A compact multi-planet system transiting HIP 29442 (TOI-469) discovered by TESS and ESPRESSO. Radial velocities lead to the detection of transits with low signal-to-noise ratio

Abstract

We followed-up with ESPRESSO the K0V star HIP 29442 (TOI-469), already known to host a validated sub-Neptune companion TOI-469.01. We aim to verify the planetary nature of TOI-469.01. We modelled radial velocity and photometric time series to measure the dynamical mass, radius, and ephemeris, and to characterise the internal structure and composition of TOI-469.01. We confirmed the planetary nature of TOI-469.01. Thanks to ESPRESSO we discovered two additional close-in companions. We also detected their low signal-to-noise transit signals in the TESS light curve. HIP 29442 is a compact multi-planet system, and the three planets have orbital periods $P_{\mathrm{orb},\mathrm{b}}=13.63083\pm 0.00003$, $P_{\mathrm{orb},\mathrm{c}}=3.53796\pm 0.00003$, and $P_{\mathrm{orb},\mathrm{d}}=6.42975_{-0.00010}^{+0.00009}$ days, and we measured their masses with high precision: $m_{\mathrm{p},\mathrm{b}}=9.6\pm 0.8M_{\oplus }$, $m_{\mathrm{p},\mathrm{c}}=4.5\pm 0.3M_{\oplus }$, and $m_{\mathrm{p},\mathrm{d}}=5.1\pm 0.4M_{\oplus }$. We measured radii and bulk densities of all the planets (the 3$\ddot{I}ƒ$ confidence intervals are shown in parenthesis): $R_{\mathrm{p},\mathrm{b}}=3.48_{-0.08(-0.28)}^{+0.07(+0.19)}{R}_{\oplus }$ and $\ddot{I}{}_{\mathrm{p},\mathrm{b}}=1.3\pm 0.2(0.3)gc{m}^{-3}$; $R_{\mathrm{p},\mathrm{c}}=1.58_{-0.11(-0.34)}^{+0.10(+0.30)}{R}_{\oplus }$ and $\ddot{I}{}_{\mathrm{p},\mathrm{c}}=6.3_{-1.3(-2.7)}^{+1.7(+6.0)}gc{m}^{-3}$; $R_{\mathrm{p},\mathrm{d}}=1.37\pm 0.11_{(-0.43)}^{(+0.32)}{R}_{\oplus }$ and $\ddot{I}{}_{\mathrm{p},\mathrm{d}}=11.0_{-2.4(-6.3)}^{+3.4(+21.0)}gc{m}^{-3}$. We used the more conservative 3$\ddot{I}ƒ$ confidence intervals for the radii as input to the interior structure modelling. We find that HIP 29442 $b$ appears as a typical sub-Neptune, likely surrounded by a gas layer of pure H-He with a mass of $0.27_{-0.17}^{+0.24}{M}_{\oplus }$ and a thickness of $1.4\pm 0.5R_{\oplus }$. For the innermost companions HIP 29442 $c$ HIP 29442 $d$, the model supports an Earth-like composition.

Accepted for publication on A&A