"If those civilizations are out there – and we don't know that they are – those that inhabit star systems that lie close to the plane of the Earth's orbit around the sun will be the most motivated to send communications signals toward Earth, because those civilizations will surely have detected our annual transit across the face of the sun, telling them that Earth lies in a habitable zone, where liquid water is stable," says Richard Conn Henry, of Johns Hopkins University. "Through spectroscopic analysis of our atmosphere, they will know that Earth likely bears life. Knowing where to look tremendously reduces the amount of radio telescope time we will need to conduct the search.”
Henry and colleagues think that we limit our search for extra-terrestrial intelligence to the ecliptic plane in which our solar system's planets orbit. This ecliptic band comprises only about 3 percent of the sky, which could make it easier for scientists to effectively narrow their search for intelligent ET.
The logic behind it postulates that if there is another, perhaps more advanced alien civilization in our galaxy out there; they may be trying to contact us, as well. If this is the case, Henry says a search focused on the ecliptic "should lead rapidly to the detection of other civilizations".
Exoplanets in the ecliptic should be able to see Earth passing in front of the Sun. These transits are what Earth astronomers rely on to identify a variety of information about the transiting planets, such as radius, density and composition. Transits also reveal the secret’s of a planet's atmosphere, therefore any potential alien astronomers studying the Earth's spectrum would theoretically find the indicators of life in our atmospheric oxygen, letting them know—just as we long to know—that they are not alone.
Henry, along with his colleagues, plan to search the ecliptic for these advanced alien civilizations with the Allen Telescope Array, a set of dozens of antennae in Hat Creek, California, US.
According to Greg Laughlin, an astronomer and extrasolar planet hunter at the University of California, Santa Cruz, if there is a stargazing civilization trying to make contact with us within 50 light years, its inhabitants would see the Earth as a bluish dot. All they would need is an 8-metre space-based telescope with a good coronagraph along with a set of space-based infrared telescopes, which would enable them to detect ozone and water vapor in our atmosphere.
Most of the 100 billion stars in our Milky Way galaxy are located in the galactic plane, forming another great circle around the sky. The two great circles intersect near Taurus and Sagittarius, two constellations opposite each other in the Earth's sky – areas where the search will initially concentrate.
"We have no idea how many – if any – other civilizations there are in our galaxy,” Henry noted. “One critical factor is how long a civilization – for example, our own – remains in existence. If, as we dearly hope, the answer is many millions of years, then even if civilizations are fairly rare, those in our ecliptic plane will have learned of our existence. They will know that life exists on Earth and they will have the patience to beam easily detectable radio (or optical) signals in our direction, if necessary, for millions of years in the hope, now realized, that a technological civilization will appear on Earth."
Posted by Rebecca Sato
Source: Johns Hopkins University
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