TOI-2141: Compact Three-Planet System

• TOI-2141 is a compact three-planet system around a solar analog, featuring a dense, transiting sub-Neptune and two non-transiting radial-velocity companions.
• The system was characterized through combined TESS, CHEOPS, HARPS-N, and SOPHIE observations, using transit modeling and Bayesian RV analysis to refine planetary parameters.
• Its detailed dynamical and interior structure analyses provide a benchmark for studying sub-Neptune formation and evolution in Sun-like environments.

Searching arXiv for TOI-2141 papers to ground the article in the cited literature. TOI-2141 is a planetary system around a solar analog, initially reported as a single transiting sub-Neptune system and later recharacterized as a compact three-planet system comprising the transiting TOI-2141 b and the non-transiting radial-velocity companions TOI-2141 c and TOI-2141 d. Across the discovery and follow-up analyses, the system has been used to examine planet formation and evolution in Sun-like environments, with particular emphasis on the internal structure of a dense sub-Neptune orbiting a mature, slightly metal-poor solar analog (Martioli et al., 2023, Luque et al., 31 Aug 2025).

1. Discovery history and changing system picture

The discovery study presented TOI-2141 as the simpler of two TESS/SOPHIE systems around solar analogs. In that formulation, the system consisted of a single transiting sub-Neptune, TOI-2141 b, with no long-term trends or additional companions in the SOPHIE radial velocities. The host was characterized as a near-solar G-type dwarf, close to the Sun in fundamental properties but not a solar twin, and the system was described as a useful benchmark for sub-Neptune formation around Sun-like stars because it lacked the hierarchical complexity inferred for TOI-1736 (Martioli et al., 2023).

A later reanalysis substantially revised that picture. By combining new TESS and CHEOPS photometry with HARPS-N radial velocities and the earlier SOPHIE data, the system was found to contain three planets: the known transiting TOI-2141 b, an inner non-transiting companion TOI-2141 c with a period of approximately $5.46$ d, and an outer non-transiting companion TOI-2141 d with a period of approximately $60.45$ d. The follow-up work therefore shifted TOI-2141 from a single-planet benchmark to a compact three-planet system with low mutual inclinations and no exact resonant lock (Luque et al., 31 Aug 2025).

This evolution in the system model is important methodologically. The earlier “clean” interpretation was not an error in the transit signal; rather, it reflected the information content of the initial data set. The later work resolved additional low-amplitude RV signals and refined the transiting planet’s mass and radius, illustrating how extended RV baselines and higher-precision follow-up can alter the inferred architecture of apparently simple TESS systems.

2. Host star: solar-analog status and stellar characterization

The host star, also identified as BD+18 3330, is a bright, Sun-like star at a Gaia EDR3 distance of $77.7\pm0.2$ pc, with $V=9.46$ mag. In the discovery analysis, the authors’ preferred strictly differential SOPHIE analysis against a solar spectrum from Moon observations yielded $T_{\rm eff}=5659\pm12$ K, $\log{g}=4.42\pm0.04$ dex, and $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$ dex. The summary table gave a stellar mass of $0.94\pm0.02\,M_\odot$ and radius of $0.98\pm0.06\,R_\odot$, together with a bolometric flux of $4.65\pm0.11\times10^{-9}$ erg s$60.45$0 cm$60.45$1, luminosity $60.45$2, rotation velocity $60.45$3 km s$60.45$4, activity index $60.45$5, rotation period $60.45$6 d, and age $60.45$7 Gyr. The authors explicitly concluded that TOI-2141 is a mature solar analog rather than a solar twin and described it as “more similiar to our Sun, being poorer in metals and more active than normal for its age” (Martioli et al., 2023).

