Detecting planets around stars in nearby galaxies

The only way to detect planets around stars at distances greater than or similar to several kpc is by (photometric or astrometric) microlensing (mu L) observations. In this paper, we show that the capability of photometric mu L extends to the detection of signals caused by planets around stars in nearby galaxies (e.g. M31) and that there is no other method that can achieve this. Due to the large crowding, mu L experiments towards M31 can only observe the high-magnification part of a lensing light curve. Therefore, the dominating channel for mu L signals by planets is in distortions near the peak of high-magnification events as discussed by Griest & Safizadeh (1998). We calculate the probability to detect planetary anomalies for mu L experiments towards M31 and find that jupiter-like planets around stars in M31 can be detected. Though the characterization of the planet(s) involved in this signal will be difficult, the absence of such signals can yield strong constraints on the abundance of jupiter-like planets.