Key takeaways

  • Earth’s location and characteristics make it hard for aliens to detect using microlensing.
  • This method detects planets by observing light changes when a star passes in front of another.
  • Microlensing can find planets much farther away than other methods, up to seven times the distance.
  • Defined areas in the galaxy where Earth’s signal is most visible to potential observers.
  • Only about 15 observers per year could detect Earth, assuming advanced civilizations exist.

An international team of researchers has found that Earth is “well-hidden” from alien observers using photometric microlensing to seek for habitable planets that may sustain life. The discoveries may also help us narrow down the best locations of the galaxy to target in our own searches for extraterrestrial intelligence (SETI).

Astronomers use a range of technologies to search for potentially habitable worlds outside the solar system. Currently, the transit approach has been the most successful, accounting for around 75% of all exoplanet discoveries. This method includes observing for periodic dimming of a star’s brightness as a planet passes between it and an observer on Earth.

However, the transit approach has flaws, the most notable of which is that transits occur only for a small proportion of planets whose orbital planes are almost edge-on to Earth. Photometric microlensing is an alternate strategy that uses the gravitational lens effect when one star passes in front of another, briefly intensifying the light from the more distant, “source” star. If the nearer star has a planet circling it, the light can be further perturbed, resulting in recognizable spikes.

Long-distance technique

The microlensing technology has the benefit of being able to work across quite large distances. While previous exoplanet detection methods have often only found planets up to one kiloparsec (approximately 3200 light-years) away from Earth, the bulk of the 130 exoplanets discovered so far using microlensing are up to seven times that distance away. Given the Milky Way’s diameter of roughly 30 kiloparsecs, it is possible that other technological civilizations will employ the microlensing approach to discover Earth across galactic distances.

It has long been assumed that those locations from which Earth could be detected via the transit method are also good candidates for targeted SETI searches, following a game theoretic, “Schelling Point” cooperation strategy for two parties looking for each other who have no means of communication. Applying the same rationale to the microlensing technology offers the potential to reveal new and distant targets in the quest for alien intelligence.

Astronomer Eamonn Kerins of the University of Manchester and his colleagues conducted a new study in which they examined the Earth’s photometric microlensing signal as it may seem to other potentially technologically sophisticated civilizations.

Defining the EMZ

“[We] dub the regions of our galaxy from which Earth’s photometric microlensing signal is most readily observable as the ‘Earth microlensing zone’ (EMZ),” the researchers explain, adding: “The EMZ can be thought of as the microlensing analogue of the Earth Transit Zone (ETZ) from where observers see Earth transit the Sun”.

The researchers examined data from the European Space Agency’s Gaia telescope, notably its second data release (DR2), which contains information on over 1.1 billion stars. The researchers divided the sky into small sections and figured out where Earth’s microlensing signal would be visible. Even if technologically advanced alien civilizations were found around every star studied, the team discovered that the total Earth discovery rate across the entire sky is only 14.7 observers per year, implying that, assuming technological life is actually relatively rare, it is “very doubtful” that anyone has spotted us using microlensing.

“Earth would be a challenging target,” Kerins told Physics World, citing the fact that it is too close to the Sun to provide a significant lensing signal for most possible observers. Added to that, “our location 27,000 light-years from the [Milky Way’s] galactic centre is something of a blind spot for any observer using microlensing.”

Background stars needed

To have a decent chance of identifying us, Kerins stated, an extraterrestrial civilization would need to be positioned so that there were many background stars behind us, giving the Earth a strong chance of diverting light from one. “The best position for an observer to be is right at the edge of our galaxy with us on a line of sight towards the galactic centre,” he said. He further added: “But there are very few stars at the edge of our galaxy and so presumably few observers.”

Defining EMZs as those locations with the top 1% of discovery rates, ideal regions for detection emerge in the Large and Small Magellanic Clouds, as well as at low galactic latitudes around the galactic center — where there is some overlap with ETZs.

According to the researchers, the direction from Earth in which prospective extraterrestrial civilizations have the best probability of detecting Earth is toward the Milky Way’s Orion-Cygnus arm on the galactic plane. The Earth’s microlensing probability and discovery rate are 3.28×10−10 and 2.35×10−2 observers per year per square degree, respectively.

The findings are as follows: “Overall, it seems the Earth is very dark to photometric microlensing discovery by other observers, unless they have sensitivity well beyond our own present capabilities.”

Martin Dominik, an astronomer at the University of St Andrews who was not involved in the current work, speculates, “Clever aliens might want to use gravitational microlensing for finding candidate planets to search for other civilizations.” He continues: “It appears intriguing that they won’t be able to detect Earth transiting in front of the Sun unless they are in a narrow strip close to the ecliptic plane, which does not make it a good option to get to know each other!”

The study is described in Monthly Notices of the Royal Astronomical Society.

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