The Super-Puff WASP-193b is On A Well-Aligned Orbit

TL;DR

This paper presents detailed measurements of the super-puff planet WASP-193b, revealing it has a well-aligned orbit and a composition consistent with a metal-H/He mix, contributing to understanding the diversity of super-puff formation.

Contribution

It provides the first measurement of a well-aligned orbit for a Jupiter-sized super-puff, challenging previous assumptions about their dynamical histories.

Findings

WASP-193b is a low-density, Jupiter-sized super-puff.

The planet's orbit is well-aligned with its host star.

Interior models suggest a metal-H/He composition.

Abstract

The "super-puffs" are a population of planets that have masses comparable to that of Neptune but radii similar to Jupiter, leading to extremely low bulk densities ($\rho_p \lesssim 0.2\,\mathrm{g}\,\mathrm{cm}^{-3}$) that are not easily explained by standard core accretion models. Interestingly, several of these super-puffs are found in orbits significantly misaligned with their host stars' spin axes, indicating past dynamical excitation that may be connected to their low densities. Here, we present new Magellan/PFS RV measurements of WASP-193, a late F star hosting one of the least dense transiting planets known to date ($M_p = 0.112^{+0.029}_{-0.034}\,M_J$, $R_p = 1.319^{+0.056}_{-0.048}\,R_J$, $\rho_p = 0.060\pm0.019\,\mathrm{g}\,\mathrm{cm}^{-3}$). We refine the bulk properties of WASP-193 b and use interior structure models to determine that the planet can be explained if it consists of roughly equal amounts of metals and H/He, with a metal fraction of $Z = 0.42$. The planet is likely substantially re-inflated due to its host star's evolution, and expected to be actively undergoing mass loss. We also measure the projected stellar obliquity using the Rossiter-McLaughlin effect, finding that WASP-193 b is on an orbit well-aligned with the stellar equator, with $\lambda = 17^{+17}_{-16}$ degrees. WASP-193 b is the first Jupiter-sized super-puff on a relatively well-aligned orbit, suggesting a diversity of formation pathways for this population of planets.