Would We Notice a Nuclear War on an Exoplanet?
Imagine a distant Earth-twin exoplanet on which a global nuclear war erupts. How far can we detect the explosions associated with this extraterrestrial war?
Our imagination is often tied to down-to-Earth experiences. To calibrate what we might expect from the war zone of an Earth-twin, let us consider a plausible scenario for our own planet.
In her book titled “Nuclear War: A Scenario”, the brilliant journalist Annie Jacobsen describes a realistic scenario in which North Korea first launches a nuclear ballistic missile towards Washington, D.C., shortly followed by a missile from a submarine towards a nuclear power plant in California. In retaliation, the U.S. sends nuclear ballistic missiles over the North Pole and Russian airspace. Consequently, Russia misinterprets this move and decides to launch 900 nuclear warheads towards the U.S. This escalation triggers the U.S. and NATO allies to target their full warhead stockpile at Russia. This doomsday scenario could result in explosions totaling the equivalent of a few thousand megaton of TNT in a matter of a couple of hours. Assuming a radiative efficiency of order 50%, a global nuclear war of this magnitude would result in a radiation flare with a total luminosity of about 10^{15} Watts, which is about a percent of the total luminosity of sunlight reflected from Earth.
Given that the nuclear war’s glow would have unique signatures in ultraviolet and infrared light and last for a few hours, the extraterrestrial flare resulting from Annie’s scenario on an Earth-twin could potentially be distinguished from stellar flares by UV observations with the Hubble Space Telescope or infrared data from the Webb telescope. For a nearby Earth-twin within a distance of a few tens of light years, space telescopes can potentially separate the planetary flare spatially from the location of the star. Masking the bright starlight is challenging but potentially feasible for faint dwarfs, the most common type of stars.
Was any candidate flare detected from the vicinity of a nearby dwarf star? Unfortunately, no search was dedicated so far to finding Annie’s flares. There was also no report about a serendipitous detection of an anomalous flare with nuclear war characteristics.
Based on our geopolitical experience, a global nuclear war is expected to be extremely rare. Even if world wars of this magnitude repeat once per century on any particular Earth-twin, detecting Annie’s flare once per year would require monitoring a hundred Earth-twins continuously. Since the flare lasts only hours out of a century, the probability of observing it at any given time is one in a million. In other words, a snapshot survey would show Annie’s flare in one out of a million Earth-twins that exhibit global nuclear wars once per century. It is unlikely that the Milky-Way galaxy hosts over a million Earth-twins where global nuclear wars repeat once per century. The likelihood for that could be assessed using a variant of the Drake equation for nuclear war flares as technological signatures.
However, even after a serendipitous discovery of Annie’s flare, the discoverer would likely suggest that the flare might have been the result of a magnetic reconnection event in the magnetosphere of the exoplanet. Astronomers tend to adhere to the least exotic scenario and neglect technological alternatives, brushing technological anomalies under the carpet of conservative interpretations. For example, when a near-Earth object was reported on January 2, 2025, it was first classified as a rocky asteroid and only after its orbit matched that of the Tesla Roadster car — it was instead recognized as a technological object.
In addition to its devastating effects on the ground, a global nuclear war would also change the chemical composition of the planet’s atmosphere, which could be probed many years after the war ended. This signature would be detectable for a transiting exoplanet whose orbit is oriented such that it passes in front of the face of the star, so that a telescope can infer the composition of its atmosphere. In particular, the heat released by nuclear fireballs is expected to trigger the copious production of nitride oxide molecules, in addition to the unique radioactive isotopes released in the explosion. Long-lived molecules could flag a war that occurred a while ago, enhancing its detection prospects beyond the few hours of Annie’s flare.
One might argue that detecting Annie’s flare would not flag extraterrestrial intelligence but rather extraterrestrial stupidity, because it signifies a geopolitical tendency for self-destruction. If astronomers choose to search for Annie’s flares, they should define the research effort as SETS instead of SETI, since it manifests a Search for Interstellar Stupidity.
In a recent public event, I was asked about the importance of money. I explained that money is important up to a certain threshold amount, as long as it buys you freedom. By freedom, I mean the ability to pursue what you enjoy doing. Beyond that threshold, the preoccupation with getting more money or using it beyond your needs robs you of your freedom. The same lesson might apply also to the technological advances of an intelligent civilization. Up to a certain level, new technological advances promote the longevity of intelligent beings. But beyond that threshold, advanced technologies make the civilization more vulnerable to self-destruction.
Have we already crossed that threshold? Are we approaching the inevitable doomsday of a global nuclear war as envisioned by Annie or some other catastrophe triggered by artificial intelligence or robots?
To gain a better perspective, we may conduct a census of extraterrestrial civilizations and plot their lifespan as a function of their technological level. Once we recognize the threshold for self-destruction, we must stop our own technological development at that level. Without knowing the threshold, our self-driving car will metaphorically fall off the cliff, like the car in the ending scene from the movie Thelma & Louise.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
