How the heck can we find, let alone communicate with, extraterrestrial intelligences when there's billions of potentially life-friendly planets to survey?

Some searches look for radio transmissions from stars that are close by, where signals are very strong. Others focus on star clusters, where the law of averages dictates you may have a lot of potential targets in a narrow field of view. A new study, however, proposes a different tactic. It suggests looking for aliens that live in the equivalent of Earth's solar transit zone.

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Most planets that we see orbit in front of their stars (an event known as a "transit") in relatively short amounts of time -- perhaps every few dozen days, or shorter. This is partly because we have limited telescope time and can only look at a star target for so long for the telltale dimming of light that shows a planet.

Someone looking at Earth from far away, however, would need to be more patient. Earth would only cross the sun's face every 365 days -- and only from a narrow field of view of 0.262 degrees when projected on to the universe at large. In other words, an intelligent alien on a distant world would need to be in just the right spot to see our planet transit the sun.

"The initial moment for this study was during a chat over coffee with my former fellow Ph.D. student Dimitris Mislis, during our time at the Hamburg Observatory in Germany," Rene Heller, of the Max Planck Institute for Solar System Research, in Gottingen, Germany, wrote Discovery News via email.

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"It must have been in 2009. Dimitris and I were both working on exoplanet transits at that time. He studied and actually observed the transits of exo-planet TrES-2b, who was suspected to show some weird transit timing variations at that time."

As part of the discussion, the two started speculating on what somebody far away from the solar system would see if they were looking at the planets of the solar system transiting the sun. They realized that Earth is only visible from a small area of the sky. Heller kept some notes, but stashed them away. He came across them again while doing postdoctoral research at the Origins Institute of McMaster University in Canada. With the support of his supervisor, Ralph Pudritz, Heller began researching the subject in more detail.

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Heller and Pudritz constructed a study where they implemented a "target list" of potential stars close to our own sun (including K- and G- type stars). These targets, identified through the well-known Hipparcos catalogue, lie within the Earth transit zone (ETZ) and are within 3,260 light-years from us. Next, the researchers created a galactic disk model to identify the number of potential stars they could survey. Their estimates show there's approximately 10,000 potential K and G dwarf stars that fall within the ETZ. But how to find them all?

Luckily, there's a star-survey mission going on right now. It's called Gaia and the European Space Agency mission has been scanning the entire sky since 2014. The first data release, Heller said, will be sometime this summer. Officials estimate that it will eventually chart about one billion stars that are brighter than a magnitude of around 20, which is really faint and includes stars only visible in high-powered telescopes.

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"Among those stars, there will be millions of G stars like the sun," Heller said. "So within the next few years, we will 'find' more and more sun-like stars in our galactic neighborhood, many of which will be located in the ETZ."

It's common for exoplanet searches to look for Earth-like planets around M dwarfs, which are fainter and smaller stars than our own sun. It's a little easier to see an Earth-like planet since the star is not so bright. Heller points out that there are about 10,000 M dwarfs found extremely close to our own sun (within 326 light-years).

It's not much of a surprise given that about 75 percent of the stars nearby us are M dwarfs, but to search for them requires a different telescope that can look in infrared wavelengths. He suggested using the Sloan Digital Sky Survey which, using a 2.5-meter telescope in New Mexico, which has already found more than 7,000 dwarfs. Heller added he believed there are already at least a few hundred M dwarfs with a visible ETZ, but he has not done a precise count.

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Heller said there is no reason to wait on looking for radio sources from intelligent civilizations in the ETZ. He said the radio telescopes that are available right now could do the search.

"It can be done now," he said, "and given the very limited amount of sun-like stars in the ETZ (limited here means 100,000) a complete ETZ SETI could be performed within a human life time. In other words, we could find out very soon if others have found us using our transits in front of the sun and then started to contact us."

A paper based on the research is available in Astrobiology.