  Planet formation theories predict the existence of free-floating planets that
have been ejected from their parent systems. Although they emit little or no
light, they can be detected during gravitational microlensing events.
Microlensing events caused by rogue planets are characterized by very short
timescales $t_{\rm E}$ (typically below two days) and small angular Einstein
radii $\theta_{\rm E}$ (up to several uas). Here we present the discovery and
characterization of two ultra-short microlensing events identified in data from
the Optical Gravitational Lensing Experiment (OGLE) survey, which may have been
caused by free-floating or wide-orbit planets. OGLE-2012-BLG-1323 is one of the
shortest events discovered thus far ($t_{\rm E}$=0.155 +/- 0.005 d,
$\theta_{\rm E}$=2.37 +/- 0.10 uas) and was caused by an Earth-mass object in
the Galactic disk or a Neptune-mass planet in the Galactic bulge.
OGLE-2017-BLG-0560 ($t_{\rm E}$=0.905 +/- 0.005 d, $\theta_{\rm E}$=38.7 +/-
1.6 uas) was caused by a Jupiter-mass planet in the Galactic disk or a brown
dwarf in the bulge. We rule out stellar companions up to a distance of 6.0 and
3.9 au, respectively. We suggest that the lensing objects, whether located on
very wide orbits or free-floating, may originate from the same physical
mechanism. Although the sample of ultrashort microlensing events is small,
these detections are consistent with low-mass wide-orbit or unbound planets
being more common than stars in the Milky Way.
