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September 5, 2025Open Access

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

Key Points

Two sub-neptunes were confirmed in the TOI-1438 system, with distinct volatile-rich interiors and specific mass ratios.
Both planets present orbital periods of 5.1 and 9.4 days, indicating close proximity to their host star, a K0V type.
Radial velocity data suggests a third planetary signal possibly tied to a non-transiting planet, which may be influenced by stellar activity.
This system illustrates a rare configuration involving small, short-period planets alongside a massive, long-period companion.

Abstract

We present the detection and characterisation of the multi-planet system discovered by the Transiting Exoplanet Survey Satellite (TESS). To confirm the planetary nature of the candidates and determine their masses, we collected a series of follow-up observations including high-spectral resolution observations with HARPS-N and HIRES over a period of five years. Our combined modelling shows that the star hosts two transiting sub-Neptunes with R_ b = ̊pb ̊earth, R_ c = ̊pc ̊earth, M_ b and M_ c The orbital periods of planets b and c are 5. 1 and 9. 4 days, respectively, corresponding to instellations of and The bulk densities are and respectively, suggesting a volatile-rich interior composition. By combining the planet and stellar parameters, we were able to compute 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 years and an RV semi-amplitude of ̨d Our best-fit model finds a minimum mass of and an eccentricity of 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 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.

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Persson et al. (Mon,) studied this question.

synapsesocial.com/papers/68bb4e016d6d5674bcd027e2 https://doi.org/https://doi.org/10.1051/0004-6361/202555318

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