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The formation of giant planets is best studied through direct imaging byobserving planets both during and after formation. Giant planets are expectedto form either by core accretion, which is typically associated with lowinitial entropy (cold-start models) or by gravitational instability, whichcorresponds to a high initial entropy of the gas (hot-start models). Thus,constraining the initial entropy provides insight into the planet formationmechanism and determines the resultant brightness evolution. We find that, byobserving planets in nearby moving groups of known age both through directimaging and astrometry with Gaia, it will be possible to constrain the initialentropy of giant planets. We simulate a set of planetary systems in stars innearby moving groups identified by BANYAN $\Sigma$ and assume a model forplanet distribution consistent with radial velocity detections. We find thatGaia should be able to detect approximately 50% of planets in nearby movinggroups greater than ~0.3 M$_\text{J}$. Using 5$\sigma$ contrast limits ofcurrent and future instruments, we calculate the flux uncertainty, and usingmodels for the evolution of the planet brightness, we convert this to aninitial entropy uncertainty. We find that, for future instruments such asMICADO and METIS on E-ELT and VIKiNG with VLTI, the entropy uncertainty is lessthan 0.5 $k_{B}$/baryon, showing that these instruments should be able todistinguish between hot and cold-start models.

Constraints on Planets in Nearby Young Moving Groups Detectable by High-Contrast Imaging and Gaia Astrometry