researchI am currently employed by the Las Cumbres Observatory and my research activities are associated with the ongoing RoboNet microlensing campaign and related collaborative work within the microlensing community. The RoboNet project is an international effort to detect exo-planets via robotic follow-up observations of microlensing events towards the Galactic Bulge alerted by the OGLE and MOA surveys.
The microlensing method
Einstein predicted that the gravitational field of any massive star will act as a gravitational lens and bend the path followed by the light rays originating from any bright star that happens to pass behind the lens. The effect of lensing at cosmological distances is practically observed as multiple distorted images of the background star around the edge of the gravitational influence of the lensing star. However, lensing also occurs on smaller scales in our galaxy and then the resulting images cannot be individually resolved. We call this phenomenon microlensing. What we see in this case instead, is a brightening of the background star that can last from a few days to several weeks. Then the star fades back to it's normal brightness. If the lensing star hosts a planetary companion, there is a chance that the planet can also act as a mini-lens and thereby reveal it's presence.
Microlensing is unique in it's capability to rapidly survey the population of cold planets, with a sensitivity to planetary mass that goes down to just below the mass of the Earth. The population of stars that it surveys are low-mass stars, typically M-dwarfs, between here and the centre of the Galaxy. Other methods are capable of detecting planets up to a few hundred light years away but microlensing is the only method that can probe the galactic population of planets. The planets discovered by this method are typically located between 0.6 and 6 AU from the host star, which corresponds to a cold zone that is more conductive to planet formation and which nicely overlaps the colder outer edge of the Habitable Zone. This region of parameter space is still largely inaccessible to other methods.
The RoboNet project
Intensive monitoring of the lightcurves of Galactic Bulge microlens events is probably the fastest way to discover "cool planets" in 1-10 AU orbits around late-type stars, with sensitivity to small planets approaching the mass of the Earth. Hundreds of such events are routinely identified each year by the OGLE and MOA experiments, and the lightcurves of several dozen are followed up by the PLANET, RoboNet, microFUN and MiNDSTEp teams. The RoboNet follow-up network links three 2m robotic telescopes: the Liverpool Telescope in La Palma, Canary Islands and the two Faulkes Telescopes in Maui, Hawaii and Siding Springs, Australia. The complete LCOGT robotic network will feature fifteen 1m and twenty-four 0.4m telescopes distributed around the globe allowing continuous monitoring of selected microlensing events.
More than a dozen planets have already been discovered by microlensing (Jan 2011). Of these planets, most are Jupiter-analogs, but a few have masses comparable to that of Neptune and below. Theoretical predictions estimate that small, cold planets are abundant and these can be detected by microlensing surveys. Microlensing is also sentitive to multiple planet systems and free-floating planets.
Sensitivities of search methods
Each of the five different techniques used to find exoplanets is most sensitive to configurations that are different from our own Solar system. Most detections to date have been through the radial velocity method, with transits coming a close second but soon expected to take over as fresh detections from the Kepler space mission are announced. Microlensing and direct imaging are finding colder planets further away from their host stars. For the time being no detections by astrometry have been confirmed.
- Tsapras, Y. et al., 2009, AN, 330, 4T: RoboNet-II: Follow-up observations of microlensing events with a robotic network of telescopes
- Horne, K. Snodgrass, C. Tsapras, Y., 2009, MNRAS, 396, 2087H: A metric and optimization scheme for microlens planet searches
- Shporer, A. Brown, T. Lister, T. Street, R. Tsapras, Y. Bianco, F. Fulton, B. Howell, A., 2009, arXiv:1011.6394: The LCOGT Network
- Gaudi, S. et al (including Tsapras, Y.), 2008, Science, 319, 927G: Discovery of a Jupiter/Saturn Analog with Gravitational Microlensing