Search for an alien communication from the Solar System to a neighbor star

TL;DR

This study investigates the possibility of alien interstellar communication from our Solar System to Wolf 359 using gravitational lensing, but finds no evidence of such signals or moving objects in the observed data.

Contribution

First observational search for alien signals from Solar System to Wolf 359 based on gravitational lensing hypothesis, including analysis of potential moving transmitters.

Findings

No detected optical signals from Solar System to Wolf 359.

No moving objects consistent with alien transmitters up to magnitude ~23.5.

Constraints on the presence of nearby alien probes within 20 au.

Abstract

Under the hypothesis that self-reproducing probes have formed a galactic-scale communication network by direct Gravitationally-Lensed (GL) links between neighboring systems, we identify Wolf 359, the third nearest stellar system, as an excellent target for a search for alien interstellar communication emitted from our Solar System. Indeed, the Earth is a transiting planet as seen from Wolf 359, meaning that our planet could pass in an alien communication beam once per orbit. We present a first attempt to detect optical messages emitted from the Solar System to this star, based on observations gathered by the TRAPPIST-South and SPECULOOS-South robotic telescopes. While sensitive enough to detect constant emission with emitting power as small as 1W, this search led to a null result. We note that the GL-based interstellar communication method does not necessarily require to emit from the so-called "Solar Gravitational Line" (SGL), starting at ~550 au from the Sun, and that the probe(s) could be located closer to the Sun and off-center relative to the SGL, at the cost of a smaller but still significant gain compared to a non-GL-boosted communication strategy. Basing on this consideration, we searched in our data for a moving object whose motion would be consistent with the one of the hypothesized alien transmitter, assuming it to use a solar sail to maintain its distance to the Sun. We could not reliably identify any such object up to magnitude ~23.5, which corresponds to an explored zone extending as far as Uranus' orbit (20 au).