A Mini-Neptune from TESS and CHEOPS Around the 120 Myr Old AB Dor member HIP 94235

TL;DR

This paper reports the discovery of a mini-Neptune orbiting a young star in the AB Doradus group, providing valuable insights into planetary evolution in early stellar systems through TESS and CHEOPS observations.

Contribution

It presents the first confirmed mini-Neptune around a young star in the AB Doradus group, combining TESS and CHEOPS data with long-term astrometric observations to characterize the system.

Findings

Discovered a 3 Earth-radius mini-Neptune around HIP 94235.

Confirmed the transit signal with CHEOPS, ensuring precise ephemeris.

HIP 94235 is part of a tight binary system with a 50 AU separation.

Abstract

The TESS mission has enabled discoveries of the brightest transiting planet systems around young stars. These systems are the benchmarks for testing theories of planetary evolution. We report the discovery of a mini-Neptune transiting a bright star in the AB Doradus moving group. HIP 94235 (TOI-4399, TIC 464646604) is a Vmag=8.31 G-dwarf hosting a 3.00 -0.28/+0.32 Rearth mini-Neptune in a 7.7 day period orbit. HIP 94235 is part of the AB Doradus moving group, one of the youngest and closest associations. Due to its youth, the host star exhibits significant photometric spot modulation, lithium absorption, and X-ray emission. Three 0.06% transits were observed during Sector-27 of the TESS Extended Mission, though these transit signals are dwarfed by the 2% peak-to-peak photometric variability exhibited by the host star. Follow-up observations with CHEOPS confirmed the transit signal and prevented the erosion of the transit ephemeris. HIP 94235 is part of a 50 AU G-M binary system. We make use of diffraction limited observations spanning 11 years, and astrometric accelerations from Hipparchos and Gaia, to constrain the orbit of HIP 94235 B. HIP 94235 is one of the tightest stellar binaries to host an inner planet. As part of a growing sample of bright, young planet systems, HIP 94235 b is ideal for follow-up transit observations, such as those that investigate the evaporative processes driven by high-energy radiation that may sculpt the valleys and deserts in the Neptune population.