So I’ve learned that rogue planets have actually been discovered (2 to 7 of them, the first confirmed in 2013). All of them are very large (sub-brown dwarfs) and young, and so warm enough to be seen directly (usually in infrared); discovering an earth-sized rogue planet would be far harder. Estimates of the number of rogue planets in the galaxy vary from 2 to 100000 per star.
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