The Kepler Space Telescope has found several potential habitable exoplanets that exist in the same solar system—so, could there be intelligent life living on them that communicate back and forth with each other? Some experts in the Search for Extraterrestrial Intelligence think it's possible, and that intercepting their communications might be our best bet for proving the existence of aliens.
The idea is called "eavesdropping SETI," and its foundation relies on the principle that, if intelligent life is common in solar systems with Earth-like planets in the habitable zone, then it's not out of the realm of possibility that there may be two distinct alien civilizations living in solar systems with multiple planets in a star's habitable zone.
Alternatively, maybe an alien civilization had the wherewithal to colonize another planet in its solar system, kind of like how some humans would like to do with Mars. Either way, if there is intelligent life on multiple planets in a given solar system, then it only follows that they'd probably want to communicate with each other.
And that's where SETI scientists come in. Data from Kepler has already been used to identify several possible solar systems, and we already have the technology to track the planets' locations with respect to Earth. When two planets line up with Earth, we can point all of our radio telescopes at them, with hopes of "overhearing" communication between the two.
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