A Cost-Effective Search for Extraterrestrial Probes in the Solar System

TL;DR

The study tackles the problem of detecting extraterrestrial probes or artifacts in the Solar System by proposing four filtering strategies and focusing on using the Earth’s shadow as a clean search domain. It demonstrates a proof‑of‑concept with ZTF data, showing that transients within the shadow are rare and largely attributable to known asteroids or artifacts, while identifying uncatalogued or unusual objects that merit follow‑up. The work highlights the need for higher time‑resolution, multi‑telescope triangulation (ExoProbe) to obtain parallax and spectra, enabling robust distance and material characterization. Overall, the paper establishes a concrete, auditable framework for near‑Earth SETI searches and motivates expanded, coordinated observations to distinguish ET artifacts from mundane solar-system objects.

Abstract

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.

Figures (8)

• Figure 1: Earth's umbra out to an orbit radius $r_{\rm orb}$ (not to scale). The Sun is in the direction of the bottom of the page. The lightest shading marks the "refraction fringe" into which sunlight is refracted by the atmosphere (angular size $\gamma$). Intermediate shading is used to mark the "dark full shadow" (DFS). The darkest shading highlights the shadow volume visible to an observer on the Earth's surface (this shading implies nothing about darkness in this volume, which is the same as the DFS). DFS radius is given by $\varrho$; parallax of the shadow center with respect to the antisolar point is $\vartheta$; angular separation of the observer from the geocenter-antisolar axis is $\varphi$. (a) Shows the shadow geometry for an observer on the Earth's surface beneath the antisolar point (the "subantisolar point" or SAP), and (b) shows the shadow geometry for an observer at an arbitrary position on Earth's surface, with angular separation $\varphi$ from the SAP.
• Figure 2: Distribution on the sky. The distribution of the Sample A transient candidates on the sky in Galactic coordinates. Upper panel (a) all one-off transient candidates, (b) one-off transient candidates in the shadow.
• Figure 3: Asteroid 2001 VC136. A complex candidate, including asteroid 2001 VC136 and a transient was found inside a box measuring 10$\times$10 arcmin and centered at R.A. (J2000)=6.3634206 deg. and Dec.(J2000)= 2.4341476, J.D.=2458760.7977431. See Table \ref{['tab_tripletransient']} for astrometric measurements and Table \ref{['filenames']} for data product filenames. The lower-most object (right), not visible on the left, is a spurious detection (non-psf).
• Figure 4: Near-Earth object or triple transient? The object(s) of interest can be found within a 1000 arcsecond box centered at R.A. (J2000)= 145.2448564 and Dec.(J2000)= 15.6224391, J.D.= 2459256.8096296; see Table 2 for astrometric coordinates. A luminous object is seen streaking through the upper-left corner of the third image (lower left), possibly a meteor or aircraft. A fourth image in the $g$ band with no object detected is added to the sequence (lower right), to show the deviation from a straight path. See Table \ref{['UncataloguedAsteroid']} for astrometric measurements and Table \ref{['filenames']} for data product filenames.
• Figure 5: Example with multiple asteroids. From uppermost to lowermost in each image, these objects are the catalogued asteroids 237629 (2001 RZ128), 34921 (4801 P-L), 468722 (2010 GF35), and 408570 (2013 LO5), respectively; they are shown here moving between these image, captured 34 min apart. Both images are $r$ band images and were observed on 2021-03-19. See Table \ref{['Measurements1']} for astrometric parameters, and Table \ref{['filenames']} for ZTF data product filenames.