Recovering Long-Period TESS planets with Cheops: The TOI-2076 system

TESS has proved revolutionary for the detection of transiting exoplanets in many key areas, including vast increases in the number of confirmed small planets around both bright and young stars. However, compared to its predecessors, the impact of TESS observations on the long-period regime remains limited due to short continuous observations (27 d) on much of the sky. Here we present the first of a series of CHEOPS observations specifically designed to fill this gap by recovering the transits of long-period TESS planets & candidates.

TOI-2076 is a validated 3-planet system of sub-Neptunes orbiting a bright (G = 8.9 mag), young (340±80 Myr) K-type star (Hedges et al 2021). The orbits of the two outer planets were unconstrained as only two non-consecutive transits were seen in TESS photometry. This left 11 & 7 possible period aliases for each, which we targeted for precise photometry using both Cheops and ground-based instruments.

CHEOPS observations revealed a clear detection for TOI-2076 c at P=21.0 d, and ruled out three of the most likely period aliases for TOI-2076 d. Ground-based photometry further ruled out remaining aliases, and confirmed the P=35.1 d alias. These observations also improved the radius precision of all three sub-Neptunes to 2.52±0.04, 3.50±0.04 and 3.23±-0.06 R_⊕ and revealed the presence of large-amplitude (~30 min) TTVs for the first time which may enable precise mass determination in the near future despite an active star not ideal for PRV analysis. Their inflated radii, likely due to extended H-He atmospheres, combined with low insolation makes all three planets excellent candidates for future comparative transmission spectroscopy with e.g. JWST.