RVs of TOI-1438. Top: The HARPS-N (blue) and HIRES (orange) RV time series with the best-fitting three Keplerian model in grey with residuals shown in the panel below. Lower: The phasefolded RV curves for planet b (left) and c (middle) as well as signal d (right). The best-fitting models are shown as the grey lines with the shaded area denoting the 1 and 2σ intervals in the K-amplitude. Residuals are given below. — astro-ph.EP
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.
Carina M. Persson, Emil Knudstrup, Ilaria Carleo, Lorena Acuña-Aguirre, Grzegorz Nowak, Alexandra Muresan, Dawid Jankowski, Krzysztof Gozdziewski, Rafael A. García, Savita Mathur, Dinil B. Palakkatharappil, Lina Borg, Alexander J. Mustill, Rafael Barrena, Malcolm Fridlund, Davide Gandolfi, Artie P. Hatzes, Judith Korth, Rafael Luque, Eduardo L. Martín, Thomas Masseron, Giuseppe Morello, Felipe Murgas, Jaume Orell-Miquel, Enric Palle, Simon H. Albrecht, Allyson Bieryla, William D. Cochran, Ian J. M. Crossfield, Hans J. Deeg, Elise Furlan, Eike W. Guenther, Steve B. Howell, Howard Isaacson, Kristine W. F. Lam, John Livingston, Rachel A. Matson, Elisabeth C. Matthews, Seth Redfield, Joshua E. Schlieder, Sara Seager, Alexis M. S. Smith, Keivan G. Stassun, Joseph D. Twicken, Vincent Van Eylen, Cristilyn N. Watkins, Lauren M. Weiss
Comments: 31 pages, 21 figures. Accepted by Astronomy & Astrophysics (A&A) 31 July 2025
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2508.21533 [astro-ph.EP] (or arXiv:2508.21533v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.21533
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Submission history
From: Carina Persson M
[v1] Fri, 29 Aug 2025 11:43:06 UTC (25,156 KB)
https://arxiv.org/abs/2508.21533
Astrobiology,