Mass determination of the three long-period Neptune- and sub-Neptune-sized planets transiting TOI-282

TL;DR

TOI-282 hosts three long-period transiting planets around a bright F8 star. The authors combine 7-year-spanning TESS photometry with extensive HARPS and ESPRESSO RV data to jointly model transits and Doppler signals, and perform dynamical analyses to extract masses, orbital elements, and stability. They measure $M_b=6.7^{+1.7}_{-0.8}$, $M_c=10^{+1}_{-2}$, and $M_d=5.8^{+0.9}_{-1.1}$ $M_{\oplus}$, with radii of $R_b=2.69\pm0.23$, $R_c=4.13^{+0.16}_{-0.14}$, and $R_d=3.11\pm0.15$ $R_{\oplus}$, implying low to moderate densities and suggesting water-rich compositions for some planets. While the outer pair is near a 3:2 period ratio, the resonant angles circulate, indicating near-resonant but not a locked MMR; the system is dynamically stable (AMD-Hill and MEGNO analysis) over long timescales. The combination of precise masses and radii, together with transmission/emission metrics, points to the TOI-282 system as a promising target for atmospheric characterization with JWST and ELT, particularly for c and d, and possibly b, should their atmospheres persist. The study thus advances understanding of long-period sub-Neptunes, their interior compositions, and their dynamical pathways toward compact multi-planet architectures around bright stars.

Abstract

TOI-282 is a bright (V=9.38) F8 main-sequence star known to host three transiting long-period ($P_b$=22.9 d, $P_c$=56.0 d, and $P_d$=84.3 d) small ($R_p\approx$ 2-4 $R_{\oplus}$) planets. The orbital period ratio of the two outermost planets, namely TOI-282 c and d, is close to the 3:2 commensurability, suggesting that the planets might be trapped in a mean motion resonance. We combined space-borne photometry from the TESS telescope with high-precision HARPS and ESPRESSO Doppler measurements to refine orbital parameters, measure the planetary masses, and investigate the architecture and evolution of the system. We performed a Markov chain Monte Carlo joint analysis of the transit light curves and radial velocity time series, and carried out a dynamical analysis to model transit timing variations and Doppler measurements along with N-body integration. In agreement with previous results, we found that TOI-282 b, c, and d have radii of $R_b=2.69 \pm 0.23 \ R_{\oplus}$, $R_c=4.13^{+0.16}_{-0.14} \ R_{\oplus}$, and $R_d=3.11 \pm 0.15 \ R_{\oplus}$, respectively. We measured planetary masses of $M_b=6.2\pm1.6 \ M_{\oplus}$, $M_c=9.2\pm2.0 \ M_{\oplus}$, and $M_d=5.8^{+0.9}_{-1.1} \ M_{\oplus}$, which imply mean densities of $ρ_b=1.8^{+0.7}_{-0.6} \ \text{g cm}^{-3}$, $ρ_c=0.7 \pm 0.2 \ \text{g cm}^{-3}$, and $ρ_d=1.1^{+0.3}_{-0.2} \ \text{g cm}^{-3}$, respectively. The three planets may be water worlds, making TOI-282 an interesting system for future atmospheric follow-up observations with JWST and ELT.

Figures (13)

• Figure 1: From top to bottom: folded TESS transit light curves of TOI-282 b, c, and d. The photometric data points are plotted with light-blue circles. The dark blue circles mark the 30-minute binned data points. The best-fitting models are over-plotted with red curves. The lower panels display the residuals of the models.
• Figure 2: From top to bottom: folded RV stellar signals induced by TOI-282 b, c, and d, and best-fitting RV models (red curve). The data includes 37 HARPS spectra (blue circular points) and 8 + 36 ESPRESSO spectra (cyan triangular and green square points, respectively). The lower panels show the residuals to the models.
• Figure 3: Observed minus calculated (O$-$C) TTV diagram for TOI-282 b, showing TTV as a function of time. The shaded regions represent the $1, 2,~\mathrm{and} ~3\sigma$ confidence interval derived from the posterior distributions of the model parameters of our dynamical analysis.
• Figure 4: Observed minus calculated (O$-$C) TTV diagrams for TOI-282 c (upper panel) and TOI-282 d (lower panel), showing TTV as a function of time. The shaded regions represent the $1, 2,~\mathrm{and} ~3\sigma$ confidence interval derived from the posterior distributions of the model parameters of our dynamical analysis. Both planets exhibit anti-correlated TTV exceeding the transit time uncertainties.
• Figure 5: Simulated evolution of the orbital parameters over $10^5$ years. Figure b shows a zoom over the first $10^4$ years on the eccentricity and argument of peri-centre plots. The colours blue, red, cyan refer respectively to planets TOI-282 b, TOI-282 c, TOI-282 d.