TRAPPIST-1 is a cool M8, borderline brown dwarf. Does anyone know whether it's a flare star?
I am puzzled about the bolometric correction for stars like this. The planets receive enough IR to have Goldilocks temperatures, but their visual illuminance can only be a few hundred to a couple of thousand lux. Which means that their full sunlight is only as bright as office lighting, and their partial shade correspondingly stygian. We know that the availability of light constraints plant growth on Earth; there is no reason to believe that an efficient photosynthesis is possible with IR and it seems unlikely on photo-chemical grounds. So if there is life on these planets its photosynthesis can only be crawling along at under 1% of the areic rate on Earth. And that means an oxygen catastrophe at ~200 billion years system age rather than ~2 billion years system age. To find free oxygen in their atmospheres seems very unlikely even if they are life-bearing, but no-one ever seems to mention this.
There may well be more efficient forms of photosynthesis, or other processes serving the same function for plants, which never took off on Earth because photosynthesis was good enough and the ancestors of our extant photosynthetic life outperformed organisms using those hypothetical mechanisms under the light-rich conditions on Earth.
On the other hand, I am more concerned that all over these planets are likely to be tidally locked to their parent star, as close as they are to it. That has got to be a serious obstacle for life.