  Multi-planet systems provide important laboratories for exploring dynamical
interactions within the range of known exoplanetary system architectures. One
such system is GJ 357, consisting of a low-mass host star and three orbiting
planets, the outermost (planet d) of which does not transit but lies within the
Habitable Zone (HZ) of the host star. The minimum mass of planet d causes its
nature to be unknown, both in terms of whether it is truly terrestrial and if
it is a candidate for harboring surface liquid water. Here, we use three
sectors of photometry from the Transiting Exoplanet Survey Satellite (TESS) to
show that planets c and d do not transit the host star, and therefore may have
masses higher than the derived minimum masses. We present the results for a
suite of dynamical simulations that inject an Earth-mass planet within the HZ
of the system for three different orbital and mass configurations of planet d.
These results show that planet d, rather than being a potentially habitable
planet, is likely a source of significant orbital instability for other
potential terrestrial planets within the HZ. We find that relatively small
eccentricities of planet d cause a majority of the HZ to be unstable for an
Earth-mass planet. These results highlight the importance of dynamical
stability for systems that are prioritized in the context of planetary
habitability.
