Mass estimates of the young TOI-451 transiting planets: Multidimensional Gaussian Process on stellar spectroscopic and photometric signals

TL;DR

This study leverages a multidimensional Gaussian Process framework to disentangle stellar activity from planetary signals in the young TOI-451 system, combining ESPRESSO RVs with contemporaneous NGTS photometry and TESS transits. The final joint model yields 2-sigma masses for TOI-451 b and d (Mb ≈ 4.7 ± 2.2 M⊕, Md ≈ 10.2 ± 4.6 M⊕) and an upper limit for TOI-451 c (Mc < 11.5 M⊕), with circular orbits preferred. The inferred radii and masses suggest that c and d host hydrogen-rich envelopes, while b could be a rocky planet with a thin envelope or a water world; TOI-451 b sits near the radius valley, offering a crucial test for atmospheric mass loss theories. The findings position TOI-451 as a key laboratory for studying early planetary evolution, mass-radius relationships, and atmospheric escape, with imminent JWST and PLATO follow-ups to refine atmospheric characteristics and formation scenarios.

Abstract

The young TOI-451 planetary system, aged 125 Myr, provides a unique opportunity to test theories of planetary internal structures and atmospheric mass loss through examination of its three transiting planets. We present an exhaustive photometric and spectroscopic follow-up to determine the orbital and physical properties of the system. We perform multidimensional Gaussian Process regression with the code pyaneti on spectroscopic time-series and NGTS/LCO light curves to disentangle the stellar and planetary signal in ESPRESSO radial velocities. We show how contemporaneous photometry serves as an activity indicator to inform RV modelling within a multidimensional Gaussian Processes framework. We argue that this can be exploited when spectroscopic observations are adversely affected by low signal-to-noise and/or poor sampling. We estimate the Doppler semi-amplitudes of Kb = 2.6(+1.1,-1.2) m/s, Kc = 1.2(+1.0,-0.8) m/s and Kd = 2.7 +/- 1.2 m/s. This translates into 2-sigma mass estimates for TOI-451 b and d of Mb = 4.7(+2.1,-2.2) Earth masses and Md = 10.2(+4.6,-4.5) Earth masses, as well as a mass upper limit for TOI-451 c of Mc < 11.5 Earth masses. The derived planetary properties suggest that planets c and d contain significant hydrogen-rich envelopes. The inferred parameters of TOI-451 b are consistent with either a rocky world that still retains a small hydrogen envelope or a water world. These insights make the TOI-451 system an ideal laboratory for future follow-up studies aimed at measuring atmospheric compositions, detecting atmospheric mass-loss signatures, and further exploring planetary formation and evolution processes.

Figures (10)

• Figure 1: Mean-subtracted RV time-series as obtained with YARARA (vermilion circles), SERVAL (red diamonds), and DRS (teal green hexagons).
• Figure 2: Detrended light curves phase-folded at the period of each planet (individual data points in transparent grey, phase-binned data in solid colour) together with the best-fit transit model (black line). Residuals are shown in the lower inset. The $x$- and $y$-axes in each panel are shown with the same range to facilitate signal comparisons.
• Figure 3: GLS periodograms of the stellar time-series. The horizontal dashed line indicates the 1% False Alarm Probability (FAP). The vertical red dashed lines represent the fundamental, first and second harmonics of the rotation period of the star. stellar-related signals, while the vertical black dotted lines mark the orbital periods of TOI-451 b, c and d.
• Figure 4: 1D GP regression on each of the contemporaneous TOI-451 time-series. Measurements are shown as black circles with error bars, with semitransparent extension accounting for the inferred jitter. Solid coloured lines show the posterior predictive mean and 3-$\sigma$ credible interval of the inferred GP model.
• Figure 5: Posterior distributions for $P_{\rm GP}$ (left), $\lambda_{\rm e}$ (centre), and $\lambda_{\rm p}$ (right). Different time-series results are differentiated with a label and offset.