[gus_massa]

But you should be able to use it as a "parabolic" mirror, to make a very directed ray to the planet. (Assuming diffraction is not a problem.) (Assuming no time delay, because to see the planet you should look to were it was many years ago, but to send a message you should aim to where it will be many years in the future.) (Assuming I'm not missing a few more technical problems that are not impossible to solve, but extremely difficult.)

[largbae]

Wouldn't you have to have very accurate information about where the planet is going to be when the light arrives?