Most known extrasolar planets (exoplanets) have been discovered using the radial velocity$^{bf 1,2}$ or transit$^{bf 3}$ methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17--30% (r
efs 4, 5) of solar-like stars host a planet. Gravitational microlensing$^{bf 6rm{bf -}bf 9}$, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing$^{bf 10}$. These planets are at least as numerous as the stars in the Milky Way$^{bf 10}$. Here we report a statistical analysis of microlensing data (gathered in 2002--07) that reveals the fraction of bound planets 0.5--10 AU (Sun--Earth distance) from their stars. We find that 17$_{bf -9}^{bf +6}$% of stars host Jupiter-mass planets (0.3--10 $MJ$, where $MJ {bf = 318}$ $Mearth$ and $Mearth$ is Earths mass). Cool Neptunes (10--30 $Mearth$) and super-Earths (5--10 $Mearth$) are even more common: their respective abundances per star are 52$_{bf -29}^{bf +22}$% and 62$_{bf -37}^{bf +35}$%. We conclude that stars are orbited by planets as a rule, rather than the exception.
We observed the microlensing event MOA-2007-BLG-192 at high angular resolution in JHKs with the NACO adaptive optics system on the VLT while the object was still amplified by a factor 1.23 and then at baseline 18 months later. We analyzed and calibra
ted the NACO photometry in the standard 2MASS system in order to accurately constrain the source and the lens star fluxes. We detect light from the host star of MOA-2007-BLG-192Lb, which significantly reduces the uncertainties in its char- acteristics as compared to earlier analyses. We find that MOA-2007-BLG-192L is most likely a very low mass late type M-dwarf (0.084 [+0.015] [-0.012] Modot) at a distance of 660 [+100] [-70] pc orbited by a 3.2 [+5.2] [-1.8] Moplus super-Earth at 0.66 [+0.51] [-0.22] AU. We then discuss the properties of this cold planetary system.
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 st
udy 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.
