The TESS-Keck Survey. VI. Two Eccentric sub-Neptunes Orbiting HIP-97166

TL;DR

This paper reports the discovery and characterization of two eccentric sub-Neptune planets orbiting HIP-97166, highlighting their orbital dynamics, formation history, and the system's stability through combined photometry and radial velocity data.

Contribution

It presents the first detailed analysis of an eccentric sub-Neptune system with two planets, including their masses, radii, eccentricities, and possible formation scenarios.

Findings

HIP-97166b has a mass of 20 ± 2 Earth masses and a radius of 2.7 ± 0.1 Earth radii.

An additional non-transiting planet was detected with a mass of 10 ± 2 Earth masses on a 16.8-day orbit.

The system's dynamics suggest moderate eccentricities and a history of early instability and mergers.

Abstract

We report the discovery of HIP-97166b (TOI-1255b), a transiting sub-Neptune on a 10.3-day orbit around a K0 dwarf 68 pc from Earth. This planet was identified in a systematic search of TESS Objects of Interest for planets with eccentric orbits, based on a mismatch between the observed transit duration and the expected duration for a circular orbit. We confirmed the planetary nature of HIP-97166b with ground-based radial velocity measurements and measured a mass of $M_{b} =$ 20 $\pm$ 2 $M_\bigoplus$ along with a radius of $R_{b} =$ 2.7 $\pm$ 0.1 $R_\bigoplus$ from photometry. We detected an additional non-transiting planetary companion with $M_{c}$ sin$i =$ 10 $\pm$ 2 $M_\bigoplus$ on a 16.8-day orbit. While the short transit duration of the inner planet initially suggested a high eccentricity, a joint RV-photometry analysis revealed a high impact parameter $b = 0.84 \pm 0.03$ and a moderate eccentricity. Modeling the dynamics with the condition that the system remain stable over $>$10$^5$ orbits yielded eccentricity constraints $e_b = 0.16 \pm 0.03$ and $e_c < 0.25$. The eccentricity we find for planet b is above average for the small population of sub-Neptunes with well-measured eccentricities. We explored the plausible formation pathways of this system, proposing an early instability and merger event to explain the high density of the inner planet at $5.3 \pm 0.9$ g/cc as well as its moderate eccentricity and proximity to a 5:3 mean-motion resonance.