Earth's Shadow Hunts for Alien Probes express
Back in the 18th century, William Herschel was searching for life on Mars. Today, astronomers are targeting not planets but artifacts — for example, alien probes lurking in the Solar System. The problem is that spotting a tiny object next to the blinding Sun is nearly impossible. A new idea: turn Earth's shadow into a natural filter.
The shadow works like a deep well. During the day, you can see stars from the bottom of a well because the walls block out direct light. Similarly, the cone of Earth's shadow, stretching 1.5 million kilometers into space, blocks the Sun and makes faint flashes visible. A team of scientists analyzed images from the automated telescope Zwicky Transient Facility (ZTF), which photographs the sky every night. They searched for objects flickering near the shadow's edge for just a couple of seconds.
In the data, they found dozens of unknown rapidly changing flashes, previously uncatalogued. Most likely, these are space junk or asteroids, but the technique itself proved its power. A systematic sweep of the "well" could turn the search for extraterrestrial intelligence from a dream into observational science.
Original abstract
as written by the authors arXiv:2510.17907
For centuries, astronomers have discussed the possibility of inhabited worlds - from Herschel's 18th-century observations suggesting Mars may host life, to the systematic search for technosignatures that began in the 1960s using radio telescopes. Searching for artifacts in the solar system has received relatively little formal scientific interest and has faced significant technical and social challenges. Automated surveys and new observational techniques developed over the past decade now enable astronomers to survey parts of the sky for anomalous objects. We briefly describe four methods for detecting extraterrestrial artifacts and probes within the Solar System and then focus on demonstrating one of these. The first makes use of pre-Sputnik images to search for flashes from glinting objects. The second method makes use of space-borne telescopes to search for artificial objects. A third approach involves examining the reflectance spectra of objects in Earth orbit, in search of the characteristic reddening that may imply long-term exposure of metallic surfaces to space weathering. We focus here on a fourth approach, which involves using Earth's shadow as a filter when searching for optically luminous objects in near-Earth space. We demonstrate a proof-of-concept of this method by conducting two searches for transients in images acquired by the Zwicky Transient Facility (ZTF), which has generated many repeated 30-second exposures of the same fields. In this way, we identified previously uncatalogued events at short angular separations from the center of the shadow, motivating more extensive searches using this technique. We conclude that the Earth's shadow presents a new and exciting search domain for near-Earth SETI.
Earth's shadow reaches about 1.4 million kilometers — almost four times the distance to the Moon.