High Five From ASTEP: Three Validated Planets and Two Eclipsing Binaries in a Diverse Set of Long-Period Candidates

TL;DR

This work targets five long-period exoplanet candidates initially identified by TESS, leveraging Antarctic-ground-based ASTEP alongside the TFOP network to overcome sparse transit sampling. By integrating speckle imaging, reconnaissance spectroscopy, SED fitting, multi-band photometry, and RV data, the authors apply a Bayesian validation with TRICERATOPS and perform a joint allesfitter fitting to derive robust planetary parameters. They validate three planets—TOI-4507 b ($P\approx 104.6$ d, $R_p\approx 8.13\,R_\oplus$), TOI-707 b ($P\approx 52.8$ d, $R_p\approx 2.4\,R_\oplus$), and TOI-3457 b ($P\approx 32.6$ d, $R_p\approx 10.64\,R_\oplus$)—while identifying TOI-2404 and TOI-4404 as eclipsing binaries, illustrating the power of long-term, ground-based follow-up for long-period signals. The results populate the sparsely sampled warm-giant and warm-sub-Neptune regimes, offering crucial empirical constraints on planetary formation, migration, and atmospheric retention in less-irradiated environments. The study also demonstrates ASTEP’s capability as a high-value follow-up instrument for characterizing long-period exoplanets and identifying false positives in challenging parameter spaces.

Abstract

We present the analysis of five long-period TESS Objects of Interest (TOIs), each with orbital periods exceeding one month. Initially identified by the Transiting Exoplanet Survey Satellite (TESS), we extensively monitored these targets with the Antarctic Search for Transiting Exoplanets (ASTEP), supported by other facilities in the TESS Follow-Up (TFOP) network. These targets occupy a relatively underexplored region of the period-radius parameter space, offering valuable primordial probes for planetary formation and migration as warm planets better maintain their evolutionary fingerprints. To characterise these systems, we leverage high-resolution speckle imaging to search for nearby stellar companions, and refine stellar parameters using both reconnaissance spectroscopy and spectral energy distribution (SED) fitting. We combine TESS photometry with high-precision ground-based observations from ASTEP, and when available, include additional photometry and radial velocity data. We apply statistical validation to assess the planetary nature of each candidate and use to jointly model the photometric and spectroscopic datasets with Markov Chain Monte Carlo (MCMC) sampling to derive robust posterior distributions. With this, we validate the planetary nature of three TOIs, including the two warm Saturns TOI-4507 b (104 d) and TOI-3457 b (32.6 d), as well as the warm sub-Neptune TOI-707 b (52.8 d). The remaining two candidates are identified as eclipsing binaries, namely TOI-2404 and TOI-4404. These results help populate the sparse regime of warm planets, which serve as key tracers of planetary evolution, and demonstrate ASTEP's effectiveness as a ground-based follow-up instrument for long-period systems.

Figures (9)

• Figure 1: TESS lightcurves of our five targets span from 2018 to 2025. Each panel shows the normalised flux over time (gray) and highlights the transit events in colour. Section \ref{['subsubsec:TESS_photometry']} and Table \ref{['tab:tess_obs']} provide further details.
• Figure 2: CORALIE CCFs of TOI-2404 show no evidence of multiple star, presenting a puzzling contrast with its photometric features showing a potential planetary transit alongside clear eclipsing binary signatures.
• Figure 3: CORALIE CCF of TOI-4404 unveil it to be a double-lined spectroscopic binary.
• Figure 4: Hertzsprung–Russell diagram showing all targets cluster in the same region, close to the main-sequence turn-off. The x-axis represents the Color Index (B–V), the y-axis the absolute magnitude, and colour maps the population density.
• Figure 5: The three validated planets and two eclipsing binaries (coloured symbols) put into context of all known planets (gray points) drawn from the NASA Exoplanet Archive. The gray markers represent the previously reported values prior to our analysis, while the arrows indicate lower limits on the stellar radii in the case of eclipsing binaries. Note the targets' intriguing positions in the radius-period parameter space relative to the most densely populated regions CastroGonzalez_2024.