We report the detection of an extrasolar planet of mass ratio q \sim 2 \times 10 ^ { -4 } in microlensing event MOA-2007-BLG-192 .
The best fit microlensing model shows both the microlensing parallax and finite source effects , and these can be combined to obtain the lens masses of M = 0.060 { +0.028 \atop - 0.021 } M _ { \odot } for the primary and m = 3.3 { +4.9 \atop - 1.6 } { M _ { \oplus } } for the planet .
However , the observational coverage of the planetary deviation is sparse and incomplete , and the radius of the source was estimated without the benefit of a source star color measurement .
As a result , the 2- \sigma limits on the mass ratio and finite source measurements are weak .
Nevertheless , the microlensing parallax signal clearly favors a sub-stellar mass planetary host , and the measurement of finite source effects in the light curve supports this conclusion .
Adaptive optics images taken with the Very Large Telescope ( VLT ) NACO instrument are consistent with a lens star that is either a brown dwarf or a star at the bottom of the main sequence .
Follow-up VLT and/or Hubble Space Telescope ( HST ) observations will either confirm that the primary is a brown dwarf or detect the low-mass lens star and enable a precise determination of its mass .
In either case , the lens star , MOA-2007-BLG-192L , is the lowest mass primary known to have a companion with a planetary mass ratio , and the planet , MOA-2007-BLG-192Lb , is probably the lowest mass exoplanet found to date , aside from the lowest mass pulsar planet .