In a new peer-reviewed study published in the Monthly Notices of the Royal Astronomical Society, a team of astronomers has laid out a practical, testable method to search for non-human artifacts in Earth’s vicinity. The paper, titled “A Cost-Effective Search for Extraterrestrial Probes in the Solar System”, outlines a data-driven framework that operates on an unconventional but straightforward premise: if technological civilizations exist elsewhere, some of their machines may already be in orbit within our solar system. The study does not speculate on intent or origin. It instead proposes and executes a methodology designed to search for unknown luminous objects within Earth’s shadow, leveraging existing sky surveys to reduce cost, increase observational coverage, and minimize contamination from human-made satellites and debris.
The lead author, Beatriz Villarroel, along with collaborators from institutions including Stockholm University, the Instituto de Astrofísica de Canarias, and the Gran Telescopio Canarias, approaches the subject with academic restraint. The paper details how standard SETI efforts have concentrated for over six decades on detecting artificial signals via radio or optical frequencies from distant star systems. Despite advancements in sensitivity and coverage, these searches have failed to detect any confirmed non-natural transmission. As that effort continues, another overlooked possibility remains unaddressed: that non-human probes or objects might already be inside the solar system, potentially even in Earth orbit.
This concept is not novel in itself. Researchers dating back to Carl Sagan have acknowledged the plausibility of physical probes as a logical extension of interstellar communication strategies. However, technical limitations and institutional taboos have historically obstructed systematic searches for local non-human artifacts. With the rising density of satellites and orbital debris, traditional methods of surveying near-Earth space have become increasingly complex. To solve this, Villarroel’s team turns attention to a natural filtering mechanism: the Earth’s shadow. The premise is direct. Human-made satellites are largely passive reflectors of sunlight. Inside the Earth’s shadow, sunlight does not reach these objects. Therefore, any bright object that appears within that zone is either emitting its own light or reflecting non-solar sources. That drastically narrows the field and allows researchers to search for anomalies with less risk of false positives.
The study identifies four complementary methods to search for extraterrestrial probes: analyzing pre-Sputnik astronomical plates, using space-based telescopes to observe regions beyond Earth orbit, comparing surface reflectance spectra for signs of long-term exposure to space weathering, and scanning for light-emitting objects inside the Earth’s shadow. While each method has merit, the primary focus is on the fourth strategy, which the authors executed in a proof-of-concept study using the Zwicky Transient Facility, or ZTF. This sky survey collects massive quantities of repeated exposures and is optimized for detecting transient events. The authors designed a series of filters to exclude known satellites, asteroids, aircraft, and noise artifacts.
Across nearly a decade of ZTF data, the researchers examined three sets of images. Sample A consisted of 678 curated transient alerts provided by an external team, filtered to include only objects located inside the Earth’s shadow at geosynchronous orbital altitude. From over 11,000 single-detection transients, 262 were found to lie within the calculated umbra where sunlight cannot reflect. The researchers eliminated known satellites, asteroids, and false positives, leaving behind a small subset of candidate events. Among these, they identified one particularly unusual object which did not appear in any catalog maintained by the Minor Planet Center or JPL databases. This object exhibited motion inconsistent with main belt asteroids and showed apparent changes in location across multiple images.
The object’s behavior, brightness, and movement could not be attributed to known artificial satellites, meteors, or standard optical phenomena. While the team refrains from drawing conclusions, they emphasize that the object’s angular motion was too rapid for a typical main belt asteroid and too slow for a meteor. It was also located inside the shadow cone, where no reflected solar light should be visible. Since satellites and debris typically rely on reflected light, this detection raises the possibility of intrinsic luminosity or an unknown propulsion-related emission source.
Sample B and Sample C extended the study’s reach. Sample B was designed to automate the detection process within the Earth’s shadow using over 224,000 images. Sample C served as a control, focused on regions near the ecliptic pole outside of the Earth’s shadow, encompassing more than 326,000 images. The researchers developed a pipeline named NEOrion to analyze transient detections and assess morphological features such as shape, elongation, and brightness profiles. Using strict filters, they isolated objects that were bright, streaking, or clustered in formation, all of which are rare in ordinary astrophysical imagery. In one example from Sample B, four catalogued minor planets were observed moving in formation, confirming the pipeline’s capacity to detect real transients and validate known objects.
