Precise mass and radius determination for two new and one known Neptune-sized planets around G Dwarf hosts

TL;DR

This work presents precise masses and radii for two newly discovered sub-Neptunes TOI-1472 c and TOI-1648 b, along with an improved mass for TOI-1472 b, orbiting G-dwarf hosts. The team combines TESS transit photometry with extensive HARPS-N RVs, HIRES data, and high-resolution imaging to perform a joint transit–RV analysis with Gaussian Process activity modeling. Interior-structure and atmospheric-evolution modeling reveals a range of outcomes, from a planet with a substantial primordial envelope still undergoing loss to a water-dominated interior, situating them at key regions of the exo-Neptunian landscape. TOI-1648 b, with a high Transmission Spectroscopy Metric, is a prime target for atmospheric studies, while TOI-1472 b–c provide valuable constraints on formation, migration, and atmospheric evolution of Neptunian planets. Collectively, the results expand the sample of well-characterized Neptunian planets and illuminate the diversity of evolutionary paths in small-to-intermediate-sized exoplanets.

Abstract

As part of the KESPRINT collaboration, we present the discovery and characterization of three exoplanets in the sub-Neptune to super-Neptune regime, spanning key regions of the exo-Neptunian landscape. TOI-1472c and TOI-1648b are newly discovered sub-Neptunes, while TOI-1472b is a previously known super-Neptune for which we provide an improved mass measurement. These planets have orbital periods of 6--15 days and radii of 2.5--4.1 R$_\oplus$, probing regions where planet formation and atmospheric evolution remain poorly understood. We combine TESS transit photometry with ground-based radial velocities to determine precise masses, radii, and orbital properties. TOI-1472b has a mass of $18.0^{+0.84}_{-0.85}$ M$_\oplus$ and a radius of $4.06 \pm 0.10$ R$_\oplus$, TOI-1472c has a mass of $21.1^{+0.96}_{-0.99}$ M$_\oplus$ and a radius of $3.33 \pm 0.08$ R$_\oplus$, and TOI-1648b has a mass of $7.4^{+1.1}_{-1.3}$ M$_\oplus$ and a radius of $2.54^{+0.14}_{-0.12}$ R$_\oplus$. The planets exhibit a range of eccentricities (0.041--0.178), indicating diverse evolutionary histories. TOI-1648b, with a high Transmission Spectroscopy Metric (TSM $\sim$59), is a promising target for atmospheric characterization. Together, these three planets provide precise constraints on the structure, composition, and dynamical evolution of small to intermediate-sized exoplanets, enriching our understanding of the exo-Neptunian landscape.

Figures (16)

• Figure 1: Upper: TESS TPF (Target Pixel Files) of Sector 17 for TOI-1472, and Sector 25 for TOI-1648. The color scale represents electron counts per pixel. Orange squares mark the pixels selected by the TESS pipeline for aperture photometry. Gaia DR3 sources are overlaid as circles with sizes corresponding to their G-band magnitude difference relative to the target, as indicated in the legend. This visualization was created using the tpfplotter code aller2020. Gray arrows show the proper motion vectors of all sources in the field. Bottom: TESS heatmaps for the two targets, produced with TESS-contCastroGonzalez2024b, illustrating the percentage of the target star’s flux contained within each pixel.
• Figure 2: Reconstructed ‘Alopeke speckle images and $5\sigma$ contrast limits from simultaneous diffraction-limited exposure sequences using the 562 nm filter on the blue camera and the 832 nm filter on the red camera. No nearby sources are detected.
• Figure 3: Spectral energy distributions of TOI-1472 (top) and TOI-1648 (bottom). The red markers indicate the observed photometric data, with horizontal bars denoting the effective bandwidth of each filter. Blue points represent the synthetic fluxes derived from the best-fitting PHOENIX stellar atmosphere model, shown in black. In the inset, the absolute flux-calibrated Gaia spectrum is displayed as a gray shaded region.
• Figure 4: GLS periodograms for TOI-1472 (left), TOI-1648 (right). Periodograms are shown for the RV data, the residuals after subtracting the fitted planetary signal, and all stellar activity indicators derived from the HARPS-N DRS ($\rm log\,R^{\prime}_\mathrm{HK}$, S-index, bisector span, CCF contrast, and CCF FWHM) and from the serval pipeline (CRX, dLW, H$\alpha$, and the Na$_1$ and Na$_2$ lines). For TOI-1472, where two datasets (HARPS-N and HIRES) are available, the RVs have been shifted to a common median RV prior to computing the GLS periodogram. The orange shaded area denotes the stellar rotation period inferred from the joint Gaussian Process (GP) analysis using wide period priors (2–100 days), as detailed in Section \ref{['sec:planet']}. The red shaded area marks the rotation period derived from the SED-based analysis (Section \ref{['sec:sed']}). The red dashed line indicates the location of the dominant peak in each periodogram, while the green and yellow lines denote the orbital periods of planets b and c, respectively. For TOI-1648, the rotational modulation exhibits a double-peaked structure in the GP posterior.
• Figure 5: TESS time series for TOI-1472 during sectors 17, 57, 58, 84 and 85. The green triangle indicates the transits of TOI-1472 b, while the orange dots indicate the planet c's transits.