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Context. The 3-planet system that contains two super Earths has recently been verified around the M dwarf Gliese 581. Recent theoretical work into the system's habitable zone (HZ) still raises questions as to the habitability of the known planets; however, the system remains the best candidate for potentially habitable planets. Aims. We address the possible existence of an undetected, lower mass planet inside the conservative HZ through the dynamical stability evolution of the system. This is important since recent studies consider planets c and d to be at the edges of the HZ, while the region in between might harbor a better candidate for habitability. These planets have a radial velocity contribution just below the measurement uncertainty, which would be the reason they have not been detected previously. Thus, our considered planetary masses are maximum masses for the undetected planet. Methods. Using reported planetary parameters of the three detected planets, we numerically integrate the system for 10 6  yr with mass-less test particles scattered inside the HZ. This specifies possible regions for stable orbits. We then integrate the system for 10 7  yr with a fourth lower mass planet inside the specified stability regions. Results. Our study found a high probability that stable systems occurred for a fourth planet of ~$2.5~M_{\oplus}$ with a semi-major axis between 0.11 AU and 0.21 AU and eccentricity between 0.01 and 0.25. In stable systems, the fourth planet had very little effect on the overall stability of the system. There, the dynamical evolution of the three detected planets was comparable to that of 3-planet models. The eccentricity of the undetected planet had small amplitude variations and its semi axis remained extremely stable. Thus, Gliese 581 remains a good candidate for future detection of habitable Earth-mass planets.

Additional planets in the habitable zone of Gliese 581?