Tightly packed planetary systems

Many exoplanet systems are tightly packed on orbits unnervingly near instability. Here we review theories for the origin of tightly packed systems and how their properties constrain how planetary systems form and evolve. We begin with a pedagogical overview of packing metrics and their connection to resonance overlap and instability. We highlight an additional scale, the escape velocity from the surface of the planet, that causes dynamical behavior to vary with host star separation. Next we discuss how tightly packed systems inform us about planet formation conditions, if and how planets migrate, and survival in chaotic configurations. Along the way, we highlight several examples of packed exoplanet systems-the Kepler-11, Kepler-36, and HR 8799-and compare them to Solar System benchmarks. We conclude with a discussion about unifying the three packing perspectives of formation, migration, and stability.