The later study rederived the stellar properties from a combined HARPS-N spectrum using the ARES+MOOG EW analysis and refined the radius with an IRFM/SED-based method anchored to Gaia parallax. Its adopted stellar parameters were $60.45$8 K, $60.45$9, $77.7\pm0.2$0, $77.7\pm0.2$1, $77.7\pm0.2$2, $77.7\pm0.2$3, and age $77.7\pm0.2$4 Gyr. The kinematics $77.7\pm0.2$5 indicate a thin-disk star, while the stellar rotation period estimate of $77.7\pm0.2$6 d from earlier activity-rotation relations was not significantly refined by the new data (Luque et al., 31 Aug 2025).

Taken together, these analyses support a consistent astrophysical classification: TOI-2141 is a slightly metal-poor, mature solar analog with well-constrained bulk properties. This is significant because differential or solar-anchored stellar characterization reduces the dominant host-star contribution to the uncertainty budget in planetary radius and density inference.

3. Photometric and spectroscopic data sets

The discovery paper used TESS photometry from Sectors 25, 26, and 52 at 2-minute cadence, specifically the SPOC PDC/PDCSAP light curves from MAST. The TESS Data Validation Report initially flagged two candidates, but only one passed validation tests. The other signal, with a periodicity of $77.7\pm0.2$7 d, was attributed to contamination from scattered light and an odd/even depth inconsistency. The validated signal, TOI-2141 b, had TESS-estimated parameters $77.7\pm0.2$8 and $77.7\pm0.2$9 d, and centroid offsets localized the signal within $V=9.46$0 arcsec of the target. SOPHIE follow-up then obtained 90 high-resolution RVs in HR mode ($V=9.46$1) between 2021-02-25 and 2022-09-18, with an average peak S/N of 55; the RVs had a median of about $V=9.46$2 m s$V=9.46$3 and an rms of 7 m s$V=9.46$4, and the GLS periodogram showed its strongest peak at $V=9.46$5 d, matching the TESS transit period (Martioli et al., 2023).

The initial joint analysis proceeded in two stages. Transit windows around four transits of TOI-2141 b were fit with BATMAN transit models and an MCMC approach with emcee, using uninformative priors and assuming circular orbits at the outset. After division by the best-fit transit model, the light curve was detrended with a quasi-periodic Gaussian-process baseline fitted to the binned data, although the GP periodicity was noted to be not robustly constrained and used mainly for detrending. Alternative baselines, including a squared-exponential kernel and no GP, yielded key transit parameters consistent within $V=9.46$6. For the final planet solution, the authors combined RVs and photometry with scipy.optimize.leastsq and then explored the posterior with a Bayesian MCMC using 50 walkers, 10000 iterations, and 3000 burn-in samples. Circular orbits were adopted for TOI-2141 b because allowing eccentricity did not improve the fit and produced larger BIC values (Martioli et al., 2023).

The follow-up study expanded the observational basis to TESS Sectors 25, 26, 52, and 79, five CHEOPS visits between May 2023 and July 2024 with 60 s exposures, and 61 HARPS-N RVs taken between October 2022 and September 2024. RVs were extracted with both YABI and SERVAL, but the analysis used the higher-precision YABI RVs, with mean internal uncertainty $V=9.46$7. The inference framework was correspondingly broader: stellar characterization from spectroscopy, IRFM/SED fitting, Gaia parallax, and two evolutionary model approaches; CHEOPS detrending and transit fitting with pycheops; RV model comparison with juliet and dynesty; final joint photometry + RV fitting with juliet; interior modeling with ExoMDN; atmospheric evolution with PASTA; and N-body stability analysis with the symplectic integrator SABAC4 and frequency-map diagnostics. For RV model comparison, the authors adopted

$V=9.46$8

interpreting $V=9.46$9 as “greatly favored.” The preferred model was a 3-planet fit with $T_{\rm eff}=5659\pm12$0, compared with $T_{\rm eff}=5659\pm12$1 for the 1-planet model and $T_{\rm eff}=5659\pm12$2 to $T_{\rm eff}=5659\pm12$3 for 2-planet alternatives; a 3-planet + GP model was disfavored with $T_{\rm eff}=5659\pm12$4 (Luque et al., 31 Aug 2025).

