A Framework for Applying the Loeb-Turner $\alpha$-Slope Test to Archival Photometry of Trans-Neptunian Objects

TL;DR

This paper applies the Loeb-Turner α-slope photometric test to archival data of trans-Neptunian objects, formalizes a selection pipeline, and discusses implications for detecting extraterrestrial technosignatures.

Contribution

It formalizes a six-criterion pipeline for the α-slope test and applies it across all TNO data, revealing calibration issues and prospects for future surveys.

Findings

No archival data conclusively supports the reflected sunlight flux--distance slope.

Identified calibration systematic in Pan-STARRS data affecting slope measurements.

Future Rubin Observatory survey can resolve or falsify the calibration systematic.

Abstract

Reflected sunlight from a solar-system body produces a flux at Earth that scales as the heliocentric distance to the negative fourth power, whereas self-luminous emission scales as the negative second power. This difference defines the $\alpha$-slope test proposed by Loeb & Turner (2012), a photometric technosignature diagnostic applicable to any solar-system body observed at multiple heliocentric distances. Of 22 (observatory $\times$ band) analysis bins for Pluto in the Minor Planet Center (MPC) archive, none recovers the reflected-sunlight flux--distance slope predicted when photometry is restricted to a single instrument and band. The archive cannot cleanly execute the $\alpha$-slope test on the brightest, most-observed trans-Neptunian object. We formalize a six-criterion eligibility pipeline (Q1--Q6) for the Loeb & Turner technosignature test and apply it to every numbered TNO. Of 8,557 candidate bins (KBO $\times$ observatory $\times$ band), 1,089 pass Q1--Q3 and 186 additionally pass Q4--Q6, splitting into 53 consistent with reflected sunlight ($\alpha = -4$), 24 with self-luminous emission ($\alpha = -2$), and 109 anomalous. The anomalous bins exhibit slopes steeper than $\alpha = -4$ or shallower than $-2$, consistent with uncorrected per-instrument calibration offsets rather than any single physical mechanism. All 24 self-luminous-like bins originate from Pan-STARRS PS1/PS2; no other observatory contributes any. This indicates a per-instrument calibration systematic. The Rubin Observatory's ten-year survey will deliver uniform single-instrument calibration on a tenfold larger sample and either resolve the test at $>10\sigma$ on hundreds of TNOs or, by reproducing the same clustering, falsify the calibration-systematic interpretation.