  One important strength of the microlensing method in detecting extrasolar
planets is its high sensitivity to low-mass planets. However, it is often
believed that microlensing detections of Earth-mass planets from ground-based
observation would be difficult due to the limit set by finite-source effects.
This view comes from the previous estimation of the planet detection
probability based on the fractional deviation of planetary signals, but proper
probability estimation requires to additionally consider the source brightness,
which is directly related to the photometric precision. In this paper, we
reevaluate the feasibility of low-mass planet detections considering
photometric precision for different populations of source stars. From this, it
is found that contribution of the improved photometric precision to the
planetary signal of a giant-source event is big enough to compensate the
decrease of the magnification excess caused by finite-source effects. As a
result, we find that giant-source events are suitable targets for Earth-mass
planet detections with significantly higher detection probability than events
involved with source stars of smaller radii and predict that Earth-mass planets
would be detected by prospective high-cadence surveys.
