Many rocky exoplanets are heavier and larger than the Earth and have higher surface gravity. This makes space-flight on these worlds very challenging because the required fuel mass for a given payload is an exponential function of planetary surface gravity, exp(g0). We find that chemical rockets still allow for escape velocities on Super-Earths up to 10× Earth mass. More massive rocky worlds, if they exist, would require other means to leave the planet, such as nuclear propulsion. This is relevant for space colonization and the search for extraterrestrial intelligence.

A decade of exoplanet search has led to surprising discoveries, from giant planets close to their star, to planets orbiting two stars, all the way to the first extremely hot, rocky worlds with potentially permanent lava on their surfaces due to the star's proximity. Observation techniques have reached the sensitivity to explore the chemical composition of the atmospheres as well as physical structure of some detected gas planets and detect planets of less than 10 Earth masses (MEarth), the so-called super-Earths, among them some that may potentially be habitable. Three confirmed non-transiting planets, and several transiting Kepler planetary candidates, orbit in the habitable zone (HZ) of their host star. The detection and characterization of rocky and potentially Earth-like planets is approaching rapidly with future ground and space missions that can explore the planetary environments by analysing their atmosphere remotely. This paper discusses how to characterize a rocky exoplanet remotely.