  The orbits of close-in exoplanets provide clues to their formation and
evolutionary history. Many close-in exoplanets likely formed far out in their
protoplanetary disks and migrated to their current orbits, perhaps via
high-eccentricity migration (HEM), a process that can also excite obliquities.
A handful of known exoplanets are perhaps caught in the act of HEM, as they are
observed on highly eccentric orbits with tidal circularization timescales
shorter than their ages. One such exoplanet is Kepler-1656 b, which is also the
only known non-giant exoplanet (<100 $M_\oplus$) with an extreme eccentricity
(e=0.84). We measured the sky-projected obliquity of Kepler-1656 b by observing
the Rossiter-McLaughlin effect during a transit with the Keck Planet Finder.
Our data are consistent with an aligned orbit, but are also consistent with
moderate misalignment with $\lambda < 50$ deg at 95% confidence, with the most
likely solution of $35^{+14.9}_{-21.6}$ deg. A low obliquity would be an
unlikely outcome of most eccentricity-exciting scenarios, but we show that the
properties of the outer companion in the system are consistent with the
coplanar HEM mechanism. Alternatively, if the system is not relatively coplanar
(<20 deg mutual inclination), Kepler-1656 b may be presently at a rare snapshot
of long-lived eccentricity oscillations that do not induce migration.
Kepler-1656 b is only the fourth exoplanet with e>0.8 to have its obliquity
constrained; expanding this population will help establish the degree to which
orbital misalignment accompanies migration. Future work that constrains the
mutual inclinations of outer perturbers will be key for distinguishing
plausible mechanisms.
