  From the thousands of known exoplanets, those that transit bright host stars
provide the greatest accessibility toward detailed system characterization. The
first known such planets were generally discovered using the radial velocity
technique, then later found to transit. HD 17156b is particularly notable among
these initial discoveries because it diverged from the typical hot Jupiter
population, occupying a 21.2 day eccentric ($e = 0.68$) orbit, offering
preliminary insights into the evolution of planets in extreme orbits. Here we
present new data for this system, including ground and space-based photometry,
radial velocities, and speckle imaging, that further constrain the system
properties and stellar/planetary multiplicity. These data include photometry
from the Transiting Exoplanet Survey Satellite (TESS) that cover five transits
of the known planet. We show that the system does not harbor any additional
giant planets interior to 10 AU. The lack of stellar companions and the age of
the system indicate that the eccentricity of the known planet may have resulted
from a previous planet-planet scattering event. We provide the results from
dynamical simulations that suggest possible properties of an additional planet
that culminated in ejection from the system, leaving a legacy of the observed
high eccentricity for HD 17156b.

Earth and Planetary Astrophysics