Title: WASP-12b: A Mass-Losing Extremely Hot Jupiter,
Abstract: WASP-12b is an extreme hot Jupiter in a 1 day orbit, suffering profound
irradiation from its F type host star. The planet is surrounded by a
translucent exosphere which overfills the Roche lobe and produces
line-blanketing absorption in the near-UV. The planet is losing mass. Another
unusual property of the WASP-12 system is that observed chromospheric emission
from the star is anomalously low: WASP-12 is an extreme outlier amongst
thousands of stars when the log $R^{'}_{HK}$ chromospheric activity indicator
is considered. Occam's razor suggests these two extremely rare properties
coincide in this system because they are causally related. The absence of the
expected chromospheric emission is attributable to absorption by a diffuse
circumstellar gas shroud which surrounds the entire planetary system and fills
our line of sight to the chromospherically active regions of the star. This
circumstellar gas shroud is probably fed by mass loss from WASP-12b. The
orbital eccentricity of WASP-12b is small but may be non-zero. The planet is
part of a hierarchical quadruple system; its current orbit is consistent with
prior secular dynamical evolution leading to a highly eccentric orbit followed
by tidal circularization. When compared with the Galaxy's population of
planets, WASP-12b lies on the upper boundary of the sub-Jovian desert in both
the $(M_{\rm P}, P)$ and $(R_{\rm P}, P)$ planes. Determining the mass loss
rate for WASP-12b will illuminate the mechanism(s) responsible for the
sub-Jovian desert.