TOI-1422 c: A Warm Sub-Neptune Exoplanet

• TOI-1422 c is a sub-Neptune exoplanet with a 34.56-day orbit around a bright G2 V star, exhibiting an ‘anti-ordered’ mass-radius relationship compared to its inner planet.
• Its discovery used joint TESS photometry and HARPS-N radial velocity measurements, unveiling key transit timing variations that suggest the presence of an additional companion.
• The planet’s high density and signs of envelope loss provide insights that challenge conventional models of planetary formation and evolution in warm Neptune systems.

TOI-1422 c is a confirmed sub-Neptune exoplanet orbiting the bright G2 V star TOI-1422 ($V = 10.6$ mag), located 155 pc from the Sun. Initially identified as a candidate from residual signals in radial velocity (RV) data, it was later established as a transiting planet through the detection of multiple transits in TESS photometry and joint analysis with high-precision RV measurements. TOI-1422 c is notable for its high mass and density relative to the system’s inner planet, TOI-1422 b, rendering the system an instance of “anti-ordered” architecture where the more massive planet resides in the outer orbit yet possesses a smaller radius than the inner planet. The system exhibits prominent transit timing variations (TTVs) indicative of additional dynamical activity and possible unseen companions (Naponiello et al., 2022, Naponiello et al., 14 Nov 2025).

1. Orbital and Physical Characteristics

TOI-1422 c has an orbital period $P_c = 34.5633 \pm 0.0002$ days and a semi-major axis $a_c = 0.205 \pm 0.005$ AU, making it the exterior planet in a two confirmed-planet, near-circular configuration ($\Delta\ln \mathcal{Z} \approx 6.9$ for circular vs.\ free-eccentricity model). Its mass and radius, determined jointly from transit photometry and RVs, are $M_c = 14 \pm 3\,M_{\oplus}$ and $R_c = 2.61 \pm 0.14\,R_{\oplus}$, which imply a bulk density of $\rho_c = 4.3^{+1.3}_{-1.0}$ g cm$^{-3}$. The radial velocity semi-amplitude associated with its orbital motion is $K_c = 2.80 \pm 0.59$ m s$^{-1}$ (Naponiello et al., 14 Nov 2025).

The planet receives incident stellar flux corresponding to an equilibrium temperature of approximately 661 K (assuming zero Bond albedo and full redistribution) (Naponiello et al., 2022). Compared to the system’s inner planet TOI-1422 b ($P_c = 34.5633 \pm 0.0002$0, $P_c = 34.5633 \pm 0.0002$1, $P_c = 34.5633 \pm 0.0002$2 g cm$P_c = 34.5633 \pm 0.0002$3), TOI-1422 c resides farther out by $P_c = 34.5633 \pm 0.0002$4 and $P_c = 34.5633 \pm 0.0002$5 (Naponiello et al., 14 Nov 2025).

2. Detection and Confirmation Strategies

The initial identification of TOI-1422 c stemmed from RV residuals exhibiting a periodicity near 29 days, with supporting evidence for a Neptune-mass object undetected in the original TESS light curves (Naponiello et al., 2022). The significance of the 29.29-day signal in the HARPS-N residuals was established with a Generalized Lomb-Scargle (GLS) false-alarm probability (FAP) of $P_c = 34.5633 \pm 0.0002$6, and two-planet joint RV+transit Bayesian models were robustly favored ($P_c = 34.5633 \pm 0.0002$7) over single-planet fits.

Further TESS observations revealed three individual transits of a sub-Neptune in Sectors 16, 57, and 84 (absent in Sector 17), with the measured period refined to $P_c = 34.5633 \pm 0.0002$8 days and transit depth $P_c = 34.5633 \pm 0.0002$9683 ppm, yielding S/N $a_c = 0.205 \pm 0.005$014. This, combined with an expanded HARPS-N dataset (120 spectra), confirmed the existence of a bona fide second planet (Naponiello et al., 14 Nov 2025).

3. System Architecture and Dynamical Properties

TOI-1422 exhibits a dynamically intriguing architecture. Planet c is both more massive and denser than the inner planet, contrary to the “peas-in-a-pod” ordering usually observed in compact multiplanet systems—where the inner planet is typically either more massive or more dense. Specifically, TOI-1422 c’s density exceeds that of b by a factor of $a_c = 0.205 \pm 0.005$14. A plausible implication is that TOI-1422 c may have undergone significant evolutionary processing: scenarios such as late-stage giant impacts that stripped its envelope, migration-induced envelope loss, or strong dynamical scattering could preferentially deplete c’s H/He layer while leaving b’s envelope relatively unperturbed (Naponiello et al., 14 Nov 2025).

