Time is one of the least explored dimensions of exoplanet research; most
stars targeted by large surveys are middle-aged by necessity or statistics.
Yet the first few hundred million years of a planetary system are probably
the most formative and include accretion, migration, and escape of atmospheres.
While the Kepler prime field included a small number of young stars by chance, the
K2 mission is deliberately selecting some target stars by age, and previews the
potential of TESS and PLATO. The Zodical Exoplanets in Time (ZEIT) project studies
K2 systems in stellar clusters of established ages. Transiting planets as small as
Earth-size have been detected in the Upper Scorpius, Pleiades, Hyades, and Praesepe
clusters. Mysterious aperiodic signals related to circumstellar disks were found in
Upper Scorpius, these may be related to planet formation. We are also investigating
planets around evolved stars and report a Jupiter-mass planet inflated by irradiation
from its host star. Gaia distances, proper motions, and spectra can identify large numbers
of young stars for observation by the TESS and PLATO missions, enabling robust comparisons
across a range of ages to understand evolutionary trends, and select propitious targets for
follow-up by ELTs and space observatories such as JWST.