Earth-mass planets in exoplanetary systems

So far, all the known exoplanets are considerably more massive than the Earth. The absence of Earth-mass planets could be due largely to observational selection effects that discriminate against low mass. Orbital studies of putative Earth-mass planets in the habitable zones of known exoplanetary systems have shown that in a reasonable proportion of these systems there are stable orbits with semimajor axes that remain confined to the habitable zone. Whether the Earth-mass planets could have formed in these habitable zones is a question that needs further study.