---
title: 'TOI-2141: Compact Three-Planet System'
url: https://www.emergentmind.com/topics/toi-2141-planetary-system
type: topic
---

# TOI-2141: Compact Three-Planet System

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 [2311.07011; 2509.00762].

## 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 [2311.07011].

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 [2509.00762].

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\(^{-1}\) cm\(^{-2}\), luminosity \(\log{L_{\star}/L_{\odot}}=-0.05\pm0.06\), rotation velocity \(v_{\rm rot}\sin i_\star=2.7\pm0.6\) km s\(^{-1}\), activity index \(\log{\rm R'}_{\rm HK}=-4.78^{+0.10}_{-0.12}\), rotation period \(P_{\rm rot}=21\pm5\) d, and age \(6.4\pm1.8\) 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” [2311.07011].

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 \(T_{\rm eff}=5635\pm61\) K, \(\log g = 4.40\pm0.03\), \([\mathrm{Fe/H}] = -0.14\pm0.04\), \(v_{\rm micro}=0.92\pm0.01\ \mathrm{km\,s^{-1}}\), \(M_\star = 0.896^{+0.059}_{-0.051}\,M_\odot\), \(R_\star = 0.950\pm0.007\,R_\odot\), and age \(= 9\pm4\) Gyr. The kinematics \((U,V,W)=(15.56,-21.60,-36.53)\,\mathrm{km\,s^{-1}}\) indicate a thin-disk star, while the stellar rotation period estimate of \(21\pm5\) d from earlier activity-rotation relations was not significantly refined by the new data [2509.00762].

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 \(30.6\) d, was attributed to contamination from scattered light and an odd/even depth inconsistency. The validated signal, TOI-2141 b, had TESS-estimated parameters \(R_{\rm p}=3.2\pm0.3\,R_\oplus\) and \(P=18.26159(7)\) d, and centroid offsets localized the signal within \(1.7\pm6.3\) arcsec of the target. SOPHIE follow-up then obtained 90 high-resolution RVs in HR mode (\(R=75000\)) between 2021-02-25 and 2022-09-18, with an average peak S/N of 55; the RVs had a median of about \(-19860.8\) m s\(^{-1}\) and an rms of 7 m s\(^{-1}\), and the GLS periodogram showed its strongest peak at \(18.259\) d, matching the TESS transit period [2311.07011].

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 \(1\sigma\). 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 [2311.07011].

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 \(\sim0.98\ \mathrm{m\,s^{-1}}\). 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
\[
\Delta \ln Z = \ln Z_{\rm M2}-\ln Z_{\rm M1},
\]
interpreting \(\Delta \ln Z>5\) as “greatly favored.” The preferred model was a 3-planet fit with \(\ln Z=-459.3\), compared with \(\ln Z=-475.0\) for the 1-planet model and \(\ln Z\approx -470.8\) to \(-470.9\) for 2-planet alternatives; a 3-planet + GP model was disfavored with \(\ln Z=-465.5\) [2509.00762].

## 4. The transiting sub-Neptune TOI-2141 b

In the discovery solution, TOI-2141 b had a transit epoch of \(2458992.5033\pm0.0017\) BJD, an orbital period of \(18.26157(6)\) d, a radius of \(3.05\pm0.23\) R\(_\oplus\), and a mass of \(24\pm4\) M\(_\oplus\), derived from an RV semi-amplitude of \(6.0\pm1.0\) m s\(^{-1}\). The normalized semi-major axis was \(39^{+4}_{-9}\), corresponding to a true semi-major axis of \(0.1330\pm0.0009\) au. The orbit was consistent with being circular, with eccentricity reported as \(<0.21\); the inclination was \(>89.0^\circ\), the transit duration \(4.9\pm0.4\) h, the impact parameter \(<0.4\), and the planet-to-star radius ratio \(0.0284\pm0.0012\). Derived bulk properties included a density of \(4.6\pm1.3\) g cm\(^{-3}\), an equilibrium temperature of \(722\pm23\) K assuming uniform heat redistribution and a geometric albedo of 0.1, and an insolation of \(50\pm4\,S_\oplus\) [2311.07011].

