What's the minimum distance of the closest possible intelligent life?

[Number Six]

Forget microbes on Mars or anywhere else. I'm talking human-ish life with some semblance of a civilization. I assume that could occur only on a planet with a sun but someone correct me if that is incorrect.

The minimum distance from earth of such life is X. What is X? I think that the nearest star to earth is Alpha Centuri at 4 light years. Do we know for certain that there is no intelligent life on planets orbiting that star? If so, what is the next farthest star and can we say for sure that there is no intelligent life on the planets around it? How far away from earth can we go before we first say "There could be intelligent life there but we simply can't say for sure one way or the other?"

[Third Eye Open]

It's Proxima Centauri, but yes 4 ly is right. As far as I know, we don't have the ability to detect what kind of planets are orbiting stars, only if they are there at all or not.

Here is a list of the closest stars to us.

http://www.astro.wisc.edu/~dolan/constellations/extra/nearest.html

[BlackKat]

While we have detected over 400 planets orbiting other stars:

http://www.nytimes.com/aponline/2009/10/19/science/AP-US-SCI-New-Planets.html?_r=1&ref=science

The size of said planets is rather large, some even significantly larger than Jupiter. The method for such detection involves the wobble those large bodies give the stars from their own gravitational force.

Detecting earth sized planets, assuming one needs earth like conditions at least in size/mass to give rise to something bipedal, is still not possible.

But it is thought that smaller, rocky planets such as Earth, Mars, or Venus are not likely to be uncommon. However until this can be verified any guess on the proximity of intelligent life is conjecture.

You would also need the right sort of star. Something like Proxima Centauri is prone to frequent flares, making it more likely to wipe out intelligent life rather than host it.

[geni]

We haven't ruled out intelligent life in the jubiter system yet.

As for stars we can't rule out small planet around alpha centauri.

Proxima Centauri is unlikely since any planet getting a worthwhile amount of heat from the star would have to be so close that radation would be a serious issue.

[Roboramma]

The nearest possible intelligent life? Tough question. For all we know there could be intelligent life in the oceans of Europa, or, even less likely, floating in the Jovian skies. I don't find either of those likely, and would be pretty excited if we even found microbes in Europa, let alone animal-like or particularly intelligent life, but one never knows until one looks.

As for other stars, the best candidate so far is this one:
http://news.nationalgeographic.com/news/2009/04/090421-most-earthlike-planet.html

Gliese 581, a red dwarf star in the constellation Libra, lies around 20.5 light-years from Earth.

A rocky world found in its star's "habitable zone".
We are getting better and better at finding smaller and smaller exoplanets, so stay tuned.

[Jeff Corey]

The closest to intelligent life is sleeping about 3 meters from me right now.

[wollery]

The problem with detecting an Earth-like planet in an Earth-like orbit around a Sun-like star is that the Doppler wobble produced would be at about the same amplitude as the star's surface variability. It is just about possible to detect, but it would take tens of years of observation to find the underlying pattern.

A better bet is to find a transiting planet, but again, the change in brightness for a Sun-like star with an Earth-like planet transiting is very small, and the probability of the orbit being aligned right is relatively small.

The next possibility is direct imaging of the reflected light, which requires a decent coronagraph. We're just starting to use these in earnest, and it's going to take a while, because you can't just take a single image - any small point of light you find could be a background star, so you need to take more images over a long period of time to track the orbit.

As for the planet having the right atmosphere, you need to take a spectrum and look for the absorption lines of some specific molecules. Again, that's possible, but very difficult at present.

The parent star could be anything remotely Sun-like, provided it doesn't kick out too much UV or have strong X-ray flares. Anything from early F type to early M type could harbour life. But the planet has to be at the right distance from the star to allow for water to be liquid.