Young planets with mass measurements are particularly valuable in studying atmospheric mass-loss processes, but these planets are rare and their masses difficult to measure due to stellar activity. We report the discovery of a planetary system around TOI-6109, a young, 75 Myr-old Sun-like star in the Alpha Persei cluster. It hosts at least two transiting Neptune-like planets. Using three TESS sectors, 30 CHEOPS orbits, and photometric follow-up observations from the ground, we confirm the signals of the two planets. TOI-6109 b has an orbital period of P=5.6904−0.0004+0.0004 days and a radius of R=4.87−0.12+0.16 R⊕. The outer planet, TOI-6109 c has an orbital period of P=8.5388−0.0005+0.0006 days and a radius of R=4.83−0.06+0.07 R⊕. These planets orbit just outside a 3:2 mean motion resonance. The near-resonant configuration presents the opportunity to measure the planet's mass via TTV measurements and to bypass difficult RV measurements. Measuring the masses of the planets in this system will allow us to test theoretical models of atmospheric mass loss.
@article{arxiv.2509.15313,
title = {THYME XIII: Two young Neptunes orbiting a 75-Myr star in the Alpha Persei Cluster},
author = {Anne Dattilo and Andrew M. Vanderburg and Madyson G. Barber and Andrew W. Mann and Ronan Kerr and Adam L. Kraus and Joseph R. Livesey and Cristilyn Watkins and Karen A. Collins and Juliana García-Mejía and Patrick Tamburo and Juliette Becker and Annelies Mortier and Thomas Wilson and Nicholas Scarsdale and Emily A. Gilbert and Alex S. Polanski and Steve B. Howell and Ian Crossfield and Allyson Bieryla and David R. Ciardi and Thomas Barclay and David Charbonneau and David W. Latham and Joseph M. Akana Murphy and Elisabeth Newton and Bob Massey and Richard P. Schwarz and Chris Stockdale and Francis P. Wilkin and Roberto Zambelli},
journal= {arXiv preprint arXiv:2509.15313},
year = {2025}
}