4. The transiting sub-Neptune TOI-2141 b

In the discovery solution, TOI-2141 b had a transit epoch of $T_{\rm eff}=5659\pm12$5 BJD, an orbital period of $T_{\rm eff}=5659\pm12$6 d, a radius of $T_{\rm eff}=5659\pm12$7 R$T_{\rm eff}=5659\pm12$8, and a mass of $T_{\rm eff}=5659\pm12$9 M$\log{g}=4.42\pm0.04$0, derived from an RV semi-amplitude of $\log{g}=4.42\pm0.04$1 m s$\log{g}=4.42\pm0.04$2. The normalized semi-major axis was $\log{g}=4.42\pm0.04$3, corresponding to a true semi-major axis of $\log{g}=4.42\pm0.04$4 au. The orbit was consistent with being circular, with eccentricity reported as $\log{g}=4.42\pm0.04$5; the inclination was $\log{g}=4.42\pm0.04$6, the transit duration $\log{g}=4.42\pm0.04$7 h, the impact parameter $\log{g}=4.42\pm0.04$8, and the planet-to-star radius ratio $\log{g}=4.42\pm0.04$9. Derived bulk properties included a density of $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$0 g cm$[{\rm Fe}/{\rm H}]=-0.120\pm0.013$1, an equilibrium temperature of $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$2 K assuming uniform heat redistribution and a geometric albedo of 0.1, and an insolation of $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$3 (Martioli et al., 2023).

The later joint TESS+CHEOPS+HARPS-N+SOPHIE fit refined these parameters to $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$4 d, $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$5 BJD $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$6, $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$7, $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$8, and $[{\rm Fe}/{\rm H}]=-0.120\pm0.013$9. The derived properties are $0.94\pm0.02\,M_\odot$0, $0.94\pm0.02\,M_\odot$1, $0.94\pm0.02\,M_\odot$2 au, $0.94\pm0.02\,M_\odot$3, $0.94\pm0.02\,M_\odot$4, $0.94\pm0.02\,M_\odot$5 deg, $0.94\pm0.02\,M_\odot$6 K, insolation $0.94\pm0.02\,M_\odot$7, and Transmission Spectroscopy Metric $0.94\pm0.02\,M_\odot$8. The mass and radius uncertainties were improved by roughly a factor of 2 and 5, respectively, relative to the earlier work (Luque et al., 31 Aug 2025).

These measurements place TOI-2141 b in the dense sub-Neptune regime: larger than a super-Earth in radius, but substantially denser than a low-density gas-dominated world. A plausible implication is that TOI-2141 b is particularly informative for the transition between rocky and volatile-rich planets around Sun-like hosts.

5. Additional companions and transit geometry

The revised orbital architecture contains three planets:

Planet	Detection status	Selected parameters
TOI-2141 b	Transiting	$0.94\pm0.02\,M_\odot$9 d; $0.98\pm0.06\,R_\odot$0; $0.98\pm0.06\,R_\odot$1; $0.98\pm0.06\,R_\odot$2 au
TOI-2141 c	Non-transiting RV planet	$0.98\pm0.06\,R_\odot$3 d; $0.98\pm0.06\,R_\odot$4; $0.98\pm0.06\,R_\odot$5; $0.98\pm0.06\,R_\odot$6 au
TOI-2141 d	Non-transiting RV planet	$0.98\pm0.06\,R_\odot$7 d; $0.98\pm0.06\,R_\odot$8; $0.98\pm0.06\,R_\odot$9; $4.65\pm0.11\times10^{-9}$0 au

The two RV planets were explicitly tested for transits. No evidence of transits for either c or d was found in the TESS photometry, and the CHEOPS pointings were timed to cover b’s transits rather than the predicted windows for c and d. A forced-transiting solution for c was very strongly disfavored, with $4.65\pm0.11\times10^{-9}$1 relative to the non-transiting model. If c were forced to transit, its radius would have to be smaller than $4.65\pm0.11\times10^{-9}$2, implying an absurd density of $4.65\pm0.11\times10^{-9}$3, and d would be even denser if forced to transit. The quoted geometric transit probabilities were $4.65\pm0.11\times10^{-9}$4 for c, $4.65\pm0.11\times10^{-9}$5 for b, and $4.65\pm0.11\times10^{-9}$6 for d (Luque et al., 31 Aug 2025).

