Discovery and characterisation of two exoplanets orbiting the metal-poor, solar-type star TOI-5788 with TESS, CHEOPS, and HARPS-N

TL;DR

TOI-5788 hosts two transiting planets around a metal-poor solar-type star, with TOI-5788~b at $P=6.34$ d and $R=1.528\,R_\oplus$, $M=3.72\,M_\oplus$, and TOI-5788~c at $P=16.213$ d and $R=2.272\,R_\oplus$, $M=6.4\,M_\oplus$. The masses are derived from $125$ HARPS-N RVs, and a joint transit analysis with TESS and CHEOPS yields precise radii and orbital elements; the system is dynamically stable with a potential but unconfirmed planet between 8–14 d. Interior modelling with plaNETic places b in a rocky regime with a small or negligible envelope, while c remains degenerate between a water-world and a gas-dwarf; atmospheric-evolution models favour a scenario in which c could retain a H/He-dominated envelope under plausible histories. The planets lie near the radius valley in a metal-poor host, providing a crucial testbed for links between stellar composition and planetary interiors, and offering insight into planet formation pathways around low-metallicity stars.

Abstract

We present the discovery and characterisation of two transiting exoplanets orbiting the metal-poor, solar-type star TOI-5788. From our analysis of six \textit{TESS} sectors and a dedicated \textit{CHEOPS} programme, we identify an inner planet (TOI-5788~b; $P = 6.340758\pm0.000030\,\si{\day}$) with radius $1.528\pm0.075\,\mathrm{R_\oplus}$ and an outer planet (TOI-5788~c; $P = 16.213362\pm0.000026\,\si{\day}$) with radius $2.272\pm0.039\,\mathrm{R_\oplus}$. We obtained 125 radial-velocity spectra from HARPS-N and constrain the masses of TOI-5788~b and~c as $3.72\pm0.94\,\mathrm{M_\oplus}$ and $6.4\pm1.2\,\mathrm{M_\oplus}$, respectively. Although dynamical analyses indicate that a third planet could exist in a stable orbit between 8 and 14 days, we find no evidence of additional planets. Since the TOI-5788 system is one of the few systems with planets straddling the radius gap, and noting that there are even fewer such systems around metal poor stars, it is a promising system to constrain planet formation theories. We therefore model the interior structures of both planets. We find that TOI-5788~b is consistent with being a rocky planet with almost no envelope, or having an atmosphere of a high mean molecular weight. We find that TOI-5788~c is consistent with both gas-dwarf and water-world hypotheses of mini-Neptune formation. We model the atmospheric evolution history of both planets. Whilst both scenarios are consistent with the atmospheric evolution of TOI-5788~c, the gas-dwarf model is marginally preferred. The results of the atmospheric evolution analysis are not strongly dependent on stellar evolution. This makes the system a promising target to test internal structure and atmospheric evolution models.

Figures (13)

• Figure 1: Six sectors of TESS data for TOI-5788. The 2-minute cadence data of Sector 14 and 20-second cadence data of Sectors 80 and 81 are shown in blue. Sectors 40, 41, and 54 have a cadence of 10 minutes (red). We bin the light curves to 4.8 hours (black). For Sectors with shorter cadence than 10 minutes, we show in red the 10-minute-binned data. We show the times of mid transit for TOI-5788 b (grey dashed lines) and TOI-5788 c (blue dashed lines).
• Figure 2: Fabry--Pérot contamination of an arbitrarily selected HARPS-N spectrum of TOI-5788. The contamination manifests as regularly spaced emission lines, shown here at around 5450 and 6034 Å.
• Figure 3: From top to bottom, time series of: MM-LSD RVs, $\log{R^{\prime}_\mathrm{HK}}$, CCF FWHM, and CCF BIS. Five $\log{R^{\prime}_\mathrm{HK}}$ observations have been omitted due to unphysical S-index measurements. The corresponding RV measurements and other activity indicators are retained.
• Figure 4: Normalised BGLS periodograms for the radial velocities, S-index, CCF full width at half maximum, and CCF bisector inverse slope. The periods of the planets are shown with red dashed vertical lines, and the predicted rotation period from 1984ApJ...279..763N is shown with a black dot-dashed vertical line.
• Figure 5: (Top) Phase-folded TESS transits light curves of TOI-5788 b and TOI-5788 c. The black points show the binned transit to emphasise the transit depths of roughly 0.02 per cent and 0.06 per cent, respectively. The standard errors for the phase-binned observations are shown but are not visible on the scale of the plot. (Middle) Phase-folded CHEOPS transit light curve of TOI-5788 c. As above, black points show the phase-binned light curve with standard errors. TOI-5788 b was not observed by CHEOPS. (Bottom) The phase folded radial velocity curves. The best-fit transit and radial-velocity models are shown as solid lines.