  We have analyzed data from Campaigns 0-5 of the K2 mission and report 19
ultra-short-period candidate planets with orbital periods of less than 1 day
(nine of which have not been previously reported). Planet candidates range in
size from 0.7-16 Earth radii and in orbital period from 4.2 to 23.5 hours. One
candidate (EPIC 203533312, Kp=12.5) is among the shortest-period planet
candidates discovered to date (P=4.2 hours), and, if confirmed as a planet,
must have a density of at least rho=8.9 g/cm^3 in order to not be tidally
disrupted. Five candidates have nominal radius values in the sub-Jovian desert
(R_P=3-11 R_E and P<=1.5 days) where theoretical models do not favor their
long-term stability; the only confirmed planet in this range is in fact thought
to be disintegrating (EPIC 201637175). In addition to the planet candidates, we
report on four objects which may not be planetary, including one with
intermittent transits (EPIC 211152484) and three initially promising candidates
that are likely false positives based on characteristics of their light curves
and on radial velocity follow-up. A list of 91 suspected eclipsing binaries
identified at various stages in our vetting process is also provided. Based on
an assessment of our survey's completeness, we estimate an occurrence rate for
ultra-short period planets among K2 target stars that is about half that
estimated from the Kepler sample, raising questions as to whether K2 systems
are intrinsically different from Kepler systems, possibly as a result of their
different galactic location.
