  $K2$ greatly extended $Kepler$'s ability to find new planets, but it was
typically limited to identifying transiting planets with orbital periods below
40 days. While analyzing $K2$ data through the Exoplanet Explorers project,
citizen scientists helped discover one super-Earth and four sub-Neptune sized
planets in the relatively bright ($V=12.21$, $K=10.3$) K2-138 system, all which
orbit near 3:2 mean motion resonances. The $K2$ light curve showed two
additional transit events consistent with a sixth planet. Using $Spitzer$
photometry, we validate the sixth planet's orbital period of $41.966\pm0.006$
days and measure a radius of $3.44^{+0.32}_{-0.31}\,R_{\oplus}$, solidifying
K2-138 as the $K2$ system with the most currently known planets. There is a
sizeable gap between the outer two planets, since the fifth planet in the
system, K2-138 f, orbits at 12.76 days. We explore the possibility of
additional non-transiting planets in the gap between f and g. Due to the
relative brightness of the K2-138 host star, and the near resonance of the
inner planets, K2-138 could be a key benchmark system for both radial velocity
and transit timing variation mass measurements, and indeed radial velocity
masses for the inner four planets have already been obtained. With its five
sub-Neptunes and one super-Earth, the K2-138 system provides a unique test bed
for comparative atmospheric studies of warm to temperate planets of similar
size, dynamical studies of near resonant planets, and models of planet
formation and migration.
