A long spin period for a sub-Neptune-mass exoplanet

TL;DR

This paper introduces a GPU-enabled code for modeling transits of oblate exoplanets and applies it to HIP 41378 f, finding it likely has a long rotation period and potential for future detailed characterization.

Contribution

The paper develops a new computational tool for analyzing oblate planet transits and applies it to real data, providing constraints on the planet's flattening and rotation.

Findings

HIP 41378 f has a flattening $f \,\leq\, 0.889$ at 95% confidence.

The planet's rotation period is constrained to be at least 15.3 hours.

Future JWST data could improve oblateness measurements.

Abstract

HIP 41378 f is a sub-Neptune exoplanet with an anomalously low density. Its long orbital period and deep transit make it an ideal candidate for detecting oblateness photometrically. We present a new cross-platform, GPU-enabled code greenlantern, suitable for computing transit light curves of oblate planets at arbitrary orientations. We then use Markov Chain Monte Carlo to fit K2 data of HIP 41378 f, specifically examining its transit for possible oblateness and obliquity. We find that the flattening of HIP 41378 f is $f \leq 0.889$ at the 95% confidence level, consistent with a rotation period of $P_\text{rot} \geq 15.3$ hr. In the future, high-precision data from JWST has the potential to tighten such a constraint and can differentiate between spherical and flattened planets.