A 34.6-day transiting sub-Neptune in the TOI-1422 planetary system

TL;DR

TOI-1422 hosts two transiting planets with an unusual anti-ordered arrangement: the outer sub-Neptune TOI-1422 c is more massive yet smaller in radius than the inner Neptune-sized TOI-1422 b. Using expanded TESS data and new HARPS-N RVs, the authors confirm TOI-1422 c with $P_c=34.563\,\mathrm{d}$, $R_c=2.61\,R_\oplus$, and $M_c=14\,M_\oplus$, and refine TOI-1422 b’s parameters, while detecting significant transit timing variations on b. Dynamical modeling with TRADES shows that a plausible three-planet configuration—with an additional inner or middle perturber—can reproduce the observed TTVs and RVs, suggesting a potential planet d with $M_d\lesssim4\,M_\oplus$ between b and c. ARDENT-based detection limits indicate room for such a companion, reinforcing the view that TOI-1422 is a valuable case for studying divergent formation histories and resonant interactions among close-in planets, with atmospheric analysis offering a path to disentangle formation scenarios.

Abstract

TOI-1422 is a G2 V star ($V = 10.6$ mag) known to host a warm Neptune-sized planet, TOI-1422 b, with a mass and radius of about $9M_{\oplus}$ and $4R_{\oplus}$, on a circular orbit with a period of $12.997$ days. An outer planetary candidate in this system had previously been suggested on the basis of a residual signal in the radial velocity (RV) data with a tentative period of $\sim$29 days, along with a possible single transit-like event, although it was not clear at the time whether the two signals belonged to the same companion. In this work, we confirm the presence of a second transiting planet, TOI-1422 c, a sub-Neptune ($R=2.61\pm0.14 R_{\oplus}$) that orbits with a longer period of 34.563 days. This confirmation is based on the detection of three TESS transits, two from newly available sectors, combined with new and archival RV measurements. The sub-Neptune ($ρ_{\rm c}=4.3^{+1.3}_{-1.0}$ g cm$^{-3}$) is more massive than the inner Neptune ($ρ_{\rm b}=0.93^{+0.21}_{-0.20}$ g cm$^{-3}$), having a mass of $M_{\rm c}=14\pm3 M_{\oplus}$, making TOI-1422 a rare anti-ordered system. Furthermore, we detect transit timing variations (TTVs) on the inner planet, with amplitudes of up to 5 hours, suggesting ongoing dynamical interactions. A dynamical analysis that combined TTVs and RVs indicates that planet c alone is unlikely to account for the full TTV amplitude observed on TOI-1422 b. We investigated whether an additional, as yet undetected companion could account for the observed signal, exploring a range of plausible orbital configurations and finding that a low-mass planet located between the two known orbits may be responsible.

Figures (17)

• Figure 1: GLS periodogram of the RV residuals from the 1-planet model, once the quadratic trend has been removed. The red and green vertical lines signal, respectively, the periodicity of $34.6$ and $29.0$ days, while the horizontal dashed line marks the 1% FAP.
• Figure 2: Phased TESS transits (top panel) and RV signal (bottom panel) of TOI-1422 c, along with the best fitted models, in black, and their residuals below each panel. The red circles represent the average of $\sim31$ minutes and $\sim83$ hours, respectively, while the gray areas represent the $1\sigma$ deviation from each model.
• Figure 3: TTVs of TOI-1422 b. The epochs of the transits are labeled on the bottom right of each panel. The red and blue vertical lines indicate, respectively, the calculated and observed center times of the transits.
• Figure 4: Observed minus calculated (O-C) center times of transits for TOI-1422 b. The ephemerides published in the discovery paper are represented by the dashed line, while the black solid line represents the ephemerides as evaluated in this manuscript. The red line is a simple sinusoidal fit to the O-C times. The error bars of the O-C times are smaller than the represented blue circles.
• Figure 5: RV detection limits in the TOI-1422 system computed with ARDENT. The data-driven detection limits are presented in brown. The dynamical detection limits are illustrated in red-yellow. The known planets TOI-1422 b and c are indicated with the black diamond symbols. The red shaded area depicts the region of the period-mass space where additional candidates could be expected.