The later joint TESS+CHEOPS+HARPS-N+SOPHIE fit refined these parameters to \(P_{\rm b} = 18.261608^{+1.9\times10^{-5}}_{-2.0\times10^{-5}}\) d, \(T_{0,\rm b} = 1992.502^{+0.0014}_{-0.0013}\) BJD \(-2457000\), \(K_{\rm b} = 5.25\pm0.35\ \mathrm{m\,s^{-1}}\), \(R_{\rm p} = 3.147^{+0.043}_{-0.042}\ R_\oplus\), and \(M_{\rm p} = 20.1^{+1.6}_{-1.5}\ M_\oplus\). The derived properties are \(\rho_{\rm p} = 3.54^{+0.31}_{-0.30}\ \mathrm{g\,cm^{-3}}\), \(g_{\rm p} = 19.9^{+1.7}_{-1.6}\ \mathrm{m\,s^{-2}}\), \(a_{\rm p} = 0.1322^{+0.003}_{-0.003}\) au, \(a_{\rm p}/R_\star = 29.93^{+0.62}_{-0.64}\), \(b = 0.65^{+0.019}_{-0.02}\), \(i_{\rm b} = 88.755^{+0.063}_{-0.063}\) deg, \(T_{\rm eq,b}=728.0\pm11.0\) K, insolation \(S = 47.3^{+2.5}_{-2.4}\,S_\oplus\), and Transmission Spectroscopy Metric \(=35.6^{+3.3}_{-2.9}\). The mass and radius uncertainties were improved by roughly a factor of 2 and 5, respectively, relative to the earlier work [2509.00762].

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 | \(P_{\rm b}=18.261608^{+1.9\times10^{-5}}_{-2.0\times10^{-5}}\) d; \(R_{\rm p}=3.147^{+0.043}_{-0.042}\ R_\oplus\); \(M_{\rm p}=20.1^{+1.6}_{-1.5}\ M_\oplus\); \(a_{\rm p}=0.1322^{+0.003}_{-0.003}\) au |
| TOI-2141 c | Non-transiting RV planet | \(P_{\rm c}=5.4624^{+0.0026}_{-0.0027}\) d; \(K_{\rm c}=2.40\pm0.33\ \mathrm{m\,s^{-1}}\); \(M_{\rm p}\sin i=6.14\pm0.86\ M_\oplus\); \(a_{\rm c}=0.0585^{+0.0011}_{-0.0012}\) au |
| TOI-2141 d | Non-transiting RV planet | \(P_{\rm d}=60.45^{+0.24}_{-0.25}\) d; \(K_{\rm d}=2.61\pm0.34\ \mathrm{m\,s^{-1}}\); \(M_{\rm p}\sin i=14.9\pm2.1\ M_\oplus\); \(a_{\rm d}=0.2906^{+0.0055}_{-0.0057}\) 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 \(\Delta \ln Z = 351679.2 - 351693.6 = -14.4\) relative to the non-transiting model. If c were forced to transit, its radius would have to be smaller than \(\sim 0.34^{+0.32}_{-0.23}\,R_\oplus\), implying an absurd density of \(\sim850\ \mathrm{g\,cm^{-3}}\), and d would be even denser if forced to transit. The quoted geometric transit probabilities were \(7.5\pm0.1\%\) for c, \(3.3\%\) for b, and \(1.5\%\) for d [2509.00762].

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
\[
\Delta i_{\rm min} \approx \cos^{-1}\!\left(\frac{R_\star}{a_b}\right) - \cos^{-1}\!\left(\frac{R_\star}{a_c}\right) \ge 2.4^\circ.
\]
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\(_2\)O layer, a variant including a thin H\(_2\) envelope, or even a pure H\(_2\)O 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” [2311.07011].

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 \(0.103^{+0.144}_{-0.077}\), mantle mass fraction \(0.20^{+0.21}_{-0.14}\), water mass fraction \(0.67^{+0.14}_{-0.32}\), and atmospheric mass fraction \(0.00039^{+0.01636}_{-0.00039}\). 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 [2509.00762].

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 \(10/3\) 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 \(e\sim0.3\) for b and d, and that the planetary masses are constrained by stability to be below \(\sim 24\,M_\oplus\) for c and \(\sim 30\,M_\oplus\) for d if the system is viewed at the limiting stable inclinations [2509.00762].

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 [2311.07011].

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.

Source: https://www.emergentmind.com/topics/toi-2141-planetary-system