The GJÂ 581 planetary system was already known to harbour three planets , including two super-Earths planets which straddle its habitable zone .
We report here the detection of an additional planet – GJ 581e – with a minimum mass of 1.9 M _ { \oplus } .
With a period of 3.15 days , it is the innermost planet of the system and has a \sim 5 % transit probability .

We also correct our previous confusion of the orbital period of GJÂ 581d ( the outermost planet ) with a one-year alias , thanks to an extended time span and many more measurements .
The revised period is 66.8 days , and locates the semi-major axis inside the habitable zone of the low mass star .

The dynamical stability of the 4-planet system imposes an upper bound on the orbital plane inclination .
The planets can not be more massive than approximately 1.6 times their minimum mass .