  Recent surveys have uncovered an exciting new population of
ultra-short-period (USP) planets with orbital periods less than a day. These
planets typically have radii <1.5 Earth radii, indicating that they likely have
rocky compositions. This stands in contrast to the overall distribution of
planets out to ~100 days, which is dominated by low-density sub-Neptunes above
2 Earth radii, which must have gaseous envelopes to explain their size.
However, on ultra-short-period orbits, planets are bombarded by intense levels
of photo-ionizing radiation and consequently gaseous sub-Neptunes are extremely
vulnerable to losing their envelopes to atmospheric photo-evaporation. Using
models of planet evolution, I show that the rocky USP planets can easily be
produced as the evaporated remnants of sub-Neptunes with H/He envelopes and
that we can therefore understand the observed dearth of USP sub-Neptunes as a
natural consequence of photo-evaporation. Critically however, planets on USP
orbits could often retain their envelopes if they formed with very
high-metallicity water dominated envelopes. Such water-rich planets would
commonly be >2 Earth radii today, which is inconsistent with the observed
evaporation desert, indicating that most USP planets likely formed from
water-poor material within the snow-line. Finally, I examine the special case
of 55 Cancri e and its possible composition in the light of recent
observations, and discuss the prospects for further characterizing this
population with future observations.
