What the paper says is that if we look at a planetary system then some of the light reaching us will be light from the star that has hit the planet (or planets) and been reflected from the planet towards us.
The light from the star is unpolarised, but after being reflected from the planet the light will be partially polarised just like any reflected light.
Only very small fraction of the light reaching us will be light reflected from the planet, because most light just comes from the star straight to us without reflection.
But the paper says even though the percentage of the light reaching us that has been reflected is very small, that small polarisation may still be detectable.
e.g. suppose the light from the star is already slightly polarised for some reason, then this could swamp the polarisation due to reflection from the planets.
No, because the way the light reflects, e.g. the reflection angle, will change as the planet rotates around the star. So we should see a periodic change in the polarisation. The period would be the same as the orbital period of the planet.
