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　　Carl Sagan once referred to our home planet as a mote of dust suspended in a sunbeam."

　　And that poetic description holds true for a lot of exoplanets, too.

　　In fact one of the simplest ways we detect exoplanets is by looking at their sunbeam…and measuring how it dims, ever so slightly, as the exoplanet passes across it, called a transit.

　　Which raises an interesting question:

　　"Thinking about extraterrestrial observers—which of them would observe the Earth, moving across our own sun?"

　　Ralph Pudritz, a theoretical astrophysicist at McMaster University in Canada.

　　He and his colleague Rene Heller quantified the narrow band of space from which any observers on other worlds would be able to see the Earth transiting the sun.

　　And they determined that this line of sight would be a plane just half a degree thick,but that cuts through a slice of our galaxy that's estimated to contain 100,000 sunlike stars… along with their companion planets.

　　The analysis is in the journal Astrobiology.

　　Pudritz and Heller suggest that hunters of extraterrestrial life may want to listen particularly closely for signals originating at star systems within that narrow band of galactic sky.

　　They say that advanced civilizations there may have already detected us using the transit method, and may now be sending us a message.

　　You might think of this paper, then, like a treasure map, for intelligent life.

　　"We don't know if there's treasure out there of course, right?

　　You never know that about a treasure map.

　　But it's a good place to look.”

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