"Why focus on ultra-cool stars?
We chose to survey stars that have masses less than 15% the mass of the Sun, for planets. TRAPPIST-1A, the star at the centre of the TRAPPIST-1 system, is representative of our sample. These objects may appear exotic in comparison to our Sun, however they provide various advantages for the study and the understanding of Earth-like planets.
1 -- Ultra-cool stars are the most frequent kind of star in the Galaxy. Finding planets orbiting those stars is akin to study the most common planets that exist, which is essential to understanding the formation of Earth-like planets. They are also crucial in order to one day establish with what frequency biology has emerged in the Cosmos.
Finding planets with conditions, architectures and properties different from those of the Solar system allows us to place our Earth in context within the many outcomes of Nature.
2 -- Ultra-cool stars are small. This improves enormously our capacity to discover planets with the transit method. It also helps with the subsequent investigations that are required to learn about their atmospheres. The small size of the host star means the transit signals produced by Earth-sized planets are 80x more pronounced compared to similar planets transiting a Sun-like star (the signals from the atmosphere are also enhanced by 80 times). It is also nearly 50 times faster to confirm a planet. The Earth takes one year to orbit the Sun, whereas around ultra-cool dwarfs, a similarly temperate planet occupies an orbit lasting only a week.
3 -- It is currently beyond our means to measure the radius and the mass of an Earth-like planet with an Earth-like temperature, orbiting a Sun-like star, whereas we did it for TRAPPIST-1. Although we all think it is important to search for Earth-like planets orbiting Sun-like stars, this is not currently practical. While our technologies and our analysis tools mature, we can start the study of terrestrial planets now, by considering ultra-cool stars, instead of waiting to do so in a decade or two on solar-like stars.
4 -- Finding planets orbiting ultra-cool dwarfs means that we find planets similar to our Earth on several aspects , but different on several others. For instance, the amount and type of light the planets receive is not the same as what we receive on Earth. Also the proximity of the TRAPPSIT-1 planets to their star means that they are likely to be tidally-locked. This signifies that there is a permanent dayside and a permanent nightside. How this affects climate remains mostly unknown. Planets like those of TRAPPIST-1 will open the study of what appears to our eyes like exotic climates, but that may in fact be some of the most usual climates outside the Solar system."