Persson, Carina M.; Knudstrup, Emil; Carleo, Ilaria; Acuña-Aguirre, Lorena; Nowak, Grzegorz; Muresan, Alexandra; Jankowski, Dawid; Goździewski, Krzysztof; García, Rafael A.; Mathur, Savita; Palakkatharappil, Dinil B.; Borg, Lina; Mustill, Alexander James; Barrena, Rafael; Fridlund, Malcolm C.V.; Gandolfi, Davide; Hatzes, Artie P.; Korth, Judith; Luque, Rafael; Martín, Eduardo L.; Masseron, Thomas; Morello, Giuseppe; Murgas, Felipe; Orell-Miquel, Jaume; Palle, Enric B.; Albrecht, Simon H.; Bieryla, Allyson; Cochran, William D.; Crossfield, Ian J.M.; Deeg, Hans J.; Furlan, Elise; Guenther, Eike Wolf; Howell, Steve B.; Isaacson, Howard T.; Lam, Kristine Wai Fun; Livingston, John H.; Matson, Rachel A.; Matthews, Elisabeth C.; Redfield, Seth L.; Schlieder, Joshua E.; Seager, Sara; Smith, Alexis M.S.; Stassun, Keivan Guadalupe; Twicken, Joseph D.; van Eylen, Vincent; Watkins, Cristilyn N.; Weiss, Lauren M. (2025). TOI-1438: A rare system with two short-period sub-Neptunes and a tentative long-period Jupiter-like planet orbiting a K0V star. Astronomy and Astrophysics, 702, A69. https://doi.org/10.1051/0004-6361/202555318
The TOI-1438 system, discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS), contains multiple planets orbiting a Sun-like star. To confirm these planets and measure their properties, astronomers collected detailed follow-up observations over five years using high-resolution spectrographs, including HARPS-N and HIRES. The host star, classified as a K0V-type star, was found to have two confirmed planets—both slightly smaller than Neptune—known as TOI-1438 b and TOI-1438 c.
Planet b has a radius about 3 times that of Earth and a mass about 9 times larger, while planet c is slightly smaller and more massive, with a radius about 2.8 times and a mass about 10.6 times that of Earth. These planets orbit their star every 5.1 and 9.4 days, respectively, receiving 145 and 65 times the amount of sunlight that Earth receives from the Sun. Their calculated densities—1.8 and 2.9 grams per cubic centimeter—suggest that both planets have interiors rich in volatile materials such as water or other light molecules, rather than being purely rocky.
Modeling of their internal structures indicates that planet b likely has a hydrogen-helium (H/He) atmosphere making up to about 2.5% of its mass, while planet c has a much thinner envelope, with less than 0.2% of its mass in H/He. Regardless of the exact makeup of their cores—whether a mix of rock and metal or rock and ice—both planets appear to require a volatile-rich outer layer.
In addition to these two confirmed planets, the team detected another signal in the star’s radial velocity data that may indicate the presence of a third, much larger, non-transiting planet. This possible outer planet would orbit once every 7.6 years, have a minimum mass roughly twice that of Jupiter, and follow a slightly oval-shaped (eccentric) orbit. However, some of the same signals could also be caused by variations in the star’s own activity, so further long-term observations are needed to confirm whether this third planet truly exists.
If verified, TOI-1438 would be an unusual three-planet system featuring two small, close-in sub-Neptunes and one distant, massive planet—a configuration rarely observed among known planetary systems.
Fig. 1
Final results of the Gemini North speckle imaging of TOI-1438. The blue and red curves show the 5 σ 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.