Earth through the looking glass: how frequently are we detected by other civilizations through photometric microlensing?

Abstract

Microlensing is proving to be one of the best techniques to detect distant, low-mass planets around the most common stars in the Galaxy. In principle, Earth’s microlensing signal could offer the chance for other technological civilisations to find the Earth across Galactic distances. We consider the photometric microlensing signal of Earth to other potential technological civilisations and dub the regions of our Galaxy from which Earth’s photometric microlensing signal is most readily observable as the ‘Earth Microlensing Zone’ (EMZ). The EMZ can be thought of as the microlensing analogue of the Earth Transit Zone (ETZ) from where observers see Earth transit the Sun. Just as for the ETZ, the EMZ could represent a game-theoretic Schelling point for targeted searches for extra-terrestrial intelligence (SETI). To compute the EMZ, we use the Gaia DR2 catalogue with magnitude G < 20 to generate Earth microlensing probability and detection rate maps to other observers. Whilst our Solar system is a multi-planet system, we show that Earth’s photometric microlensing signature is almost always well approximated by a binary lens assumption. We then show that the Earth is in fact well-hidden to observers with technology comparable to our own. Specifically, even if observers are located around every Gaia DR2 star with G < 20, we expect photometric microlensing signatures from the Earth to be observable on average only tens per year by any of them. In addition, the EMZs overlap with the ETZ near the Galactic centre which could be the main areas for future SETI searches.

The Breakthrough Listen Search for Intelligent Life: Nearby Stars’ Close Encounters with the Brightest Earth Transmissions

“…Sheikh et al (2020) identify stars in the Earth Transit Zone that would potentially know of the presence of Earth by observing Earth transit the Sun. Similarly, Suphapolthaworn et al (2022) identify stars that may see the microlens signal of Earth. A corollary of the Earth Transit Zone theory has led to the SETI observations of transiting planets (Siemion et al 2013;Traas et al 2021), with their ecliptic planes aligned with our line of sight.…”

The Possibility of Detecting our Solar System through Astrometry

“…Here we propose a probing question that supposing the extraterrestrial observers are using the astrometry method and are also surveying the Galaxy for habitable worlds, which of them could discover the planets in the solar system, even the Earth? Previous works have investigated the region in which the Earth will be observed transiting in front of the Sun (Heller & Pudritz 2016;Kaltenegger & Pepper 2020;Kaltenegger & Faherty 2021) and the frequency that the Earth will be detected by other civilisations through photometric microlensing (Suphapolthaworn et al 2022).…”