Since the first detected exoplanetary transit in 1999, use of this technique of looking for tiny dips in light as planets pass in front of their parent stars has surged. On paper it is simplicity itself, in practice it requires exquisite precision in measuring the brightness of stars, along with a deep knowledge of the intrinsic behavior of stellar photospheres (the outer layer of stars where the majority of light is emitted), orbital mechanics, and statistical techniques for eking out detections and characterizations of other worlds.

To date the most prolific transit detection experiment has been NASA’s Kepler space telescope. But newer instruments, like NASA’s Transiting Exoplanet Survey Satellite, or TESS, are hot on its heels – albeit with a different set of science goals. 

Kepler’s primary accomplishment was to stare at a single patch of sky from 2009 to 2013, monitoring around 150,000 stars out of the half-million in its field of view. That effort paid off with over 5,000 candidate exoplanets detected as they transited their stars, blocking a minute fraction of the light.

Despite failing reaction wheels onboard the spacecraft, compromising its ability to point at targets, Kepler was ingeniously repurposed to perform what became known as its ‘K2’ mission starting in 2014. By using solar radiation pressure on its solar panels, the spacecraft could be kept stable – as long as it pointed at locations along the ecliptic plane, the plane of Earth’s orbit around the Sun. As a result, Kepler kept gathering data all the way until 2018 when it finally ran out of fuel to maneuver the spacecraft.

The K2 science bounty has been significant. But one of the most recent analyses of the data caught my eye in particular. In a research paper by Kruse, Agol, Luger, and Foreman-Mackey the authors apply a set of analysis tools that look to improve the level of detection sensitivity possible in the K2 data. As a result, they claim detections of over 800 transiting exoplanets, with over 370 of those not previously discovered.

But another notable piece of this lengthy and neat paper is that they report that 154 of these transiting exoplanet candidates “reciprocally transit with our Solar System”.

What that means is that from the point of view of those other worlds, our own solar system will exhibit planetary transits. If anyone was monitoring our Sun they would, in principle, be able to detect at least one of our planets. This is precisely because the K2 data is in the ecliptic plane – it is looking at the only parts of the sky where reciprocal transits are geometrically possible.

The idea that we might want to pay particular attention to places that could, in turn, be staring back at us, is not new in itself. There has even been intriguing work done by my colleagues David Kipping and Alex Teachey positing that advanced civilizations might use knowledge of reciprocal transits to signal or cloak their presence. But the Kruse et al. work is the first that I’m aware of to present a substantial list of candidates and to run the numbers on what the reciprocal transits might look like. This accounts for the small differences in orbital tilt of the planets around our Sun. For example, only one of the studied exoplanetary systems could witness the transits of three Solar System worlds – Jupiter, Saturn, and Uranus - due to the extremely close, 2 in a million, angular alignment required.

The most provocative candidate is a star with four detected exoplanets that, if anyone is looking, would be able to detect a single planet around the Sun. That planet is Earth, with a 365-day orbital period in the nominal habitable zone.

With the renewed interest in SETI these days, and the idea of looking for technosignatures, it seems that we’re starting to find some of the prime targets for proper scrutiny. Simply because these might be the places where there’s somebody, or something, already scrutinizing us.