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We investigate constraints on additional planets orbiting the distant M-dwarf star OGLE-2005-BLG-390L, around which photometric microlensing data has revealed the existence of the sub-Neptune-mass planet OGLE-2005-BLG-390Lb. We specifically aim to study potential Jovian companions and compare our findings with predictions from core-accretion and disc-instability models of planet formation. We also obtain an estimate of the detection probability for sub-Neptune mass planets similar to OGLE-2005-BLG-390Lb using a simplified simulation of a microlensing experiment. We compute the efficiency of our photometric data for detecting additional planets around OGLE-2005-BLG-390L, as a function of the microlensing model parameters and convert it into a function of the orbital axis and planet mass by means of an adopted model of the Milky Way. We find that more than 50 % of potential planets with a mass in excess of 1 M_J between 1.1 and 2.3 AU around OGLE-2005-BLG-390L would have revealed their existence, whereas for gas giants above 3 M_J in orbits between 1.5 and 2.2 AU, the detection efficiency reaches 70 % ; however, no such companion was observed. Our photometric microlensing data therefore do not contradict the existence of gas giant planets at any separation orbiting OGLE-2005-BLG-390L. Furthermore we find a detection probability for an OGLE-2005-BLG-390Lb-like planet of around 2-5 %. In agreement with current planet formation theories, this quantitatively supports the prediction that sub-Neptune mass planets are common around low-mass stars.

In order to obtain a census of planets with masses in the range of Earth to Jupiter, eight telescopes are being used by the combined microlensing campaign of the PLANET and RoboNet collaborations for high-cadence photometric round-the-clock follow-up of ongoing events, alerted by the OGLE and MOA surveys. In 2005 we detected a planet of 5.5 Earth masses at 2.6 AU from its parent 0.22 MA M star. This object is the first member of a new class of cold telluric planets. Its detection confirms the power of this method and, given our detection efficiency, suggests that these recently-detected planets may be quite common around M stars, as confirmed by subsequent detection of a ~ 13 Earth-mass planet. Using a network of dedicated 1 2-m-class telescopes, we have entered a new phase of planet discovery, and will be able to provide constraints on the abundance of frozen Super-Earths in the near future.