A Circumplanetary Dust Ring May Explain the Extreme Spectral Slope of the 10 Myr Young Exoplanet K2-33b

TL;DR

This paper proposes that a circumplanetary dust ring around the young exoplanet K2-33b could explain its unusual spectral slope, offering a new perspective on planetary atmospheres and rings in early planetary systems.

Contribution

The study introduces a novel joint atmosphere-ring retrieval method and suggests that a dust ring can account for the spectral features of K2-33b, which previous models could not fully explain.

Findings

A circumplanetary dust ring can produce a steep spectral slope with low optical depth.

The dust ring's composition influences absorption features around 10 μm.

Future JWST observations can test the ring hypothesis and determine its composition.

Abstract

Young exoplanets are attractive targets for atmospheric characterization to explore the early phase of planetary evolution and the surrounding environment. Recent observations of the 10 Myr young Neptune-sized exoplanet K2-33b revealed that the planet's transit depth drastically decreases from the optical to near-infrared wavelengths. Thao et al. (2022) suggested that a thick planetary haze and/or stellar spots may be the cause; however, even the best-fit model only barely explains the data. Here, we propose that the peculiar transmission spectrum may indicate that K2-33b possess a circumplanetary dust ring, an analog of Jupiter's dust ring. We demonstrate that the ring could produce a steep slope in the transmission spectrum even if its optical depth is as low as $\sim {10}^{-2}$. We then apply a novel joint atmosphere-ring retrieval to K2-33b and find that the ring scenario could well explain the observed spectrum for various possible ring compositions. Importantly, the dust ring also exhibits prominent absorption features of ring particles around $\sim$10 $\rm{\mu}m$, whose shape and strength depend on the composition of the ring. Thus, future observations by JWST-MIRI would be able to test not only the ring hypothesis but also, if it indeed exists, to constrain the composition of the ring -- providing a unique opportunity to explore the origins of the dust ring around its parent planet, soon after the planetary system's formation.