It could find a candidate biosphere. If we see a lot of free oxygen or other super reactive things like fluorine that would be very suggestive. It’s not proof but we don’t know many other processes that can maintain a high percentage of oxidizers in an atmosphere.

If we saw that the next step would probably be a telescope designed specifically to observe that target. There are some thoughts about using a telescope out near Pluto that could use the sun as a gigantic gravitational lens to photograph an exoplanet and get very detailed spectroscopic information.

If we have a planet nine that is a primordial black hole that would be an absolute killer gravitational lens.