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A new paper (accessible here) that I co-authored with the brilliant graduate student at Vanderbilt University, Oem Trivedi, provides a framework for quantifying the Loeb Scale in the classification of interstellar objects as natural or technological.
The recent astronomical discovery of a third interstellar object, 3I/ATLAS, following 1I/‘Oumuamua and 2I/Borisov, has raised questions about the nature and origin of these enigmatic objects. While 2I/Borisov appeared as a conventional comet, both 1I/‘Oumuamua and 3I/ATLAS exhibited anomalous properties that deviate from familiar solar system objects. With the advent of the Vera C. Rubin Observatory, it is anticipated that the detection rate of interstellar objects (ISOs) will increase by up to two orders of magnitude, raising the need for a systematic framework to classify and interpret their nature and distinguish between natural icy rocks and possible technological artifacts.
The Loeb Scale was recently formulated in a detailed paper that I co-authored with Omer Eldadi and Gershon Tenenbaum (accessible here). It provides a structured, ten-level classification scheme to evaluate ISOs based on the level of anomalies that they exhibit relative to icy rocks, raising the possibility that they might be artificial in origin. Much like the Kardashev scale offers a way to classify technological civilizations according to their energy usage, the Loeb Scale serves as a tool for ranking interstellar artifacts along a continuum ranging from ordinary natural bodies (Level 0) to confirmed technological artifacts that potentially pose a risk to humanity (Levels 8–10). Whereas the Loeb Scale offers a systematic classification scheme, it does not apply to the civilizations themselves but rather to their products based on observational surveys near Earth.
Carl Sagan refined the Kardashev framework by introducing a logarithmic interpolation scheme that relates a civilization type to its total energy consumption, thereby enabling a continuous quantitative mapping as a substitute to a coarse categorical distinction. The new paper applies a similar approach to the Loeb Scale for ISOs by mapping their measured anomalies and physical characteristics to a quantitative rank. The paper refines the Loeb Scale by translating observable features of ISOs into a continuous score.
The Loeb scale begins at level 0, corresponding to natural astrophysical bodies whose properties are well understood and require no appeal to exotic explanations, and rises to level 10, reserved for catastrophic scenarios involving confirmed technological objects that pose an existential threat to humanity on Earth. At the lower levels, the scale is meant to separate trivial or explainable anomalies from those requiring closer scrutiny, with Level 0 referring to ISOs that exhibit properties consistent with known classes of comets or asteroids, such as the interstellar comet 2I/Borisov. Level 1 captures objects with minor deviations or inconclusive data that do not strongly challenge a natural interpretation, whereas level 2 describes an ISO that exhibits some unusual properties but still falls within the range of natural phenomena given current understanding. Level 3 marks the point where persistent anomalies appear, with features such as repeated but unexplained variations in trajectory or unusual albedo or morphology, without yet triggering serious consideration of artificial origin.
The middle of the scale is where the possibility of technosignatures is formally acknowledged, where Level 4 is described as the “critical threshold,” where the combination of anomalies is strong enough that a technological explanation must be considered alongside natural hypotheses. This was the classification assigned to 1I/‘Oumuamua, whose non- gravitational acceleration without a coma of gas or dust, spectral properties, and extreme shape remain unexplained by conventional models. Level 5 corresponds to a suspected passive technology, such as derelict probes, light sails, or debris, where evidence suggests artificial origin but no active behavior is detected. Levels 6 and 7 escalate to suspected active technologies, including evidence of propulsion, maneuvering, or directed electromagnetic emissions. At these levels, the possibility of interaction or intent must be contemplated but the distinction between minor and major threats to humanity can be ignored.
The uppermost part of the scale deals with confirmation and risk, with Level 8 being assigned when multiple lines of evidence establish beyond a reasonable doubt that the object is artificial but it poses no immediate danger, for example a flyby of a confirmed probe near Earth. Level 9 is reached if the confirmed artificial object is on a trajectory with the potential to cause regional harm, such as an impact equivalent to a major nuclear strike. Finally, Level 10 is the highest designation and is reserved for a confirmed artificial body on a collision course with Earth that would carry global consequences, akin to existential threats in planetary defense assessments. By laying out this graded sequence from the mundane to the catastrophic, the Loeb scale provides both a scientific framework for classifying ISOs and a practical guide for allocating observational and policy attention in proportion to the level of the risk.
The proposed framework of the new paper for quantifying the Loeb Scale is modular, in the sense that the metric definitions, transforms, calibration constants, and weights are all explicitly configurable and can be refined by a working group of the International Astronomical Union or an equivalent community process. Hard-trigger override rules ensure that decisive evidence is acted upon promptly and an anomaly count bonus encourages detection teams to value corroboration across independent data types (trajectory, spectrum, geometry, electromagnetic signals) without letting counting alone dominate the weighted evidence.
Finally, governance and policy decisions follow naturally from this mapping. For the range corresponding to Loeb Level 4, the framework suggests enhanced global observational campaigns, prioritized telescope time, and transparent rapid data release. For Levels 5–7, the framework recommends escalating coordination with agencies responsible for planetary defense, and for Levels 8–10 it prescribes emergency protocols analogous to those used in planetary-impact scenarios but adapted to the additional complications related to artificial causation. The mathematics provides a reproducible, transparent, and tunable bridge between raw measurements and the Loeb Scale integer classification, allowing the astronomical community to communicate consistently and to exercise preparedness while avoiding premature actions.
It is important to emphasize that the Loeb Scale is conceptually independent of the Drake Equation as the Drake Equation was originally formulated as a probabilistic tool to estimate the number of contemporaneous radio communicating civilizations in the Milky-Way galaxy. The terms of the Drake equation explicitly encode astrophysical, biological, and sociological factors that govern the occurrence rate of detectable electromagnetic signals, and are therefore tied to the question of how many civilizations might be actively transmitting detectable signals. In contrast, the Loeb Scale is not a measure of occurrence rates but a classification framework for interpreting the technosignature significance of physical artifacts that are detected near-Earth, regardless of when or by whom they were produced.
This distinction has practical implications as one may imagine scenarios in which the Milky-Way galaxy contains no radio emitting civilizations at present, either because advanced societies perished by now or because they no longer engage in radio communication. Under these circumstances, it could still be possible that long-lived artifacts such as probes, fragments, or derelict spacecraft are present for billions of years, thus entering the Solar System as interstellar objects. The Loeb Scale is designed precisely for such cases as it quantifies the anomaly level and possible artificial nature of nearby physical objects without making any assumptions about the present-day activity of their creators. Thus, the Loeb Scale complements, but does not depend on, the Drake framework, because it deals with surviving physical technosignatures rather than transient electromagnetic signals.
ABOUT THE AUTHOR
Press enter or click to view image in full size(Image Credit: Chris Michel, National Academy of Sciences, 2023)
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.