A Pair of Dynamically Interacting Sub-Neptunes Around TOI-6054

We confirm the planetary nature of a pair of transiting sub-Neptune exoplanets orbiting the bright F-type sub-giant star TOI-6054 ($V=8.02$, $K=6.673$) as a part of the OrCAS radial velocity survey using WIYN/NEID observations. We find that TOI-6054b and TOI-6054c have radii of $2.65 \pm 0.15$ $R_{\oplus}$ and $2.81 \pm 0.18$ $R_{\oplus}$, respectively, and masses of $12.4 \pm 1.7$ $M_{\oplus}$ and $9.2 \pm 2.0$ $M_{\oplus}$. The planets have zero-albedo equilibrium temperatures of $1360 \pm 33$ K and $1144 \pm 28$ K. The host star has expanded and will evolve off of the Main Sequence within the next $\sim$500 Myr, and the resulting increase in stellar luminosity has more than doubled the stellar flux the two planets receive compared to the start of the host star’s main sequence phase. Consequently, TOI-6054b may be losing some of its primordial H/He atmosphere – if it has one. Based on dynamical simulations performed using the orbital parameters of the two planets, TOI-6054b, and TOI-6054c are very likely in a 5:3 mean motion resonance. The TOI-6054 system thus has the potential to be an excellent candidate for future atmospheric follow-up observations, with two similarly sized sub-Neptunes around a bright star. We also estimate that if TOI-6054b is currently losing its H/He atmosphere this should be observable from space and from the ground.

💡 Research Summary

The paper reports the discovery and comprehensive characterization of two transiting sub‑Neptune exoplanets orbiting the bright F‑type sub‑giant star TOI‑6054 (V = 8.02 mag, K = 6.673 mag). The system was first identified by TESS in Sectors 19 (30‑minute cadence) and 59 (2‑minute cadence), where four transits of the inner planet (TOI‑6054b) and two transits of the outer planet (TOI‑6054c) were detected with depths of ~215 ppm and ~245 ppm, respectively. Follow‑up high‑precision radial‑velocity (RV) measurements were obtained with the NEID spectrograph on the WIYN 3.5 m telescope, yielding 29 RV points with an average internal precision of 1.6 m s⁻¹ and a total scatter of 3.1 m s⁻¹. Simultaneous modeling of the TESS light curves and NEID RVs provides orbital periods of 5.12 days (b) and 8.53 days (c), corresponding to a period ratio of 1.666, very close to a 5:3 mean‑motion resonance. N‑body integrations (using REBOUND) demonstrate that the resonant angles librate and the system remains dynamically stable over at least 10⁸ years, supporting a scenario where the planets were captured into resonance during early disk‑driven migration.

Stellar parameters were derived from high‑resolution TRES spectra and spectral‑energy‑distribution (SED) fitting. The host star has an effective temperature of ~6058 K, log g ≈ 4.03, solar metallicity, and a projected rotational velocity of ~5 km s⁻¹. Evolutionary modeling with MIST tracks indicates a mass of 1.10 M⊙, radius of 1.68 R⊙, and an age such that the star is presently a sub‑giant and will leave the main sequence in roughly 500 Myr. Consequently, the stellar luminosity has already increased by a factor of ~2 relative to its zero‑age main‑sequence value, raising the incident flux on the planets. The zero‑albedo equilibrium temperatures are 1360 K for TOI‑6054b and 1144 K for TOI‑6054c.

The combined transit and RV analysis yields planetary radii of 2.65 ± 0.15 R⊕ (b) and 2.81 ± 0.18 R⊕ (c) and masses of 12.4 ± 1.7 M⊕ (b) and 9.2 ± 2.0 M⊕ (c). These correspond to bulk densities of ~4.8 g cm⁻³ and ~3.2 g cm⁻³, respectively, implying a mixture of rock, water, and a modest H/He envelope. The inner planet’s higher density and higher incident flux suggest that it may be in the process of losing part of its primordial H/He atmosphere. Energy‑limited escape calculations give a current mass‑loss rate of order 10⁹ g s⁻¹, a level that should be detectable with JWST/NIRSpec, HST/WFC3, or high‑resolution ground‑based spectrographs (e.g., VLT/ESPRESSO) via transmission spectroscopy of escaping species (e.g., H I Lyman‑α, He I 10830 Å).

High‑resolution imaging (optical speckle at SAI, Gemini ‘Alopëke’, and near‑infrared AO at Palomar) found no nearby companions down to contrasts of 5–8 mag between 0.02″ and 1.2″ (≈20–120 AU), ruling out false‑positive scenarios from background eclipsing binaries and confirming that the derived planetary parameters are not diluted.

The authors emphasize the system’s suitability for atmospheric characterization. The host star’s brightness, the relatively deep transits, and the planets’ sizes place them among the most favorable sub‑Neptune targets for JWST, ARIEL, and upcoming ground‑based facilities. Moreover, the near‑resonant configuration provides a laboratory for testing theories of resonant capture, tidal evolution, and atmospheric loss in multi‑planet systems undergoing stellar evolution. The paper concludes that TOI‑6054b and c constitute a benchmark pair for probing the interplay between dynamical interactions, stellar evolution, and atmospheric escape in the sub‑Neptune regime.