[Paper Review] The TESS-Keck Survey. XI. Mass Measurements for Four Transiting Sub-Neptunes Orbiting K Dwarf TOI-1246

Emma V. Turtelboom, Lauren M. Weiss+79 more|arXiv (Cornell University)|Jan 1, 2022

Astro and Planetary Science155 references17 citations

TL;DR

This study presents mass measurements for four transiting sub-Neptunes orbiting the K dwarf TOI-1246 using radial velocity data from Keck/HIRES and TNG/HARPS-N, combined with refined TESS photometry. The outermost planet, TOI-1246 e, is significantly more massive (14.8 ± 2.3 M⊕) than the inner three, and the two outer planets lie near a 2:1 orbital resonance, suggesting dynamical stability and migration history.

ABSTRACT

Multiplanet systems are valuable arenas for investigating exoplanet architectures and comparing planetary siblings. TOI-1246 is one such system, with a moderately bright K dwarf (V = 11.6, K = 9.9) and four transiting sub-Neptunes identified by TESS with orbital periods of 4.31, 5.90, 18.66, and 37.92 days. We collected 130 radial velocity observations with Keck/HIRES and TNG/HARPS-N to measure planet masses. We refit the 14 sectors of TESS photometry to refine planet radii (2.97 +/- 0.06 R (circle plus), 2.47 +/- 0.08 R (circle plus), 3.46 +/- 0.09 R (circle plus), and 3.72 +/- 0.16 R (circle plus)) and confirm the four planets. We find that TOI-1246 e is substantially more massive than the three inner planets (8.1 +/- 1.1 M (circle plus), 8.8 +/- 1.2 M (circle plus), 5.3 +/- 1.7 M (circle plus), and 14.8 +/- 2.3 M (circle plus)). The two outer planets, TOI-1246 d and TOI-1246 e, lie near to the 2:1 resonance (P (e)/P ( d ) = 2.03) and exhibit transit-timing variations. TOI-1246 is one of the brightest four-planet systems, making it amenable for continued observations. It is one of only five systems with measured masses and radii for all four transiting planets. The planet densities range from 0.70 +/- 0.24 to 3.21 +/- 0.44 g cm(-3), implying a range of bulk and atmospheric compositions. We also report a fifth planet candidate found in the RV data with a minimum mass of 25.6 +/- 3.6 M (circle plus). This planet candidate is exterior to TOI-1246 e, with a candidate period of 93.8 days, and we discuss the implications if it is confirmed to be planetary in nature.

Motivation & Objective

• To measure precise masses of four transiting sub-Neptunes orbiting the K dwarf TOI-1246 using radial velocity monitoring.
• To refine planetary radii and orbital parameters using 14 sectors of TESS photometry.
• To investigate the dynamical architecture of the system, particularly the near-2:1 resonance between the outer two planets.
• To constrain planetary composition and formation history through mass-radius relationships.
• To contribute to the broader characterization of multi-planet systems around K dwarfs.

Proposed method

• Acquired 130 radial velocity observations using Keck/HIRES and TNG/HARPS-N to measure planetary masses via Doppler shifts.
• Re-analyzed 14 sectors of TESS light curves to improve transit depth and refine planet radii (2.97 ± 0.06 R⊕ to 3.72 ± 0.16 R⊕).
• Performed joint photometric and radial velocity modeling using the exoplanet and RadVel packages to derive orbital and physical parameters.
• Applied transit timing variation (TTV) analysis to detect and characterize dynamical interactions, particularly between TOI-1246 d and e.
• Used celerite and Gaussian process regression to model stellar activity and improve radial velocity fitting.
• Constrained planetary densities and compositions via mass-radius relationships and interior modeling.

Experimental results

Research questions

• RQ1 What are the precise masses of the four transiting sub-Neptunes in the TOI-1246 system?
• RQ2 How do the planetary radii and orbital periods refine when re-analyzing 14 sectors of TESS photometry?
• RQ3 Is there evidence of dynamical interaction between the outer two planets, particularly near a 2:1 mean-motion resonance?
• RQ4 What is the bulk composition of the planets, especially the massive outer planet TOI-1246 e?
• RQ5 How does the system's architecture inform theories of planetary migration and formation in K-dwarf systems?

Key findings

• TOI-1246 e has a mass of 14.8 ± 2.3 M⊕, making it substantially more massive than the three inner planets (8.1 ± 1.1 M⊕, 8.8 ± 1.2 M⊕, 5.3 ± 1.7 M⊕).
• The outer two planets, TOI-1246 d and e, have an orbital period ratio of 2.03, placing them near a 2:1 mean-motion resonance.
• Re-analysis of TESS photometry refined the planet radii to 2.97 ± 0.06 R⊕, 2.47 ± 0.08 R⊕, 3.46 ± 0.09 R⊕, and 3.72 ± 0.16 R⊕.
• The system exhibits transit timing variations consistent with gravitational interactions between the outer planets.
• The high mass of TOI-1246 e suggests a composition enriched in volatiles or a substantial hydrogen-helium envelope.
• The system's architecture supports a history of in situ formation or migration, with dynamical stability maintained near resonance.