Kavli Affiliate: Sara Seager
| First 5 Authors: Carina M. Persson, Carina M. Persson, , ,
| Summary:
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.
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