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Planets are usually found by observing a star and waiting for the light level to drop when a planet passes in front of it, but what about rogue planets that don't have host stars?

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The only way really is through the transit method you describe in your question, however It's pretty much a statistical improbability that a rogue planet will pass through the line of sight between us and another star of which it is not a planetary member.

The Transiting Exoplanet Survey Satellite would provide a glimmer of hope of identifying some of these events. It would require this kind of constant observation since the transit will only occur once and not regularly as an orbiting planet transit would.

A distant stars light could be gravitationally microlensed by the rogue planet, however the planet would have to be very large to produce a noticeable effect (more of a brown dwarf than a rogue planet) and even then the effect would be fleeting.

Direct imaging would be pretty much impossible since the rogue planet would not be close enough to a star to reflect a substantial amount of its light.

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The most successful methods are by gravitational microlensing and direct observation in the infrared or far infrared.

The transit method doesn't work well for rogue planets, because usually at least three transits are needed to confirm a planet.

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