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Do we know for certain that the nearest star to Earth, excluding the Sun and the theorized Nemesis, is the Alpha Centauri multiple star system, at +/-4.4 light-years away? Have we been able to identify every star that is <5 light-years from Earth or is there a possibilty a faint star could be nearer? Are there stars that we haven't looked at yet that could be closer?

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It's begining to look like there aren't as many brown dwarfs as was once thought:

NASA's Wide-field Infrared Survey Explorer, or WISE, has been turning up a new crowd of stars close to home:

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Previous estimates had predicted as many brown dwarfs as typical stars, but the new initial tally from WISE shows just one brown dwarf for every six stars. It's the cosmic equivalent to finally being able to see down a mysterious, gated block and finding only a few homes.

Reasonably current data puts the current closest brown dwarf at 6.5LY (2 parsec) and less than 0.08 solar masses.

A nearer one could show up, but it's not looking likely.

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It's pretty easy to put upper limits on how bright any nearer stars could be. As a rough guide, consider a survey like Hipparcos, which is complete to something like 9th magnitude. If there's another star closer than Alpha Centauri, the brightest possibility is if it's at the same distance. Wolfram|Alpha tells us that a magnitude 9 star at Alpha Centauri's distance would have a luminosity of about 0.0004 times solar. Using the rough main-sequence mass-luminosity relation, $L\propto M^{3.5}$, that would give a mass of about 0.11 times solar. So already, you can see that if there's anything out there it's barely a star even if it's as distant as Alpha Centauri, and otherwise almost certainly a brown dwarf. I'm sure more recent surveys will place even stronger limits.

Admittedly, the catch here is that the brightness will be concentrated at longer and longer wavelengths as the mass decreases, so things could be a bit bigger on the ground that we may have only looked in the wrong band so far. But even so, I think the sum of present observations make it nearly impossible that there are any stars closer to us than Alpha Centauri.

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In this answer you can find a calculation for how bright "Nemesis" would be at near-infrared wavelengths. This calculation assumed we were looking for a 20 Jupiter mass object with a similar age to the Sun at a distance of 1.4 light years (to fit in with the Nemesis hypothesis). The calculated magnitudes were H=14 and W2=8 (in the WISE infrared satellite system).

If we relax the assumptions and let the object be 4.4 light years away, we have to add 2.5 magnitudes to these numbers. i.e. H=16.5 and W2=10.5. Whilst the former is right on the edge of detectability in the 2MASS survey, the former is comfortably detectable in the WISE survey (limit is about W2=15.6 see http://wise2.ipac.caltech.edu/docs/release/allsky/ ) and the object would have a large parallax/proper motion. Whether this would be detectable might depend on the presence of a good "first epoch" image. In the case of the brown dwarf Luhman 16 at 6.5 light years (but which is maybe 40-50 Jupiter masses), well it showed up easily in first epoch 2MASS images (H=9.6).

Now, your question asks whether a star may have been missed. Such an object would be much brighter than the hypothesised brown dwarf above, or Luhman 16. The "industry-standard" models of Baraffe et al. (1998) suggest that a $0.075\ M_{\odot}$ minimum mass stellar object at 4.4 light years would have H=7.5. It is hard to imagine how such an object would have been missed, unless it has such a small proper motion that it has not moved significantly between the 2MASS survey in the 90s to the WISE survey in ~2010. This is unlikely (but not impossible).

Once Gaia results are published in 2017, it will have complete parallax data for all stars down to about V=19. This should include even the lowest mass M-dwarf stars out to around 10 parsecs (30 light years).

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  • $\begingroup$ The hypothetical Nemesis would be a brown dwarf in orbit around the Sun. A nearby brown dwarf that's not in orbit around the Sun would meet the question's criteria, and would not necessarily be "many magnitudes brighter" than Nemesis. $\endgroup$ – Keith Thompson Apr 20 '15 at 18:26
  • $\begingroup$ @KeithThompson If it was at 4.4 light years away it would be 2.5 magnitudes fainter than the hypothesised Nemesis object (which would be at 1.4 ly for a 20 Mjup object) as I stated. However, the OP asks about whether a star could have eluded detection - which on checking, as it turns out you are right, would not be "many magnitudes brighter", it would be about 2.1 magnitudes brighter at H. I have edited to give chapter and verse. $\endgroup$ – Rob Jeffries Apr 20 '15 at 20:14
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    $\begingroup$ Sorry, I was assuming that a "brown dwarf" is a kind of star. I just checked Wikipedia, and it says that brown dwarfs are "substellar objects". Still, finding a brown dwarf closer than Alpha Centauri would be quite interesting. $\endgroup$ – Keith Thompson Apr 20 '15 at 22:04

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