  Two of TESS's major science goals are to measure masses for 50 planets
smaller than 4 Earth radii and to discover high-quality targets for atmospheric
characterization efforts. It is important that these two goals are linked by
quantifying what precision of mass constraint is required to yield robust
atmospheric properties of planets. Here, we address this by conducting
retrievals on simulated JWST transmission spectra under various assumptions for
the degree of uncertainty in the planet's mass for a representative population
of seven planets ranging from terrestrials to warm Neptunes to hot Jupiters.
Only for the cloud-free, low metallicity gas giants are we able to infer
exoplanet mass from transmission spectroscopy alone, to ~10% accuracy. For low
metallicity cases (<4xSolar) we are able to accurately constrain atmospheric
properties without prior knowledge of the planet's mass. For all other cases
(including terrestrial-like planets), atmospheric properties can only be
inferred with a mass precision of better than ~50%. At this level, though, the
widths of the posterior distributions of the atmospheric properties are
dominated by the uncertainties in mass. With a precision of ~20%, the widths of
the posterior distributions are dominated by the spectroscopic data quality.
Therefore, as a rule-of-thumb, we recommend: a ~50% mass precision for initial
atmospheric characterization and a ~20% mass precision for more detailed
atmospheric analyses.
