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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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3 Answers 3

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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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Can you tell us how big a "glimmer" is? And how tihs could possibly identify a planet - which is what the question asks? –  Rob Jeffries Dec 29 '14 at 16:25
    
It's a figure of speech referring to the continuing unlikely probability of a detection. The events being referred to are transits, described in the question and named in the first paragraph –  polyphant Dec 29 '14 at 18:21
    
Fair enough. I think Transit identification of isolated planets will be unfeasible, though a statistical survey might be possible. My -1 was for saying direct imaging was impossible, since this how almost all candidate isolated planets have been found. –  Rob Jeffries Dec 29 '14 at 21:37

Arguably, "rogue planets" have already been discovered by direct imaging.

Giant planets when first formed are big and hot. They radiate their own light, mostly in the infrared. So young isolated planets can be seen directly.

There have been various claims in the literature that objects as small as a few Jupiter masses have been identified in young star forming regions. See various papers by the IAC brown dwarf research group

http://adsabs.harvard.edu/abs/2000Sci...290..103Z

http://adsabs.harvard.edu/abs/2002ApJ...578..536Z

http://adsabs.harvard.edu/abs/2014A%26A...568A..77Z

http://adsabs.harvard.edu/abs/2013MmSAI..84..926Z

These claims are open to criticism - sometimes it is hard to tell whether a faint object really belongs to the star forming region observed, rather than being an unassociated background object. The claimed masses also depend heavily on models for the luminosity-mass relation as a function of age, and the ages of these objects are not easily constrained.

Nevertheless it would not be surprising if, in the maelstrom of the formation of a cluster of stars, some planetary systems were stripped from their parent stars by close encounters with other objects.

The chances of seeing older, isolated, planetary mass objects are slim, but microlensing appears to be the only technique presently available. The microlensing signature of a free-floating planet is of course unrepeatable so a discovered planet could not be followed up in any way. However, surveys of microlensing events could be a way of saying something statistically about how common such objects are. See for example http://astrobites.org/2011/05/24/free-floating-planets-might-outnumber-stars/

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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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