On the Formation of Planets in the Milky Way's Thick Disk

• DOI: 10.48550/arxiv.2408.09319
• Publication Date: 2024-08-17
• Authors: Tim Hallatt; Eve J. Lee
• Keywords: Protoplanetary disks; QB460-466; Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics - Solar and Stellar Astrophysics; Exoplanets; Astrophysics of Galaxies (astro-ph.GA); the Milky Way; FOS: Physical sciences; Astrophysics; Astrophysics - Astrophysics of Galaxies; Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics - Earth and Planetary Astrophysics

Exoplanet demographic surveys have revealed that close-in (${\lesssim}$1 au) small planets orbiting stars in the Milky Way's thick disk are ${\sim}50\%$ less abundant than those Galactic thin disk. One key difference between two stellar populations is time at which they emerged: likely product of cosmic noon (redshift $z {\sim}2$), an era characterized by high star formation rate, massive and dense molecular clouds, strong supersonic turbulence. Solving for background radiation field these early star-forming regions, we demonstrate protoplanetary disks experienced fields up to ${\sim}7$ orders magnitude more intense solar neighborhood conditions. Coupling a one-dimensional evolution model, find external UV photoevaporation destroys just ${\sim}$0.2--0.5 Myr, limiting timescale over can assemble. Disk temperatures exceed sublimation common volatile species ${\gtrsim}$Myr timescales, predicting spatial homogeneity gas chemical composition. Our calculations imply deficit planet occurrence around should be even pronounced giant planets, particularly wide orbital separations, higher rocky-to-giant ratio vs.~thin