Cool Earths: Discovering exoplanets beyond the snow-line with gravitational microlensing

The prolific discoveries of planets orbiting distant stars over the past two decades have had a radical effect on our understanding of planetary systems. Current formation theories postulate that proto-planetary cores form in dusty accretion discs surrounding the host star. These proto-planets co-evolve with the disk and may undergo orbital decay due to torque asymmetries in the surrounding disk material.
The snow-line defines the distance from a star beyond which the disk temperature drops low enough that liquid water turns to ice. Theory predicts that beyond the snow-line, the formation of ice grains allows planetary embryos to develop sufficiently massive solid cores and gradually grow by accreting material from the surrounding gaseous disk, transforming them into gas giants. Should the available disk material be depleted during this process, planetary growth stops, leaving behind smaller bodies. Simulations suggest that such bodies should exist in abundance, yet they lie beyond the sensitivity limits of most planet-finding techniques.
Besides the tantalizing possibility of discovering cold Earths, discovering these planets is critical for improving our understanding of the physical processes that drive planetary formation. The gravitational microlensing technique provides a unique window at and beyond the snow-line, allowing us to probe their distribution.
This project represents an international effort to detect distant exo-planets beyond the snow-line via robotic observations of microlensing events towards the Galactic Bulge. One of its central aspects is the extensive use of advanced target selection and prioritization systems that optimize observations over the global robotic telescope network of the Las Cumbres Observatory.
Through intensive monitoring of the light curves of microlensing events it is possible to identify the tell-tale signs of cold planets in 1-10 AU orbits around late-type stars, with a detection sensitivity down to about one Earth-mass. To achieve this, our team has pioneered the development of adaptive robotic scheduling of microlensing targets by employing a prioritization algorithm that continuously updates the list of observable microlensing targets, giving higher priority to those more likely to reveal a planetary signal. The algorithm calculates the optimal sampling frequency for each microlensing in order to maximize the planet detection probability.
Microlensing is the fastest and most cost-efficient way to probe and understand the population of cold exoplanets as it relies on observations that are directly related to the mass of the planetary system and not its brightness. It can therefore detect planets around extremely faint or unseen stars and is uniquely sensitive to planetary systems at distances of several kilo-parsec, allowing us to get an unbiased glimpse of the true Galactic population of planets.