DSM:One thing that we cannot do is study the planetary history of a planet. We always see ti at a specific time. This is especially important with Mars. Current Mars, in its planetary habitability, is below Titan. But ancient mars, 500 million years after its formation, had an ocean on it, as it looks. There was sediment, there were flowing rivers. At that time, it still had a magnetic field, it still had a thicker atmosphere. So ancient Mars would have scaled a lot higher up on that index. But this we cannot know from any exoplanet that we look at right now. Mars is still reasonably high, but it used to be higher in the past, and from this we can infer that there is a good chance that there is still life on Mars. But with exoplanets we cannot do that.