TOI-283 b: A transiting mini-Neptune in a 17.6-day orbit discovered with TESS and ESPRESSO

TL;DR

TOI-283 b is a transiting mini-Neptune around a bright K-dwarf, characterized by $R_p = 2.34 \pm 0.09\,R_\oplus$, $M_p = 6.54 \pm 2.04\,M_\oplus$, and $P = 17.61745 \pm 0.00002$ days, inferred from a joint analysis of 36-sector TESS photometry and 84 ESPRESSO RVs with Gaussian-process noise modeling. The host star TOI-283 is well-constrained: $T_{\rm eff} = 5213 \pm 70$ K, $M_* = 0.80 \pm 0.01\,M_\odot$, $R_* = 0.85 \pm 0.03\,R_\odot$, at $d = 82.4$ pc; the measured bulk density $ρ_p = 2.81 \pm 0.93$ g cm$^{-3}$ places TOI-283 b in a regime where models predict $H_2O$- and $H/He$-rich envelopes. A long-term RV trend near $P_{\rm trend} ≈ 1356$ days is observed and likely tied to stellar activity, with the possibility of an outer companion. Interior-structure inferences using ExoMDN suggest a water-rich composition and a gas envelope, though the high equilibrium temperature implies water is predominantly in vapor form; transmission spectroscopy with JWST could probe the atmosphere, albeit with challenges due to the modest TSM of 35. Overall, TOI-283 b enriches the population of long-period sub-Neptunes and provides a valuable data point for understanding the mass-radius valley and envelope compositions around K-type hosts.

Abstract

Super-Earths and mini-Neptunes are missing from our Solar System, yet they appear to be the most abundant planetary types in our Galaxy. A detailed characterization of key planets within this population is important for understanding the formation mechanisms of rocky and gas giant planets and the diversity of planetary interior structures. In 2019, NASA's TESS satellite found a transiting planet candidate in a 17.6-day orbit around the star TOI-283. We started radial velocity (RV) follow-up observations with ESPRESSO to obtain a mass measurement. Mass and radius are measurements critical for planetary classification and internal composition modeling. We used ESPRESSO spectra to derive the stellar parameters of the planet candidate host star TOI-283. We then performed a joint analysis of the photometric and RV data of this star, using Gaussian processes to model the systematic noise present in both datasets. We find that the host is a bright K-type star ($d = 82.4$ pc, $\mathrm{T}_\mathrm{eff} = 5213 \pm 70$ K, $V = 10.4$ mag) with a mass and radius of $\mathrm{M}_\star = 0.80 \pm 0.01\; \mathrm{M}_\odot$ and $\mathrm{R}_\star = 0.85 \pm 0.03\; \mathrm{R}_\odot$. The planet has an orbital period of $P = 17.617$ days, a size of $\mathrm{R}_\mathrm{p} = 2.34 \pm 0.09\; \mathrm{R}_\oplus$, and a mass of $\mathrm{M}_\mathrm{p} = 6.54 \pm 2.04\; \mathrm{M}_\oplus$. With an equilibrium temperature of $\sim$600 K and a bulk density of $ρ_\mathrm{p} = 2.81 \pm 0.93$ g cm$^{-3}$, this planet is positioned in the mass-radius diagram where planetary models predict H$_2$O- and H/He-rich envelopes. The ESPRESSO RV data also reveal a long-term trend that is probably related to the star's activity cycle. Further RV observations are required to confirm whether this signal originates from stellar activity or another planetary body in the system.

Figures (20)

• Figure 1: High-resolution images of TOI-283. Left panel: SOAR/HRCam observation of TOI-283 taken on 18 February 2019. No nearby stars were detected within 3$\arcsec$. Inset: Thumbnail image of TOI-283. Right panel: Gemini/Zorro high-resolution image of TOI-283 taken on 7 April 2024. TOI-283 is a single star to a contrast limit of 7 mag at 832 nm. Inset: $1.2\arcsec \times 1.2 \arcsec$ thumbnail image of TOI-283.
• Figure 2: TOI-283 b RV measurements taken with ESPRESSO. Panel a: RV time series and best-fitting model. The best-fitting model was computed using the median values of the posterior distribution of the fit parameters; the shaded area represents the 1$\sigma$ uncertainty limits of the best-fitting model. Panel b: Residuals of the fit after subtracting the single-planet Keplerian model. The uncertainties shown here include the RV jitter values (added in quadrature) for each set.
• Figure 3: TLS analysis of TESS light curves. Top panel: Signal detection efficiency (SDE) versus period. The peak with the maximum SDE corresponds to $P = 17.61731 \pm 0.00184$ days. Bottom panel: TESS light curve folded to the maximum SDE period. The blue line corresponds to the TLS transit model.
• Figure 4: Stellar rotation from activity indices. Top panel: Histograms of the posterior distribution of the fit rotation period for the ESPRESSO activity indices. Bottom panel: Kernel density estimation of the posterior distributions.
• Figure 5: GLS periodograms of the ESPRESSO RVs, activity indices, and the observational window function. The RV and activity index values were median-subtracted for each dataset before (E18) and after (E19) the June–July 2019 intervention. Horizontal lines indicate the FAP levels at 10% (dash-dotted green line) and 1% (dashed orange line). The vertical dotted red line marks the orbital period of TOI-283 b ($P = 17.61745$ days), while the vertical dash-dotted gray line indicates the period of the long-term signal ($P = 1356$ days; see Sect.\ref{['Sec:JointFit']}).