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takyon writes:

Astronomers have identified exoplanets from which potential life forms are likely to be able to observe a transit of one of our solar system's planets:

"The detection of thousands of extrasolar planets by the transit method naturally raises the question of whether potential extrasolar observers could detect the transits of the Solar System planets," they wrote in a paper published [open, DOI: 10.1093/mnras/stx2077] [DX] last month in the Monthly Notices of the Royal Astronomical Society.

[...] The transit method only works if a planet is aligned in a way that it crosses the star. In the Solar System, the terrestrial planets – Mercury, Mars, Earth and Venus – are more likely to be spotted in this way than the gas and ice giants – Jupiter, Saturn, Uranus and Neptune. Up to three planets in various combinations can be seen at any one time, the researchers found. The next step is to find which boundaries are located in the best positions to observe more than one of the terrestrial planets crossing the Sun, and count up the number of exoplanets inside these "transit zones."

Katja Poppenhaeger, co‑author of the study and assistant professor at Queen's University Belfast, estimated that "a randomly positioned observer would have roughly a 1 in 40 chance of observing at least one planet. 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." A full sweep shows there are currently 68 known exoplanets that are in a good spot to catch a planet transiting the Sun. From this list, nine of them are temperate and have sizes similar to Earth, but none are considered to be habitable. That doesn't mean the chances of aliens potentially spying on Earth are completely zero. The researchers estimate that there are ten other unconfirmed exoplanets that have more favorable conditions of sustaining life, and are within the transit zones.

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