The non-detection of c’s transit also constrains the mutual geometry. The minimum inclination offset between b and c implied by the data is

$4.65\pm0.11\times10^{-9}$7

This indicates only modest mutual inclination, not a highly tilted configuration. It also resolves a potential misconception: the presence of a transiting outer planet does not, by itself, require an inner companion with higher geometric transit probability to transit as well unless the orbits are sufficiently coplanar.

6. Interior structure and atmospheric evolution

The discovery paper interpreted TOI-2141 b through its location in the mass-radius plane. In that analysis, the planet was compatible with either a 50\% Earth-like rocky core plus a 50\% H$4.65\pm0.11\times10^{-9}$8O layer, a variant including a thin H$4.65\pm0.11\times10^{-9}$9 envelope, or even a pure H$60.45$00O composition; the summary statement was that TOI-2141 b is “likely a water-rich planet.” At the abstract level, the authors concluded that both TOI-1736 b and TOI-2141 b “likely have an Earth-like dense rocky core and a water-rich envelope” (Martioli et al., 2023).

The later work refined this interpretation with ExoMDN and PASTA. Under a four-layer internal model consisting of an iron core, Earth-like silicate mantle, water layer, and H/He atmosphere, the inferred mass fractions for TOI-2141 b were approximately core mass fraction $60.45$01, mantle mass fraction $60.45$02, water mass fraction $60.45$03, and atmospheric mass fraction $60.45$04. The bulk properties were therefore interpreted as indicating a substantial volatile layer above an Earth-like rocky core, together with a likely hydrogen/helium atmosphere. PASTA atmospheric-evolution models, assuming H-dominated primordial atmospheres and no post-disk migration in the model, suggested that TOI-2141 b has lost only about 1\% of its total mass in atmospheric content over its lifetime, consistent with mild photoevaporation rather than complete stripping; the framework also suggested that the star was probably born as a slow rotator (Luque et al., 31 Aug 2025).

This combination of high bulk density for a sub-Neptune, substantial inferred volatiles, and only mild atmospheric erosion makes TOI-2141 b a useful case for distinguishing between water-rich and H/He-bearing evolutionary pathways in the warm sub-Neptune regime.

7. Dynamical architecture and broader significance

The later dynamical analysis found that the system lies close to a $60.45$05 mean-motion resonance chain, but the best-fit solution is outside exact resonance. N-body stability calculations showed that the best-fit architecture is stable and not in resonant lock, that stability persists for eccentricities up to about $60.45$06 for b and d, and that the planetary masses are constrained by stability to be below $60.45$07 for c and $60.45$08 for d if the system is viewed at the limiting stable inclinations (Luque et al., 31 Aug 2025).

In the context of the earlier discovery paper, TOI-2141 had already been emphasized as valuable because planets around solar analogs allow stellar properties to be measured precisely relative to the Sun, thereby improving planet radii, masses, and interior-composition inferences. The later three-planet solution does not remove that significance; rather, it broadens it. The system now links several research themes at once: precise stellar benchmarking for Sun-like hosts, joint transit-plus-RV inference for dense sub-Neptunes, non-transiting companions recovered through Bayesian RV model comparison, modest mutual inclinations in compact multiplanet systems, and dynamical architectures that are near commensurabilities without being resonant (Martioli et al., 2023).

A plausible implication is that TOI-2141 occupies an observationally useful middle ground. It is not a dynamically extreme system, not a gas-giant-dominated architecture, and not an exactly coplanar transit chain. Instead, it is a bright, slightly metal-poor solar analog hosting a dense, volatile-rich transiting sub-Neptune together with two additional RV companions, making it a technically instructive system for comparative studies of sub-Neptune formation and evolution around Sun-like stars.