TRAPPIST-1 is a nearby ultra-cool dwarf that is host to a remarkable planetary system consisting of seven transiting planets. The orbital properties and radii of the planets have been well-constrained, and recently the masses of the inner six planets have been measured with additional ground and spacebased photometric observations. Large uncertainties in these mass measurements have prevented a robust analysis of the planetary compositions. Here we perform many thousands of N-body dynamical simulations with planet properties perturbed from the observed values and identify those that are stable for millions of years. This allows us to identify self-consistent orbital solutions that can be used in future studies. From our range of dynamical masses, we find that most of the planets are consistent with an Earth-like composition, where TRAPPIST-1f is likely to have a volatile-rich envelope.

TESS has begun fulfilling its promise of delivering thousands of new transiting planets orbiting nearby bright stars. The mission's legacy will fuel exoplanet science for many years to come, but much of this science relies on precisely predicted transit times that are needed for many follow-up characterization studies. We investigate the severity of ephemeris deterioration for TESS planets, and find that most will have uncertainties greater than 1 hour just one year after their TESS observations. It is the mission's relatively short observing baseline that drives this fast deterioration. We identify the parameters that have the strongest impact on this deterioration. We recommend that one or two follow-up transits be observed three and/or nine months after the end of a planet's TESS observations, in order to refresh its ephemeris for two years past the follow-up observations. We find that the longer the baseline between the TESS and the follow-up observations, the longer the ephemerides will stay fresh, facilitating the scheduling of future observations with expensive facilities such as the Hubble Space Telescope, the James Webb Space Telescope, the ELTs, and Ariel.