Giant Outer Transiting Exoplanet Mass (GOTEM) Survey.VII. TOI-6041: a multi-planet system including a warm Neptune exhibiting strong TTVs

TL;DR

TOI-6041 is a bright G7 host star hosting at least two planets, including a transiting warm Neptune (TOI-6041 b) with $P_b = $26.04945$^{+0.00033}_{-0.00034}$ d and $R_b = 4.55^{+0.18}_{-0.17}\,R_igoplus$, and a non-transiting outer companion TOI-6041 c with $P_c = 88.0^{+1.6}_{-1.3}$ d and $M_c\sin i_c = 0.245^{+0.029}_{-0.028}\,M_J$. The inner planet shows significant transit timing variations with a peak-to-peak amplitude of about $57.2$ minutes, which, along with RV data, allows a 3σ upper mass limit of $28.9\,M_igoplus$ for b and a 1σ true-mass limit of $m_c \lesssim 0.80\,M_J$ for c under a dynamical stability framework. A TTV-based interpretation suggests perturbations from planet c could explain the observed variations if $e_c \sim 0.3$, but degeneracies in orbital angles and the potential for a third planet between or near b and c remain viable explanations. The TOI-6041 system lies in the Neptunian Savanna of the radius–period diagram, making it a compelling target for atmospheric characterization (Transmission Spectroscopy Metric $\text{TSM} \gtrsim 59$ for b) and spin–orbit studies to probe formation and migration histories, with future RV and photometric observations expected to refine the architecture and dynamics of this intriguing planetary system.

Abstract

We present the characterization of the TOI-6041 system, a bright ($V = 9.84 \pm 0.03$) G7-type star hosting at least two planets. The inner planet, TOI-6041b, is a warm Neptune with a radius of $4.55^{+0.18}_{-0.17}\,R_\oplus$, initially identified as a single-transit event in \textit{TESS} photometry. Subsequent observations with \textit{TESS} and \textit{CHEOPS} revealed additional transits, enabling the determination of its $26.04945^{+0.00033}_{-0.00034}$~d orbital period and the detection of significant transit timing variations (TTVs), exhibiting a peak-to-peak amplitude of about 1~hour. Radial velocity (RV) measurements obtained with the APF spectrographs allow us to place a $3σ$ upper mass limit of $28.9\,M_\oplus$ on TOI-6041b. In addition, the RV data reveal a second companion, TOI-6041c, on an 88~d orbit, with a minimum mass of $0.25\,M_{\mathrm{Jup}}$. A preliminary TTV analysis suggests that the observed variations could be caused by gravitational perturbations from planet c; however, reproducing the observed amplitudes requires a relatively high eccentricity of about 0.3 for planet c. Our dynamical stability analysis indicates that such a configuration is dynamically viable and places a $1σ$ upper limit on the mass of TOI-6041c at $0.8\,M_{\mathrm{Jup}}$. An alternative is the presence of a third, low-mass planet located between planets b and c, or on an inner orbit relative to planet b -- particularly near a mean-motion resonance with planet b -- which could account for the observed variations. These findings remain tentative, and further RV and photometric observations are essential to better constrain the mass of planet b and to refine the TTV modeling, thereby improving our understanding of the system's dynamical architecture.

Figures (18)

• Figure 1: Spectral energy distribution of TOI-6041. Red symbols represent the observed photometric measurements, where the horizontal bars represent the effective width of the pass-band. Blue symbols are the model fluxes from the best-fit PHOENIX atmosphere model (black). The absolute flux-calibrated Gaia spectrum is shown as a grey swathe in the inset figure.
• Figure 2: Periodograms of RV and S-index derived from APF data for TOI-6041. From top to bottom: the APF RVs, the residuals of the RVs after a Keplerian fit to the 88 d signal, and the S-index activity indicator. The vertical pink line marks the planet candidate with a period of 88 d, which shows no corresponding signal in the S-index periodogram. The horizontal lines indicate false alarm probability 2008MNRAS.385.1279B thresholds of 0.1%, 1%, and 10%, respectively from top to bottom.
• Figure 3: APF RV measurements of TOI-6041 (top panel); phase-folded RVs for TOI-6041c (second panel) and TOI-6041b (third panel); and the phase-folded CHEOPS and TESS light curves for TOI-6041b (fourth and fifth panels). The red lines show the best-fit models derived using EXOFASTv2. Residuals are displayed at the bottom of each panel.
• Figure 4: Stability analysis of the TOI-6041 system. The phase space of the system is explored by varying the orbital period $P_i$ and eccentricity $e_i$ of each planet independently. For each initial condition (Table \ref{['toi6041_result_exofast']}), the system is integrated over 10 kyr, and a stability criterion is derived with the frequency analysis of the mean longitude. The chaotic diffusion is measured by the variation in the frequencies. The color scale corresponds to the logarithmic variation in the stability index used in Correia2010. The red zone corresponds to highly unstable orbits, while the dark blue region can be assumed to be stable on a billion-year timescale. White dashed lines indicate the nominal orbital periods reported in Table \ref{['toi6041_result_exofast']}.
• Figure 5: Transit-timing variations of the TOI-6041b planet. We adopt $e_\mathrm{c} = 0.3$ and two values of the argument of pericenter: $\omega_\mathrm{c} = 0^\circ$ (blue) and $\omega_\mathrm{c} = 50^\circ$ (red), while all other parameters are fixed at their nominal values from Table \ref{['toi6041_result_exofast']}. The dots correspond to the observed TTVs. While the amplitudes are consistent with observations, the signal shape depends strongly on the orbital phase angles.