Gravitational microlensing is a phenomenon where the light of a background star can be magnified by a foreground mass. All stars move around, so the magnification only lasts for a month or so — we call the pattern of brightening then fading the microlensing lightcurve. If the foreground mass is a star orbited by a planet, the planet may cause a short anomaly in the lightcurve, from which it is possible to infer the presence and properties of the planet. Even looking towards the densest part of the Galaxy, a microlensing event will only occur on any given star once every hundred thousand years or so, and planets causing anomalies lasting less than a day can only be detected in a fraction of events. It is therefore necessary to monitor around a hundred million stars at least once an hour in order to catch a planet.
Most of my work on microlensing has been to build simulations of microlensing surveys to assess their effectiveness and motivate improvements to them. I have led the development of the Manchester-Besançon microLensing Simulator - MaBμLS (pronounced like the name Mabel), and used it to simulate planetary microlensing surveys by the Euclid and WFIRST space missions. These simulations show that these space missions will be able to find thousands of planets in orbits larger than the Earth's with masses down to that of Mercury.
I am a member of the MiNDSTEp microlensing consortium, which searches for microlensing exoplanets using the Danish 1.54m telescope at La Silla, Chile.
I have recently joined the KELT-North team which searches for exoplanets transiting bright stars. Transiting planets around bright stars are particularly interesting, because their brightness enables spectroscopic observations of their atmospheres. I have also been involved with work to characterize transiting planets in more detail.
While most of my work goes into finding planets, I am interested in how we can use the planets we find to learn about how they formed. There are now nearly 1000 planets that have been found from the ground and nearly 3000 candidates found using space telescopes. These planets are the final result of the planet formation process, and by studying the distribution of their properties we can develop and refine theories of planet formation.
The MiNDSTEp consortium has been pioneering the use of Lucky Imaging cameras to perform time-series photometry in crowded star fields (a crucial part of microlensing observations). Members of the collaboration are undertaking an extensive program to characterize the performance of these new instruments. In 2011 I initiated a series of "field tests" of the prototype camera using globular clusters. Globular clusters provide the perfect testing ground for lucky imaging cameras because they contain many variable stars with short periods and their dense cores usually cannot be resolved using conventional ground based CCD cameras.