Transit timing variations (TTVs) are prominent in the TOI-1422 system. The inner planet b shows observed-minus-calculated mid-transit deviations of up to 5 hours ($a_c = 0.205 \pm 0.005$20.017 days). N-body integrations coupling TTV and RV data indicate that c’s perturbations alone cannot account for this amplitude. Only a model including an additional low-mass (unseen) companion—termed planet d by the discovery group—between b and c (with $a_c = 0.205 \pm 0.005$3 days, $a_c = 0.205 \pm 0.005$4, $a_c = 0.205 \pm 0.005$5) yields a self-consistent solution for the observed masses, eccentricities, and TTVs (Naponiello et al., 14 Nov 2025). Injection-recovery tests confirm that such a companion could presently evade RV detection.

4. Significance Among Warm and Sub-Neptune Exoplanets

With its period, mass, and equilibrium temperature, TOI-1422 c is squarely classified as a “warm Neptune” or “sub-Neptune,” with $a_c = 0.205 \pm 0.005$6–100 days and $a_c = 0.205 \pm 0.005$7–20 $a_c = 0.205 \pm 0.005$8 (Naponiello et al., 2022, Naponiello et al., 14 Nov 2025). Its combination of large mass, small radius, and high density distinguishes it from prototypical Neptunes and sub-Neptunes, as such planets are generally expected to retain more extended volatile-rich envelopes, particularly on wider orbits.

The “anti-ordered” mass–radius relationship (outer planet more massive and denser) highlights the significant diversity within exoplanetary system architectures and challenges canonical models of monotonically decreasing planet density with increasing separation (“peas-in-a-pod” systems) (Naponiello et al., 14 Nov 2025). The possibility that c’s denser composition results from envelope loss by impacts or migration-driven atmospheric escape unconnected to b’s history underlines the role of dynamical events in sculpting planetary system outcomes.

5. Observational Prospects and Future Directions

The TOI-1422 system presents multiple targets for future observation. High-precision RVs at $a_c = 0.205 \pm 0.005$91 m s$\Delta\ln \mathcal{Z} \approx 6.9$0 (e.g., ESPRESSO) are required to confirm the tentative planet d and further constrain the masses of c and d. Additional time-series photometry using TESS, CHEOPS, or PLATO will enable extension of the TTV baseline, improving the identification and orbital characterization of other companions (Naponiello et al., 14 Nov 2025).

Transmission spectroscopy of c with JWST or ARIEL offers the prospect to determine atmospheric metallicity and distinguish between icy/volatile-rich vs. stripped rocky compositions. Such data would provide critical tests of formation and evolution hypotheses—particularly regarding envelope retention or loss beyond the ice line. Continued activity monitoring is essential to discriminate low-amplitude planetary RV signals from stellar activity.

6. Context and Comparative Summary

TOI-1422 c is similar in mass and period to other well-studied warm Neptunes orbiting bright G stars, such as HD 106315 c, Kepler-56 c, and HD 63935 b. However, its non-detection in earlier transit surveys and the detection of both transiting and RV signals make it a distinctive benchmark for studying planetary structure and system evolution (Naponiello et al., 2022). The presence of multiple, dynamically active planets, including at least one planet with an anomalous mass–radius relation, marks TOI-1422 as a key system for comparative exoplanetology and a testing ground for models of planet formation, migration, and envelope evolution.

Parameter	Value	Reference
$\Delta\ln \mathcal{Z} \approx 6.9$1	$\Delta\ln \mathcal{Z} \approx 6.9$2 days	(Naponiello et al., 14 Nov 2025)
$\Delta\ln \mathcal{Z} \approx 6.9$3	$\Delta\ln \mathcal{Z} \approx 6.9$4 AU	(Naponiello et al., 14 Nov 2025)
$\Delta\ln \mathcal{Z} \approx 6.9$5	$\Delta\ln \mathcal{Z} \approx 6.9$6	(Naponiello et al., 14 Nov 2025)
$\Delta\ln \mathcal{Z} \approx 6.9$7	$\Delta\ln \mathcal{Z} \approx 6.9$8	(Naponiello et al., 14 Nov 2025)
$\Delta\ln \mathcal{Z} \approx 6.9$9	$M_c = 14 \pm 3\,M_{\oplus}$0 g cm$M_c = 14 \pm 3\,M_{\oplus}$1	(Naponiello et al., 14 Nov 2025)
$M_c = 14 \pm 3\,M_{\oplus}$2	$M_c = 14 \pm 3\,M_{\oplus}$3 K	(Naponiello et al., 2022)
$M_c = 14 \pm 3\,M_{\oplus}$4	$M_c = 14 \pm 3\,M_{\oplus}$5 m s$M_c = 14 \pm 3\,M_{\oplus}$6	(Naponiello et al., 14 Nov 2025)