HD 28109 hosts a trio of transiting Neptunian planets including a near-resonant pair, confirmed by ASTEP from Antarctica

ABSTRACT We report on the discovery and characterization of three planets orbiting F8 star HD 28109, which sits comfortably in ${TESS}$’s continuous viewing zone. The two outer have periods $\rm 56.0067 \pm 0.0003~d$ 84.2597_{-0.0008}^{+0.0010}~d$, implies a period ratio very close to that first-order 3:2 mean motion resonance, exciting transit timing variations (TTVs) up 60\, min$. These were first identified by ${TESS}$, we third planet ${TESS}$photometry with 22.8911 0.0004~d$. confirm planetary nature all candidates using ground-based photometry from Hazelwood, ${ASTEP}$, LCO, including full detection \sim 9\, h$ 28109 c Antarctica. radii are ${\it R}_b=2.199_{-0.10}^{+0.098} ~{\rm R}_{\oplus }$, R}_c=4.23\pm 0.11~ {\rm R}_d=3.25\pm 0.11 }$; characterize their masses TTVs precise radial velocities ESPRESSO HARPS, find them be M}_b=18.5_{-7.6}^{+9.1}~M_{\oplus M}_c=7.9_{-3.0}^{+4.2}~{\rm M}_{\oplus M}_d=5.7_{-2.1}^{+2.7}~{\rm making b dense, massive while d both underdense. also demonstrate ripe for atmospheric transmission spectroscopy, especially given position CVZ James Webb Space Telescope. data obtained date consistent resonant (librating) non-resonant (circulating) solutions; additional observations will show whether pair is actually locked resonance or just near-resonant.