No biospheres have been detected beyond the Earth; therefore, the existence of extraterrestrial biospheres remains hypothetical. The rare Earth hypothesis suggests they should be very rare, save ones composed of microbial life only.[36] On the other hand, Earth analogs may be quite numerous, at least in the Milky Way galaxy, given the large number of planets.[37] Three of the planets discovered orbiting TRAPPIST-1 could possibly contain biospheres.[38] Given limited understanding of abiogenesis, it is currently unknown what percentage of these planets actually develop biospheres.