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There are billions of stars in the Milky Way similar to the Sun.[3][4]
With high probability, some of these stars have Earth-like planets in a circumstellar habitable zone.[5]
Many of these stars, and hence their planets, are much older than the Sun.[6][7] If the Earth is typical, some may have developed intelligent life long ago.
Some of these civilizations may have developed interstellar travel, a step humans are investigating now.
Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.[8]
And since many of the stars similar to the Sun are billions of years older, Earth should have already been visited by extraterrestrial civilizations, or at least their probes.[9]
However, there is no convincing evidence that this has happened.[8]
