By the end of 2010, eleven exoplanets in ten systems had been discovered through gravitational microlensing (Table 5.1). The first unambiguous detection of a 4MJ planet with a projected separation of ∼4 AU was reported in 2004, and the discovery of a 5Moplus; planet in 2006. With the characterisation of a two-planet system somewhat analogous to Jupiter and Saturn in 2008, in which the orbital motion of the outer planet could be detected and measured during the lensing event, these discoveries marked the emergence of the technique as a powerful and independent exoplanet probe over an important region of planetary mass and orbital radius.

Introduction

Gravitational lensing In general relativity, the presence of matter (energy density) distorts spacetime, and the path of electromagnetic radiation is deflected as a result. Under certain conditions, light rays from a distant background object (the source) are bent by the gravitational potential of a foreground object (the lens) to create images of the source which are distorted (and possibly multiple), and which may be highly focused and hence significantly amplified. Its manifestation depends upon the fortuitous alignment of the background source, the intervening lens, and the observer.

Different regimes are generally recognised for gravitational lensing, depending on whether effects are discernible at an individual object level (strong lensing), or only in a statistical sense (weak lensing).