3c7.
6. “NASA Astrobiology.” Accessed April 6, 2025. https://astrobiology.nasa.gov/news/detecting-lifes-influence-on-planetary-atmospheres/.
7. Meadows, Victoria S., Christopher T. Reinhard, Giada N. Arney, Mary N. Parenteau, Edward W. Schwieterman, Shawn D. Domagal-Goldman, Andrew P. Lincowski, et al. “Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment.” Astrobiology 18, no. 6 (June 1, 2018): 630–62. https://doi.org/10.1089/ast.2017.1727.
   This is such an exciting paper we couldn't let it pass without further comment. The authors have built a model that eliminates false biosignature positives from abiotic mechanisms for atmospheric oxygen. See their Figure 2.
8. “(PDF) Photosynthetic Fluorescence from Earthlike Planets around Sunlike and Cool Stars.” ResearchGate, December 10, 2024. https://www.researchgate.net/publication/367025228_Photosynthetic_Fluorescence_from_Earthlike_Planets_around_Sunlike_and_Cool_Stars.
9. Schwieterman, Edward W., and Michaela Leung. “An Overview of Exoplanet Biosignatures.” arXiv, April 23, 2024. https://doi.org/10.48550/arXiv.2404.15431.
10. Haqq-Misra, Jacob, Thomas J. Fauchez, Edward W. Schwieterman, and Ravi Kopparapu. “Disruption of a Planetary Nitrogen Cycle as Evidence of Extraterrestrial Agriculture.” The Astrophysical Journal Letters 929, no. 2 (April 2022): L28. https://doi.org/10.3847/2041-8213/ac65ff.
11. “A Bond Albedo Map of Europa | Astronomy & Astrophysics (A&A).” Accessed April 7, 2025. https://www.aanda.org/articles/aa/full_html/2025/01/aa53058-24/aa53058-24.html.