Separating Super-Puffs vs. Hot Jupiters Among Young Puffy Planets

Abstract

Discoveries of close-in young puffy (R$_p \gtrsim$ 6 R$_\oplus$) planets raise the question of whether they are bona fide hot Jupiters or puffed-up Neptunes, potentially placing constraints on the formation location and timescale of hot Jupiters. Obtaining mass measurements for these planets is challenging due to stellar activity and noisy spectra. Therefore, we aim to provide independent theoretical constraints on the masses of these young planets based on their radii, incident fluxes, and ages, benchmarking to the planets of age $<$1 Gyr detected by Kepler, K2 and TESS. Through a combination of interior structure models, considerations of photoevaporative mass loss, and empirical mass-metallicity trends, we present the range of possible masses for 24 planets of age $\sim$10-900 Myr and radii $\sim$6-16 R$_\oplus$. We generally find that our mass estimates are in agreement with the measured masses and upper limits where applicable. There exist some outliers including super-puffs Kepler-51 b, c and V1298 Tau d, b, e, for which we outline their likely formation conditions. Our analyses demonstrate that most of the youngest planets ($\lesssim$ 100 Myr) tend to be puffed-up, Neptune-mass planets, while the true hot Jupiters are typically found around stars aged at least a few hundred Myr, suggesting the dominant origin of hot Jupiters to be late-stage high eccentricity migration.