Sibling Sub-Neptunes Around Sibling M Dwarfs: TOI-521 and TOI-912

TL;DR

This study presents the discovery and precise characterization of two sub-Neptunes orbiting nearly identical M dwarfs TOI-521 and TOI-912 as part of the THIRSTEE program. By combining TESS photometry with extensive ground-based transit data and high-precision RVs from ESPRESSO, IRD, and HARPS, the authors determine that TOI-521 b and TOI-912 b have almost identical masses ($\approx 5.1$–$5.3\,M_\oplus$) and radii ($\approx 1.93$–$1.98\,R_\oplus$), yielding densities near $4\,\rm g\,cm^{-3}$. TOI-912 b may have a highly eccentric orbit ($e\approx0.58$) suggesting tidal evolution, while TOI-521 b is consistent with a circular orbit; an additional non-transiting candidate around TOI-521 at $P\approx 20.3$ d is identified but not yet confirmed. The results support a density-based division of sub-Neptunes around M dwarfs and identify these planets as promising targets for atmospheric studies with JWST, contributing to a population-level understanding of sub-Neptune formation and evolution in low-mass star systems.

Abstract

Sub-Neptunes are absent in the Solar System, yet they are commonly found in our Galaxy. They challenge the internal structure models and prompt investigation on their formation, evolution, and atmospheres. We report the characterisation of new sub-Neptunes orbiting two similar M dwarfs, TOI-521 (T_eff=3544 K), and TOI-912 (T_eff=3572 K). Both stars host a candidate identified by TESS and are part of the THIRSTEE follow-up program, which aims at understanding the sub-Neptune population through precise characterisation studies on a population level. We analysed light curves, ground-based photometry and ESPRESSO, HARPS and IRD RVs to infer precise orbital and physical parameters. The two stars host nearly identical planets in terms of mass and radius. TOI-521 b is a transiting sub-Neptune in a 1.5-d orbit with radius and mass of R=1.98+/-0.14 R_e and M=5.3+/-1.0 M_e respectively. Moreover, we identified an additional candidate at 20.3 d, with a minimum mass of Msini=10.7+/-2.4 M_e currently not detected to transit. Similarly, TOI-912 b is a 4.7-d sub-Neptune with R=1.93+/-0.13 R_e and M=5.1+/-0.5 M_e. Interestingly, TOI-912 b likely has an unusually high eccentricity (e=0.58+/-0.02), and it is probably undergoing strong tidal dissipation. If such eccentricity is confirmed, it would make it one of the most eccentric sub-Neptunes known to date. TOI-521 b and TOI-912 b have very similar densities (4 g/cm^3) and they lie in the degenerate region of the mass-radius diagram where different compositions are plausible, including a volatile-rich composition, or a rocky core surrounded by a H-He envelope. Our sample supports the division of sub-Neptunes into two distinct populations divided by a density gap. Both planets are interesting targets for atmospheric follow-up in the context of understanding the temperature-atmospheric feature trend that starts to emerge thanks to JWST observations.

Figures (23)

• Figure 1: GLS periodogram of IRD and ESPRESSO RVs, and ESPRESSO activity indicators of TOI-521. The vertical gray shows the planetary period of TOI-521.01 at $\sim 1.54$. In the ESPRESSO RVs periodogram, the significant peak at $f = 0.351$ d$^{-1}$ ($\sim 2.85$ d) corresponds to the daily alias of the planetary period. The horizontal dashed, dash-dotted, and dotted horizontal lines show the $10\%$, $1$%, and 0.1% FAP levels, respectively.
• Figure 2: Same as Fig. \ref{['fig:TOI521_periodogram']}, but for the HARPS and ESPRESSO RVs, and the ESPRESSO activity indicators of TOI-912. The vertical gray and red lines show the planetary signal of TOI-912.01 ($\sim 4.7$ d) and the stellar rotational period ($\sim 46$ d).
• Figure 3: RV residuals and GLS periodogram after the 1-planet (circular) model fit for TOI-521. The vertical dashed line in the periodogram highlights the most significant peak.
• Figure 4: Phase-folded TESS light curve (left) and RVs (right) of TOI-521 b from the joint photometric and RV analysis.The bottom panels show the model residuals. The best-fitting model is plotted as a solid black line. In the transit plot, the black dots show the data binned over $15$ min. In the RV plot, the error bars include the jitter term, added in quadrature to the nominal error, and the grey shaded area highlights the $\pm 1 \sigma$ region.
• Figure 5: Same as Fig. \ref{['fig:phase_plot_TOI521']}, but showing the phase-folded RVs of the candidate TOI-521 c.