TOI-1438: A rare system with two short-period sub-Neptunes and a tentative long-period Jupiter-like planet orbiting a K0V star

TL;DR

TOI-1438 hosts two transiting sub-Neptunes in close orbits around a K0V star, with radii of $R_b = 3.04 \pm 0.19\,R_\oplus$ and $R_c = 2.75 \pm 0.14\,R_\oplus$ and masses $M_b = 9.4 \pm 1.8\,M_\oplus$, $M_c = 10.6 \pm 2.1\,M_\oplus$, yielding densities of $\rho_b \approx 1.8 \pm 0.5$ and $\rho_c \approx 2.9 \pm 0.7\,\mathrm{g\,cm^{-3}}$. A joint transit-RV analysis uncovers a possible third, long-period companion with $P_d \approx 7.6$ yr and $M_p\sin i \approx 2.1\,M_J$, though activity indicators prevent a conclusive planetary interpretation. Interior-structure retrieval with three-layer models indicates volatile-rich envelopes for both planets and substantial degeneracy in core compositions, with envelope H/He masses up to about 2.5% for b and 0.2% for c, respectively. Dynamical analyses reveal largely stable, low-eccentricity inner orbits with potential long-term secular effects from the outer companion; if confirmed, TOI-1438 would exemplify a rare architecture of compact inner planets and a distant giant, offering key constraints on formation scenarios in protoplanetary discs. Overall, this system provides valuable insights into planet interiors, system architecture, and formation pathways, while underscoring the need for extended RV monitoring to confirm the outer companion.

Abstract

We present the detection and characterisation of the TOI-1438 multi-planet system discovered by TESS. We collected a series of follow-up observations including high-spectral resolution observations with HARPS-N over a period of five years. Our modelling shows that the K0V star hosts two transiting sub-Neptunes with Rb = 3.04 +/- 0.19 RE, Rc = 2.75 +/- 0.14 RE, Mb = 9.4 +/- 1.8 ME, and Mc = 10.6 +/- 2.1 ME. The orbital periods of planets b and c are 5.1 and 9.4 days, respectively, corresponding to instellations of 145 +/- 10 and 65 +/- 4 FE. The bulk densities are 1.8 +/- 0.5 and 2.9 +/- 0.7 g cm-3, respectively, suggesting a volatile-rich interior composition. We computed a set of planet interior structure models. Planet b presents a high-metallicity envelope that can accommodate up to 2.5 % in H/He in mass, while planet c cannot have more than 0.2 % as H/He in mass. For any composition of the core considered (Fe-rock or ice-rock), both planets would require a volatile-rich envelope. In addition to the two planets, the radial velocity (RV) data clearly reveal a third signal, likely coming from a non-transiting planet, with an orbital period of 7.6 +1.6 -2.4 years and a radial velocity semi-amplitude of 35+3-5 m s-1. Our best fit model finds a minimum mass of 2.1 +/- 0.3 MJ and an eccentricity of 0.25+0.08-0.11. However, several RV activity indicators also show strong signals at similar periods, suggesting this signal might (partly) originate from stellar activity. More data over a longer period of time are needed to conclusively determine the nature of this signal. If it is confirmed as a triple-planet system, TOI-1438 would be one of the few detected systems to date characterised by an architecture with two small, short-period planets and one massive, long-period planet, where the inner and outer systems are separated by an orbital period ratio of the order of a few hundred.

Figures (21)

• Figure 1: Final results of the Gemini North speckle imaging of TOI-1438. The blue and red curves show the $5\,\sigma$ contrast curves in 562 nm and 832 nm filters, respectively, as a function of the angular separation out to 1.2. The inset shows the reconstructed 832 nm image with a 1 scale bar. TOI-1438 was found to have no close companions from the diffraction limit out to 1.2 and within the magnitude contrast levels achieved.
• Figure 2: SED of TOI-1438 and the model with highest probability Castelli2004 are plotted together with the synthetic photometry (magenta diamonds) and the observed photometry (blue points). The vertical error bars outlines the $1\,\sigma$ uncertainties and the horizontal bars the effective width of the passbands. The residuals of the fit (normalised to the errors of the photometry) are shown in the lower panel.
• Figure 3: GLS periodogram of the HARPS-N and HIRES RV data. The GLS of the raw RVs are shown in the top panel and the below panels have the best-fitting models subtracted for a given planet or signal d, as described in Sect. \ref{['Section: transit and RV modelling']}. From right to left, the orange, red, and blue vertical lines denotes the orbital periods of planets b and c, and signal d, respectively.
• Figure 4: GLS periodogram of HARPS-N data with activity indicators from DRS and Serval as indicated in the legends. The preliminary stellar rotation period of 23 days and Earth's orbital period of 365 days are marked with red and green vertical dashed lines, respectively, and signal d with a blue vertical thick line.
• Figure 5: Emission in the line cores of the CaII H & K absorption lines in the co-added HARPS spectrum of TOI-1438.