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

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