Five planets are known to orbit the star 55 Cancri .
The recently-discovered planet f at 0.78 AU ( Fischer et al . 2008 ) is located at the inner edge of a previously-identified stable zone that separates the three close-in planets from planet d at 5.9 AU .
Here we map the stability of the orbital space between planets f and d using a suite of n-body integrations that include an additional , yet-to-be-discovered planet g with a radial velocity amplitude of 5 m s ^ { -1 } ( planet mass = 0.5-1.2 Saturn masses ) .
We find a large stable zone extending from 0.9 to 3.8 AU at eccentricities below 0.4 .
For each system we quantify the probability of detecting planets b - f on their current orbits given perturbations from hypothetical planet g , in order to further constrain the mass and orbit of an additional planet .
We find that large perturbations are associated with specific mean motion resonances ( MMRs ) with planets f and d .
We show that two MMRs , 3f:1g ( the 1:3 MMR between planets g and f ) and 4g:1d can not contain a planet g .
The 2f:1g MMR is unlikely to contain a planet more massive than \sim 20 { M _ { \oplus } } .
The 3g:1d and 5g:2d MMRs could contain a resonant planet but the resonant location is strongly confined .
The 3f:2g , 2g:1d and 3g:2d MMRs exert a stabilizing influence and could contain a resonant planet .
Furthermore , we show that the stable zone may in fact contain 2-3 additional planets , if they are \sim 50 { M _ { \oplus } } each .
Finally , we show that any planets exterior to planet d must reside beyond 10 AU .