You can tell a lot about a star by its twinkle. So far, thousands of exoplanets have been spotted orbiting distant stars, mostly by looking for "transits" as light from the host star dips when a planet passes in front of it. Now, scientists have examined whether Earth could be detected by other civilizations using the same technique, and according to their calculations, 68 known exoplanets are in prime positions to spot Earth and our rocky neighbors.

The transit method has uncovered a veritable treasure trove of interstellar discoveries. More than 2,300 exoplanets have been verified, including seven orbiting TRAPPIST-1, a red dwarf star just 40 light-years from Earth. Astronomers have also spotted one planet with a massive ring system, and there's still the ongoing mystery of Tabby's Star, whose irregular eclipses have led some to suggest an alien megastructure of some kind is orbiting it.

Scientists from Queen's University Belfast and the Max Planck Institute for Solar System Research wanted to turn that idea back onto our home planet, and ask "How would an alien observer see the Solar System?" To find out, they plotted out which parts of the sky would have the best view of the Solar System's planets as they pass in front of the Sun, creating a path through the sky for each one called its transit zone. Interestingly, the smaller rocky planets – Mercury, Venus, Earth, and Mars – would be easier to spot than the more distant giants.

"Larger planets would naturally block out more light as they pass in front of their star," says Robert Wells, lead author of the study. "However the more important factor is actually how close the planet is to its parent star – since the terrestrial planets are much closer to the Sun than the gas giants, they'll be more likely to be seen in transit."

To refine their search, the team focused in on sections of sky where more than one planet could be seen in transit across the Sun. Their calculations found that three planets was the maximum that could be seen from any one location, and even then only in certain combinations of three.

"We estimate that a randomly positioned observer would have roughly a 1 in 40 chance of observing at least one planet," says Katja Poppenhaeger, co-author of the study. "The probability of detecting at least two planets would be about ten times lower, and to detect three would be a further ten times smaller than this."

When the transit zones were compared to the thousands of known exoplanets, the team determined that 68 of them would be able to see at least one Solar System planet pass in front of the Sun. Earth would be visible to nine of them.

So are aliens looking back at us from these exoplanets? Probably not. Even in the unlikely event that another species has figured out the exact same method we use to detect distant planets, none of the nine perfectly-positioned exoplanets are habitable. Using their model, the team calculated that there should be about 10 exoplanets that are both habitable and capable of seeing Earth's transit, but if they're out there we haven't found them yet.

The study was published in the journal Monthly Notices of the Royal Astronomical Society.

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