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

Abstract

The Transiting Exoplanet Survey Satellite (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 V mag = 8.31 G-dwarf hosting a 3.00 − 0.28 + 0.32 R ⊕ 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 the Characterising Exoplanet Satellite 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 yr, and astrometric accelerations from Hipparcos 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.

Dynamical Architectures of S-type Transiting Planets in Binaries. I. Target Selection Using Hipparcos and Gaia Proper Motion Anomalies*

“…Another close binary HIP 94235 (a ∼ 50 au) is consistent with the possibility that the primary hosts a transiting mini-Neptune. The companion HIP94235B exhibits an inclination of around 68° ( Zhou et al 2022), which suggests a minimum misalignment of 22°between the companions and the transiting planet. Given that HIP 94235 is part of a young comoving group (∼120 Myr, Zhou et al 2022), it is possible that the realignment between the companion and the disk is still ongoing.…”

“…The companion HIP94235B exhibits an inclination of around 68° ( Zhou et al 2022), which suggests a minimum misalignment of 22°between the companions and the transiting planet. Given that HIP 94235 is part of a young comoving group (∼120 Myr, Zhou et al 2022), it is possible that the realignment between the companion and the disk is still ongoing. Alternatively, it is also possible that the LTT 1445 BC was initially aligned with the primary disk from the onset.…”

“…Both studies suggest that Kepler-444 BC may have truncated the protoplanetary disk of the primary, resulting in the small sizes of the system's five planets. Zhou et al (2022) characterize the 3D orbit of an M dwarf companion to TOI-4399 (HIP 94235), which hosts a mini-Neptune with an orbital period of 7.1 days. Their results show that the companion has a semimajor axis of ∼60 au and an inclination of ∼67°.8, indicating a modest misalignment between the companions and the transiting planet.…”

The Occurrence of Small, Short-period Planets Younger than 200 Myr with TESS

“…TESS has already detected and characterized tens of young planetary systems (e.g., Newton et al 2019;Battley et al 2020;Plavchan et al 2020;Zhou et al 2022;Wood et al 2023), providing initial insights into the differences between young planetary systems and their mature counterparts. Initial investigations into the population of young planets observed by TESS span a wider age range (Fernandes et al 2023), targeting clusters which already have confirmed planets (Fernandes et al 2022), and explored methods of light-curve extraction that best preserve transit signals in the scenario of heightened photometric variability from stellar activity (Nardiello et al 2020).…”