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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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up vote 2 down vote accepted

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

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

EDIT: It is also worth noting that the whether these things really are "planets" at all is disputed. They could either be genuine planets, formed in the same way that is hypothesised for most giant planets - that is by accretion onto a rocky core that formed around a star. They could then have been displaced from their parent star by dynamical interactions with other bodies in their system or with a third body. As I said above, N-body simulations do predict that this will happen (e.g. Liu et al. 2013).

On the other hand they could represent the very lowest mass gas fragments that are able to form during the collapse and fragmentation of a molecular cloud and that for some reason were unable to accrete further gas (i.e. they are really more like low-mass brown dwarfs). This so-called "fragmentation limit" is of order 10 Jupiter masses, but if it were a little lower it might explain the free-floating planets that have been seen so far.

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Seems sensical. Stars don't just pop out of the ether. They would all grow similar to a planet until accruing enough mass for fusion, so it would be a surprise if such bodies didn't exist. Presumably, not all of such bodies grow to stellar levels, so it would follow that some would persist as such. – Mitch Goshorn May 19 '15 at 6:03
@MitchGoshorn Most researchers think giant planets and stars form in different ways and that this is their defining characteristic. So the origin of the "free floating planets" is arguable at present. Relevant N-body codes show that planets get knocked out during early life in a cluster (where most stars form). There is a "fragmentation limit", that could be as high as 10 Jupiter masses that prevents " planets" forming in the same way as stars. But some say this lower limit could be lower. – Rob Jeffries May 19 '15 at 6:40
They may be distinct processes, but until it becomes a star or a star appears nearby, the composition of the cloud from which it is growing should be fairly consistent. They aren't limited to an orbital pathway as in typical planet formation, so they may eventually grow larger than your typical gas giant, but compositionally they should be quite similar to neighboring gas giants. – Mitch Goshorn May 19 '15 at 7:56
No argument, basically the same as your edit (I hadn't seen it, apologies). – Mitch Goshorn May 19 '15 at 8:59

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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