TESS Investigation -- Demographics of Young Exoplanets (TI-DYE) II: a second giant planet in the 17-Myr system HIP 67522

TL;DR

This paper reports the discovery of a second giant planet in the 17-Myr system HIP 67522 using TESS data, confirmed with ground-based photometry, revealing insights into young exoplanet demographics and activity.

Contribution

It presents the confirmed detection of a second giant planet in HIP 67522, demonstrating the importance of multi-wavelength follow-up and analysis of stellar activity in young exoplanet systems.

Findings

Discovery of HIP 67522 c with a 14.33-day period

Planet c is near a 2:1 resonance with planet b

Transit distortions suggest stellar activity effects

Abstract

The youngest ($<$50 Myr) planets are vital to understand planet formation and early evolution. The 17 Myr system HIP 67522 is already known to host a giant ($\simeq$10$R_\oplus$) planet on a tight orbit. In the discovery paper, Rizzuto et al. 2020 reported a tentative single transit detection of an additional planet in the system using TESS. Here, we report the discovery of HIP 67522 c which matches with that single transit event. We confirm the signal with ground-based multi-wavelength photometry from Sinistro and MuSCAT4. At a period of 14.33 days, planet c is close to a 2:1 mean motion resonance with b (6.96 days or 2.06:1). The light curve shows distortions during many of the transits, which are consistent with spot crossing events and/or flares. Fewer stellar activity events are seen in the transits of planet b, suggesting that planet c is crossing a more active latitude. Such distortions, combined with systematics in the TESS light curve extraction, likely explain why planet c was previously missed.