The Possibility of Detecting Planets in the Andromeda Galaxy

The Angstrom project is using a global network of 2 m class telescopes to conduct a high-cadence pixel microlensing survey of the bulge of the Andromeda galaxy (M31), with the primary aim of constraining its underlying bulge mass distribution and stellar mass function. Here we investigate the feasibility of using such a survey to detect planets in M31. We estimate the efficiency of detecting signals produced by planets with various masses and separations from the host star. We find that for a ~5MJ planet that is located within the lensing zone (~1-3 AU), detection is possible above 3 σ with detection efficiency ~6%. This corresponds to the yearly detection rate of ~3fLZ planets, where fLZ is the probability that a planet exists in the lensing zone. It is expected that most events with detectable planets are associated with giant source stars, and thus source size will have a significant effect on the planet detection efficiency. We also find that the planetary perturbations will be in nearly all cases caused by central caustics, and thus observational strategies focusing on these central perturbations would maximize planet detections. A dramatic improvement in the efficiency of ~30%-50% is expected if follow-up observations on an 8 m telescope are made possible by a real-time alert system.