The finding suggests there may be many ways for a planet to end up in a star's habitable zone, the astronomers added.
More detailed studies of HD 40307's brood are unlikely because the planets do not appear to transit, or pass in front of, their parent star, relative to Earth's line of sight.
The new findings are based on a re-analysis and refinement of data collected by Europe's High Accuracy Radial velocity Planet Searcher (HARPS) instrument, a light-splitting spectrograph installed on Europe's La Silla Observatory in Chile. Planets beyond the solar system can be detected by tiny gravitational tugs they exert of the light coming from their parent stars.
To find HD 40307's sixth planet, scientists had to make the difficult distinction between starlight impacted by a planet's gravity and the effects of stellar activity, such as flares and magnetic storms.
"All we know at this point is that it has a minimum mass of about 7.1 Earth-masses. We have no explicit follow-up planned, thought the HARPS team is probably still gathering more data, and may in the future be able to confirm these results, and perhaps add even more planets to the brood," astronomer Steven Vogt, with the University of California's Lick Observatory, wrote in an email to Discovery News.