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

TL;DR

This paper demonstrates that combining Gaia astrometry and high-contrast imaging of nearby young moving groups can effectively constrain the initial entropy of giant exoplanets, shedding light on their formation mechanisms.

Contribution

It introduces a method to use Gaia and direct imaging data to distinguish between hot-start and cold-start planet formation models based on initial entropy.

Findings

Gaia can detect about 50% of planets >0.3 M_J in nearby groups.

Future instruments can measure initial entropy with uncertainty less than 0.5 k_B/baryon.

The method can differentiate between planet formation scenarios.

Abstract

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