TOI-5126: A hot super-Neptune and warm Neptune pair discovered by $\text{TESS}$ and $\text{CHEOPS}$

Abstract

We present the confirmation of a hot super-Neptune with an exterior Neptune companion orbiting a bright (V = 10.1 mag) F-dwarf identified by the $\text{Transiting Exoplanet Survey Satellite}$ ($\text{TESS}$). The two planets, observed in sectors 45, 46 and 48 of the $\text{TESS}$ extended mission, are $4.74_{-0.14}^{+0.16}$ $R_{\oplus }$ and $3.86_{-0.16}^{+0.17}$ $R_{\oplus }$ with $5.4588385_{-0.0000072}^{+0.0000070}$ d and $17.8999_{-0.0013}^{+0.0018}$ d orbital periods, respectively. We also obtained precise space based photometric follow-up of the system with ESAs $\text{CHaracterising ExOplanets Satellite}$ ($\text{CHEOPS}$) to constrain the radius and ephemeris of TOI-5126 b. TOI 5126 b is located in the "hot Neptune Desert" and is an ideal candidate for follow-up transmission spectroscopy due to its high predicted equilibrium temperature ($T_{eq}=1442_{-40}^{+46}$ K) implying a cloud-free atmosphere. TOI-5126 c is a warm Neptune ($T_{eq}=971_{-27}^{+31}$ K) also suitable for follow-up. Tentative transit timing variations (TTVs) have also been identified in analysis, suggesting the presence of at least one additional planet, however this signal may be caused by spot-crossing events, necessitating further precise photometric follow-up to confirm these signals.

Accepted in MNRAS, 18 pages, 14 figures