Despite the abundance of human-made satellites, flash trains and glints caused by these objects were entirely absent from Sample B, as expected. In contrast, they appeared frequently in Sample C, reinforcing the effectiveness of the Earth-shadow filtering method. This approach significantly reduces background noise, allowing researchers to focus on objects that demonstrate active emission or unexplained optical behavior.
The authors stress that their method is not capable of detecting old or dormant probes that lack active emission. However, they note that some objects may remain luminous due to surface materials, energy sources, or other unknown factors. The possibility of objects positioned at lunar distances or beyond adds complexity, especially since such distances would imply slower angular motion and require high sensitivity to distinguish from background stars.
Throughout the study, the authors apply strict selection criteria and conservative interpretation of data. Every candidate transient was reviewed for position, motion, and potential matches to existing databases. In one instance, a candidate object was observed over several images but failed to appear in expected follow-up frames, likely because it passed through a camera dead zone or CCD seam. That object’s motion pattern suggested proximity to Earth but lacked sufficient parallax data to determine range. It remains unclassified.
The study includes detailed visual examples and tables cataloging observed coordinates, timestamps, and bandpass data for each noteworthy object. These are cross-referenced with known asteroid identifiers and previous observations. In some cases, the team observed asteroid pairs or clusters moving in near parallel formation. While these events were all ultimately matched to known catalog entries, their detection in shadow-filtered zones demonstrated the methodology’s reliability.
The researchers acknowledge the limitation of temporal resolution. Since ZTF captures only a few exposures per field per night, events lasting less than a minute are difficult to confirm without additional instruments. Nonetheless, their pipeline is capable of flagging objects for follow-up and future triangulation. They propose that this capability could support new observatories or citizen science efforts aimed at identifying recurring transients in real time. Projects like VASCO (Vanishing & Appearing Sources during a Century of Observations) are already exploring digitized pre-Sputnik plates for anomalies, including disappearing stars and non-matching objects.
Further analysis in this paper discusses the reflectance properties of orbital debris. The authors describe how long-term exposure to micrometeoroids and radiation can redden the surfaces of metallic objects. By identifying uncatalogued space objects with anomalously red spectra and no known operator, researchers could isolate long-resident objects for further study. This method adds another layer of screening when analyzing the thousands of unidentified entries in orbital catalogs.
Space-based telescopes like Kepler and TESS also offer an avenue to conduct this search in cleaner observational environments. By focusing beyond geosynchronous orbit, where fewer human objects exist, the probability of contamination drops further. However, sensitivity limits and smaller mirror sizes restrict their ability to detect faint, fast-moving objects. Despite this, their datasets remain valuable for targeted searches, especially for events reported in multiple overlapping fields.
The authors recommend a multipronged strategy combining archival plate analysis, reflectance spectral studies, deep-sky survey filtering, and targeted observations in the Earth’s shadow. Each method offers unique advantages. The shadow-filter approach is particularly promising because it minimizes known interference without requiring new infrastructure. It allows ground-based telescopes to search for candidates under conditions that filter out the majority of known contaminants.
The study concludes that searches for probes or artificial objects in the solar system are now technically viable. They do not require speculative assumptions or exotic technologies. Instead, the focus remains on verifiable, repeatable phenomena observed through established astronomical instruments. The possibility that one or more of the candidates identified in this study may represent a non-terrestrial object remains unresolved. The authors do not claim detection of alien technology. They present a framework, a set of tools, and early results that justify further observation.
With rising interest in nearby transients, unidentified aerial phenomena, and the role of spaceborne observatories, the approach outlined in this paper offers a low-cost, high-efficiency method to conduct local surveys of our near-space environment. The Earth’s shadow becomes more than an absence of light. It is a scientific filter, capable of excluding the known and drawing attention to the unknown. Whether these searches will ultimately identify alien artifacts or not, the data itself demands continued attention.
Source Paper:
Villarroel, B., Korn, A. J., Usoskin, I. G., Rydberg, C.-E., Seixas, D. F. M., Solano, E., & Guijarro, A. (2025). A cost-effective search for extraterrestrial probes in the Solar System. Monthly Notices of the Royal Astronomical Society, 527(2), 2142–2156. https://doi.org/10.1093/mnras/staf1158
