  From wispy gas giants on the verge of disruption to tiny rocky bodies already
falling apart, short-period exoplanets pose a severe puzzle to theories of
planet formation and orbital evolution. By far most of the planets known beyond
the solar system orbit their stars in much tighter orbits than the most
close-in planet in the solar system, Mercury. Short-period planets experienced
dynamical and evolutions histories distinct from their farther-out cousins, and
so it's not clear they are representative of all planets. These exoplanets
typically have radii between about 1 and 4 Earth radii, whereas the solar
system does not contain any planet in this radius range. And while the most
massive planets in the solar system occupy the icy regions beyond about 5 AU
from the sun, about 1% of sun-like stars have a Jupiter-mass planet near 0.05
AU, with just a few days of an orbital period. How did these short-period
planets get there? Did they form in-situ, or did they migrate towards their
contemporary orbits? If they migrated, what prevented them from falling into
their stars? Vice versa, could some of the remaining 99% of stars without such
a hot Jupiter show evidence of their past consumption of a close-in, massive
planet? The proximity between short-period planets and their host stars
naturally facilitates observational studies, and so short-period planets
dominate our observational constraints on planetary composition, internal
structure, meteorology, and more. This white paper discusses the unique
advantages of short-period planets for the theoretical and observational
investigations of exoplanets in general and of their host stars.
