TOI-1438 HARPS-N and HIRES radial velocities

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.

Our best fit model finds a minimum mass of 2.1+/-0.3M_J_ 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.