To spot a planet nowadays you need:<br />* either a detectable doppler effect on the light of the centralstar, due to the toggle exerted by the planet of the star (similar to the fact that Earth rotates about the CoG of Earth+Moon, i.e. about a point more than 4000km from Earth's center. With this method, you detect big planets, often very close to the star because you observe the star over a relatively short period. <br />* or a transit, but that requires the system to be seen "edge-on" (i.e that the Sun is in the plane of the star's ecliptic) = /> occurs sometimes, but seldomly<br />* or by imaging the planet = /> achieved for now on 2 or 3 of them, because they are very big, very young (hence very hot, i.e. very bright in infra-red) and far from the star (>50 AU typically)<br /><br />So we may still detect planets in the alpha centauri A-B + proxima system. But probably relatively small ones or very far ones (*) (**)...<br /><br />(*) unless you see the star above its pole. Then the radial velocity method does not work even for a big planet. I do not know whether centauri is in that scheme.<br /><br />(**) if you have a Neptune at more than 100 AU from alpha centauri A+B, with a period of about 1000 years, you will find it very hard to spot by radial velocity.<br /><br />You can see there are many ways for an alpha-centaurian planet to have escaped our detection methods.