[slg]

>The planet outputs (or reflects) vastly more total light than the laser, but the laser is pointed directly at the receiver and can output more light in that specific direction than the planet does.

So then the problem is that you need a laser than can hit a 1 km^2 target from 40 trillion km away?

[drdrey]

No, the laser would point in the general direction of the sun and as long as Earth is within the cone it has the potential to detect the signal. The laser just happens to focus all of its energy in one narrow cone as opposed to all directions.