Habitable exoplanets around low-mass stars could be the most common type of habitable planets. Chemical rockets could be inadequate for many of these exoplanets.
Researchers suggest that a “reasonable” cutoff for interstellar travel is given by m0/mf ≈ 1000. In this case, a payload with mass comparable to that of the Apollo mission (∼ 45 tons) would require a fuel mass that is one-fifth the mass of the Sears tower.
In other words, it will not be easy for technological species orbiting low-mass M-dwarfs, such as Proxima Centauri and TRAPPIST-1, to undertake interstellar travel by means of chemical rockets. Consequently, these species may opt for alternative means of propulsion such as light sails, which have the added advantages of dispensing with carrying the fuel and achieving relativistic speeds.