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.