  The detection and characterization of young planetary systems offers a direct
path to study the processes that shape planet evolution. We report on the
discovery of a sub-Neptune-size planet orbiting the young star HD 110082
(TOI-1098). Transit events we initially detected during TESS Cycle 1 are
validated with time-series photometry from Spitzer. High-contrast imaging and
high-resolution, optical spectra are also obtained to characterize the stellar
host and confirm the planetary nature of the transits. The host star is a late
F dwarf (M=1.2 Msun) with a low-mass, M dwarf binary companion (M=0.26 Msun)
separated by nearly one arcminute (~6200 AU). Based on its rapid rotation and
Lithium absorption, HD 110082 is young, but is not a member of any known group
of young stars (despite proximity to the Octans association). To measure the
age of the system, we search for coeval, phase-space neighbors and compile a
sample of candidate siblings to compare with the empirical sequences of young
clusters and to apply quantitative age-dating techniques. In doing so, we find
that HD 110082 resides in a new young stellar association we designate
MELANGE-1, with an age of 250(+50/-70) Myr. Jointly modeling the TESS and
Spitzer light curves, we measure a planetary orbital period of 10.1827 days and
radius of Rp = 3.2(+/-0.1) Earth radii. HD 110082 b's radius falls in the
largest 12% of field-age systems with similar host star mass and orbital
period. This finding supports previous studies indicating that young planets
have larger radii than their field-age counterparts.
