So if I understand this correctly, it all boils down to how far the aliens are? If they're too far, there is no way to discern that there are planets around here, but if they're close enough they can detect the presence of a large source of gravity that would potentially intrigue them enough to make a closer look?<p>Or more to the point, it means our chances of discovering inhabitable exoplanets are next to zero despite there being a near infinite number of such, simply because even when we do discover them, what we really discovered is a planet from thousands or even millions of years ago. Useful!