A One of the primary constraints we have upon planet formation is the density of exoplanets, which inform their possible compositions. Exoplanet detection biases make densities difficult to measure for temperate, terrestrial planet systems: RV detections have a low probability of transit, while transiting planets typically orbit fainter stars with poor RV precision. Radial velocities yield a lower limit on mass, and hence density, while transits yield a radius, but no mass, with a conservative upper limit if the planet were pure iron. Thus, there is a great "chasm" between density upper limits and lower limits for temperate rocky exoplanets.
The chasm can be bridged with multiple transiting exoplanet systems, such as the seven-planet system TRAPPIST-1 orbiting an ultracool M dwarf star. Transit timing can yield masses, and thus constrain the planet densities. I will show how recent technical advances and exquisite data on TRAPPIST-1 from Spitzer combine to yield the most precise densities of small, rocky exoplanets to date. I will also discuss the constraints upon the architecture of this system, and if time allows, look forward to the future where we might apply similar techniques to terrestrial planet systems more like our Solar-System: those with moons, orbiting Sun-like stars.