TOI-332 b: a super dense Neptune found deep within the Neptunian desert

TL;DR

The paper reports the discovery of TOI-332 b, a super dense Neptune-sized planet within the Neptunian desert, challenging existing planetary formation theories due to its high density and minimal gaseous envelope.

Contribution

This is the first detailed characterization of a super dense Neptune in the Neptunian desert, highlighting the need for alternative formation scenarios.

Findings

TOI-332 b has a radius of 3.20 R$_{igoplus}$ and a mass of 57.2 M$_{igoplus}$.

It has an exceptionally high density of 9.6 g/cm$^3$, with negligible hydrogen-helium envelope.

Photoevaporation cannot explain its current state, suggesting other formation or migration processes.

Abstract

To date, thousands of planets have been discovered, but there are regions of the orbital parameter space that are still bare. An example is the short period and intermediate mass/radius space known as the Neptunian desert, where planets should be easy to find but discoveries remain few. This suggests unusual formation and evolution processes are responsible for the planets residing here. We present the discovery of TOI-332 b, a planet with an ultra-short period of $0.78$ d that sits firmly within the desert. It orbits a K0 dwarf with an effective temperature of $5251 \pm 71$ K. TOI-332 b has a radius of $3.20^{+0.16}_{-0.12}$ R$_{\oplus}$, smaller than that of Neptune, but an unusually large mass of $57.2 \pm 1.6$ M$_{\oplus}$. It has one of the highest densities of any Neptune-sized planet discovered thus far at $9.6^{+1.1}_{-1.3}$ gcm$^{-3}$. A 4-layer internal structure model indicates it likely has a negligible hydrogen-helium envelope, something only found for a small handful of planets this massive, and so TOI-332 b presents an interesting challenge to planetary formation theories. We find that photoevaporation cannot account for the mass loss required to strip this planet of the Jupiter-like envelope it would have been expected to accrete. We need to look towards other scenarios, such as high-eccentricity migration, giant impacts, or gap opening in the protoplanetary disc, to try and explain this unusual discovery.