  Are there other planetary systems in our Universe? Indirect evidence has been
found for planets orbiting other stars in our galaxy: the gravity of orbiting
planets makes the star wobble, and the resulting periodic Doppler shifts have
been detected for about a dozen stars. But are there planets in other galaxies,
millions of light years away? Here we suggest a method to search for
extragalactic planetary systems: gravitational microlensing of unresolved
stars. This technique may allow us to discover planets in elliptical galaxies,
known to be far older than our own Milky Way, with broad implications for life
in the Universe.
