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

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 9 M⊕ and 4 R⊕, on a circular orbit with a period of 12.997 d. 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 ∼29 d, 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 ± 0.14 R⊕) that orbits with a longer period of 34.563 d. This confirmation is based on the detection of three TESS (Transiting Exoplanet Survey Satellite) transits, two from newly available sectors, combined with new and archival RV measurements. The sub-Neptune (ρc = 4.3+1.3−1.0 g cm−3) is more massive than the inner Neptune (ρb = 0.93+0.21 −0.20 g cm−3), having a mass of Mc = 14 ± 3 M⊕, 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 h, 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.