  We compare potential state-of-the-art experiments for detecting Earth-mass
planets around main-sequence stars using radial velocities, transits,
astrometry, and microlensing. For conventionally-discussed signal-to-noise
ratio (S/N) thresholds, S/N ~ 8, the last three methods are roughly comparable
in terms of both the total number of planets detected and the mass distribution
of their host stars. However we argue that S/N ~ 25 is a more conservative and
realistic S/N threshold. We show analytically and numerically that the decline
in the number of detections as a function of S/N is very steep for radial
velocities, transits, and astrometry, such that the number of expected
detections at S/N ~ 25 is more than an order-of-magnitude smaller than at
conventional S/N thresholds. Indeed, unless Earth-mass planets are very common
or are packed much closer to their parent stars than in the solar system,
future searches using these methods (as they are currently planned) may not
yield any reliable Earth-mass planet detections. On the other hand,
microlensing has a much shallower S/N slope than the other techniques and so
has much greater sensitivity at realistic S/N thresholds. We show that even if
all stars have Earth-mass planets at periods of one year (and adopting other
optimistic assumptions as well), the combined yield of all four techniques
would be the detection of only about five such planets at S/N ~ 25.
