Future Programs

Our experience with the existing microlensing planet search programs provides indications of how the sensitivity of future microlensing surveys can be improved. At present, the OGLE and MOA groups are each able to independently discover more than 500 microlensing events per year. There is a great overlap between the discoveries of these two groups, but the total number of events discovered every year is probably about 700. This is at least an order of magnitude larger than the global follow-up groups can hope to follow. The follow-up groups optimize their observations by focusing on high magnification events. However, many of the shorter time scale high magnification events are not recognized as such in time, and so a large fraction of the high magnification events are not searched for planets.

The solution to this problem is to observe many microlensing events in each image with a global network of very wide FOV telescopes that can observe 10 square degrees or more of the Galactic bulge at 15-20 intervals. The new 1.8m MOA-II telescope (Hearnshaw et al., 2005) with a 2.2 square degree FOV CCD camera (Yanagisawa et al., 2000) that began operation in 2006 is the first telescope that meets this requirement, and the OGLE group plans to upgrade to a 1.4 square degree OGLE-4 camera in time for the 2009 Galactic bulge observing season. With MOA-II in New Zealand, and OGLE-IV in Chile, all that is needed is a very wide-FOV microlensing survey telescope in Southern Africa. A number of groups are pursuing funding for such a telescope.

Simulations of such a system have been performed by Bennett (2004) and Gaudi (2007, private communication), and estimates of the sensitivity of the global network

# of Planet Discoveries

# of Planet Discoveries

Fig. 3.20. The number of planet detections expected per year as a function of planet mass is shown for proposed future space and ground-based microlensing surveys under the assumption of one planet per star in the indicated separation ranges. The space-based survey has its most significant advantage over the ground-based survey at separations smaller (0.5-1.5 AU) and larger (5-15 AU) than the Einstein radius, because a space-based survey is able to resolve bulge main sequence stars and detect moderate amplitude planetary signals when the magnification due to the stellar lens is small.

Fig. 3.20. The number of planet detections expected per year as a function of planet mass is shown for proposed future space and ground-based microlensing surveys under the assumption of one planet per star in the indicated separation ranges. The space-based survey has its most significant advantage over the ground-based survey at separations smaller (0.5-1.5 AU) and larger (5-15 AU) than the Einstein radius, because a space-based survey is able to resolve bulge main sequence stars and detect moderate amplitude planetary signals when the magnification due to the stellar lens is small.

consisting of MOA-II, OGLE-IV and an OGLE-IV-like system in South Africa are presented in Fig. 3.9 and Fig. 3.20. The improvement in sensitivity with such a network in the mass vs. semi-major axis plane is shown in Fig. 3.9 with the light and dark red curves showing the sensitivity of the current surveys and the future very wide-FOV network, respectively. This network will extend the sensitivity of the microlensing method down to an Earth mass at planet-star separations close to the Einstein ring radius 2-3 AU).

The separation range where ground-based microlensing is most sensitive, 1-5 AU corresponds to the vicinity of the so-called "snow-line" which is the region of the proto-planetary disk where it is cold enough for water-ice to condense. The density of solids in the proto-planetary disk increases by a factor of ~ 4 across the "snowline" and as a result, the core accretion theory predicts that this is where the most massive planets will form (Ida & Lin, 2004; Laughlin, Bodenheimer & Adams, 2004; Kennedy et al., 2006). According to this theory, giant planets form just outside the "snow line" where they can accrete ~ 10of rock and ice to form a core that grows into a gas giant like Jupiter or Saturn via the run-away accretion of hydrogen and helium onto this core. However, this theory also predicts that the hydrogen and helium gas can easily be removed from the proto-planetary disk during the millions of years that it takes to build the rock-ice core of a gas-giant. Thus, if the core accretion theory is correct, rock-ice planets of ~ 10M© that failed to grow into gas giants should be quite common, although it is possible to form such planets in the competing gravitational instability theory (Boss, 2006).

The number of planetary microlensing event detections expected per year is shown in Fig. 3.20 assuming an average of one such planet per star, with conservative assumptions regarding photometric precision. The assumption of an average of one such planet per star is certainly too optimistic for Jupiter mass planets (Gaudi et al., 2002; Butler et al., 2006), but it is closer to reality for super-Earths, like OGLE-2005-BLG-390Lb and OGLE-2005-BLG-169Lb (Beaulieu et al., 2006; Gould et al., 2006). It could very well be accurate for Earth-mass planets where the weaker two-body gravitational interactions allow two planets to orbit in the separation range corresponding to the bins in Fig. 3.20. (Our own Solar System is an example of this.)

Another future development that is already funded is a global network of robotic telescopes dedicated to monitoring transient events like planetary microlensing events, known as the Las Cumbres Global Telescope Network (Brown et al., 2007). Ideally, this network would routinely observe high magnification microlensing events and planetary deviations discovered in progress with a very high cadence, such as that provided by the MDM telescope for OGLE-2005-BLG-169 (see Fig. 3.16). This would enable the very wide-FOV survey telescopes to maintain their normal sampling strategy so that other planetary microlensing events would not be missed. This might add to the planet detection efficiency substantially, but such a system is more difficult to model.

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