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I don't think there is any reasonable doubt that comets have been ejected from ours and other solar systems and are drifting in the vast voids of space. If there where larger bodies say the size of the Earth or Jupiter drifting in space would be be able to identify their presence? If so how close to our sun would they need to be?

This question assumes that the bodies are not in orbit around a sun as part of a solar system. Rather they are drifting between solar systems or galaxies, either in orbit around the galactic center, or other wise behaving as galactic or universal asteroids.

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Yes, it is possible to detect rogue planets by observing distant stars and measuring the microlensing effect when a big enough object passes in front of them and increases their apparent brightness. If the rogue planet passes one of the observed stars, this microlensing effect will be a lot shorter than when any of the planets orbiting the star would pass in front of it.

So far, we have identified several candidate rogue planets, with the CFBDSIR 2149-0403 being the closest to our Solar system, roughly 100 light years away. There are however huge differences in estimates of how many are out there, these being somewhere between 2 to a 100,000 free-floating planets per each star in the galaxy. It should be noted though, that not all of these rogue planets are thought to have been ejected from the systems they formed in, but many might have been formed on their own, possibly in similar way to how stars are formed, and then orbit the galaxy independent of any other systems.

As for how far they have to be for us to be able to observe their presence, it's rather hard to give an estimate, because the most suitable telescopes for such tasks probably wouldn't be the ones with the highest radial resolution, but powerful yet wider angle telescopes that could monitor many thousands of stars at the same time, like for example the Kepler space observatory. Kepler's main mission was looking for habitable-zone Earth-like planets around distant star systems in our galaxy, but now that only two out of four reaction wheels are working, it might even end up being more useful for the search of rogue planets, sweeping the skies more frequently than it otherwise would have, if it could target more precisely. However, its future is yet to be determined, so let's not jump the gun on this one.

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    $\begingroup$ I personally find the planets that formed outside of a star system more fascinating than ejecta. There was a good paper recently on how it would be possible for a planet like that to have tectonic activity and possibly even form life. Imagining what life might be like in a world that never orbited a star is mind-blowing! $\endgroup$ – called2voyage♦ Sep 12 '13 at 12:52
  • $\begingroup$ It's important to note that while we can detect some rogue planets, we're far from being able to detect them all. It's sorta like detecting moths outside at night when they fly in front of the porchlight. For all we know, there's dozens of Earth-sized planets just outside of the Ort-cloud. $\endgroup$ – john3103 Sep 12 '13 at 18:56
  • $\begingroup$ @john3103 - True, we detect these free-floaters merely by chance now, and I don't know of any project specifically looking for them alone. Tho this "chance" is kinda inherent to astronomy in general, with a lot of discoveries done like that. The good thing about physics that is on our side when it comes to detecting object is, they tend to have a bigger footprint (in terms of arcseconds) the larger and closer they are. Tho it still happens, even rather frequently, that they have to hit us first before we detect them. We're still far from having enough eyes observing our cosmic neighborhood. ;) $\endgroup$ – TildalWave Sep 12 '13 at 20:00

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