What if an advanced alien civilization saw our planet across the interstellar expanse and switched on their powerful radio transmitters to send a "hello neighbor!" message... but all of our radio antennae were staring at KIC 8462852 when we really should be paying attention to Omicron Persei.
Space is big and there's billions of stars in our galaxy where hypothetical transmitting extraterrestrials could be orbiting; we may be missing out on a flood of alien phone calls every day just because we can't monitor them all.
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Enter René Heller and Ralph Pudritz of the Max Planck Institute for Solar System Research in Göttingen, Germany, and McMaster University in Canada, respectively, who pondered this very question and arrived at a wonderfully elegant solution. Granted, their strategy, which has been published in the journal Astrobiology, doesn't tell us exactly where intelligent aliens are holed up, but it may better the odds of us tapping into the galactic switchboard.
Transiting Exoplanets When astronomers look for exoplanets - planets that orbit other stars - one key method is to seek out their transit signal. NASA's Kepler Space Telescope, for example, spent its primary mission staring at the same patch of sky occupied by approximately 150,000 stars. Periodically, Kepler would detect a very slight dip in brightness in these stars indicating that an orbiting exoplanet had passed in front, blocking some starlight. This event is known as a "transit" and hundreds of new worlds have been confirmed using this method.
Of particular interest are the worlds with the smallest silhouette that pass in front of sun-like stars. These worlds are the proverbial "Holy Grail" as they could be worlds with Earth-like qualities. If they are found to exist at just the right distance from their host star that is neither too hot or too cold (a region known as the "habitable zone") liquid water may exist on their surfaces. As we know from life on Earth, liquid water is essential for life (as we know it) to evolve. So: 1) find an exoplanet orbiting a sun-like star, which is 2) approximately the same size as Earth and 3) realize it is orbiting in that star's habitable zone and you have the recipe for a world that could potentially harbor life. Possibly intelligent life.
PHOTOS: How Aliens Can Find Us (and Vice Versa)
In the future it is hoped that powerful next-generation space telescopes (like NASA's James Webb Space Telescope) will also be able to study the spectroscopic signature of chemicals produced by biological processes (a.k.a. biomarkers) in the atmospheres of these transiting exoplanets.
So far, exoplanetary studies of small worlds have gotten this far and it has been enough for us to slew radio antennae at these candidates to listen out for narrow-band radio signals leaking from that star system. As we can consider ourselves "intelligent" (more or less), we model our hypothetical aliens on our ability to use radio to communicate. Perhaps they have the same plan to communicate over the light-years.
But there's a flaw with this logic. To detect a radio signal over interstellar distances, the transmitting aliens had better be pointing their transmitters straight at us otherwise it will be highly unlikely that we'll hear them. Conversely, if we find a ripe-looking exoplanet at some random position in the sky, fire our "hello neighbor!" message at them, they had better have radio antennae pointed in our direction. If either of these two conditions aren't met, we can forget about playing galactic pen pal with anyone; the signals will be wasted on star systems that are either void of intelligent life or intelligent life has no clue someone is transmitting at them. Not cool.
An Interstellar Switchboard?
According to Heller and Pudritz some of this extraterrestrial ambiguity may be removed from the equation when we consider how aliens may discover us.
ANALYSIS: Are We Searching for Aliens in the Wrong Place?
Just as we look to other stars to detect transits of orbiting planets, perhaps intelligent extraterrestrials have adopted the same astronomical methods as us. Perhaps they scan the sky looking for small worlds that also transit stars. Perhaps, just perhaps, they also feel compelled to fire radio transmissions at those "ripe" transiting worlds. Therefore, it might be a good idea to identify which sun-like stars are at just the right angle to see our Earth pass in front of our sun from their perspective! Suddenly we've narrowed down the number of stars in our galaxy that can actually detect us and maybe we have taken the first step in establishing where we could look for incoming messages from curious aliens.
"It's impossible to predict whether extraterrestrials use the same observational techniques as we do," said Heller in a press release. "But they will have to deal with the same physical principles as we do, and Earth's solar transits are an obvious method to detect us.
The researchers have now mapped a thin band they call the Earth's "transit zone" (pictured top) that projects along the plane of the ecliptic (the plane in which the Earth orbits the sun) and reaches out into the galaxy. Any stars within this transit zone will be able to see Earth orbit in front of the sun, thus realizing there's a small rocky world orbiting within the habitable zone of a star. 82 nearby sun-like stars occupy this zone and could therefore be very inviting SETI targets.
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"The key point of this strategy is that it confines the search area to a very small part of the sky," Heller added. "As a consequence, it might take us less than a human life span to find out whether or not there are extraterrestrial astronomers who have found the Earth. They may have detected Earth's biogenic atmosphere and started to contact whoever is home."
It's interesting to turn the tables and consider that we're the aliens living on that small rocky world and other beings within the transit zone could be pondering the possibility of life on Earth. If they send us a message, hopefully we'll now be able to receive it.
Sources: MPS, McMaster University