  We present the discovery and mass measurement of the cold, low-mass planet
MOA-2009-BLG-266Lb, made with the gravitational microlensing method. This
planet has a mass of m_p = 10.4 +- 1.7 Earth masses and orbits a star of mass
M_* = 0.56 +- 0.09 Solar masses at a semi-major axis of a = 3.2 (+1.9 -0.5) AU
and an orbital period of P = 7.6 (+7.7 -1.5} yrs. The planet and host star mass
measurements are enabled by the measurement of the microlensing parallax
effect, which is seen primarily in the light curve distortion due to the
orbital motion of the Earth. But, the analysis also demonstrates the capability
to measure microlensing parallax with the Deep Impact (or EPOXI) spacecraft in
a Heliocentric orbit. The planet mass and orbital distance are similar to
predictions for the critical core mass needed to accrete a substantial gaseous
envelope, and thus may indicate that this planet is a "failed" gas giant. This
and future microlensing detections will test planet formation theory
predictions regarding the prevalence and masses of such planets.
