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October 5, 2025Open Access

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

ERErika ReaEuropean Space Agency MGMaximilian N. GüntherEuropean Space Research and Technology Centre GDGeorge DransfieldUniversity of Oxford

Key Points

Three long-period TESS Objects of Interest were validated as warm planets, enhancing the understanding of planetary evolution.
Including TOI-4507 b, a warm Saturn, and TOI-707 b, a warm sub-Neptune, improves insights into planetary formation.
Eclipsing binaries TOI-2404 and TOI-4404 were identified, illustrating the diverse nature of the observed candidates.
The work employs high-resolution speckle imaging and MCMC sampling to robustly derive stellar and planetary parameters.

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.

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