TOI-942b: A Prograde Neptune in a ~60 Myr old Multi-transiting System

TL;DR

This study measures the orbital obliquity of the young Neptune-sized planet TOI-942b, revealing a prograde orbit in a 60-million-year-old multi-planet system, providing insights into early planetary system architectures.

Contribution

First obliquity measurement of a young Neptune in a multi-planet system, demonstrating prograde orbit and informing theories of planetary system evolution.

Findings

TOI-942b has a prograde orbit with a low obliquity.

The planet's true 3D obliquity is approximately 2 degrees.

Uncertainty analysis confirms robustness of obliquity measurement.

Abstract

Mapping the orbital obliquity distribution of young planets is one avenue towards understanding mechanisms that sculpt the architectures of planetary systems. TOI-942 is a young field star, with an age of ~60 Myr, hosting a planetary system consisting of two transiting Neptune-sized planets in 4.3- and 10.1-day period orbits. We observed the spectroscopic transits of the inner Neptune TOI-942b to determine its projected orbital obliquity angle. Through two partial transits, we find the planet to be in a prograde orbit, with a projected obliquity angle of |lambda| = 1/+41-33 deg. In addition, incorporating the light curve and the stellar rotation period, we find the true three-dimensional obliquity to be 2/+27-23 deg. We explored various sources of uncertainties specific to the spectroscopic transits of planets around young active stars, and showed that our reported obliquity uncertainty fully encompassed these effects. TOI-942b is one of the youngest planets to have its obliquity characterized, and one of even fewer residing in a multi-planet system. The prograde orbital geometry of TOI-942b is in line with systems of similar ages, none of which have yet been identified to be in strongly misaligned orbits.