We report on the discovery and characterisation 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 56.0067±0.0003 days and 84.2597+0.0010−0.0008 days, 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 60mins. These two planets were first identified by TESS, and we identified a third planet in the TESS photometry with a period of 22.8911±0.0004 days. 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 ∼9h transit of HD 28109 c from Antarctica. The radii of the three planets are Rb=2.199+0.098−0.10 R⊕, Rc=4.23±0.11 R⊕ and Rd=3.25±0.11 R⊕; we characterise their masses using TTVs and precise radial velocities from ESPRESSO and HARPS, and find them to be Mb=18.5+9.1−7.6 M⊕, Mc=7.9+4.2−3.0 M⊕ and Md=5.7+2.7−2.1 M⊕, making planet b a dense, massive planet while c and d are both under-dense. We also demonstrate that the two outer planets are ripe for atmospheric characterisation using transmission spectroscopy, especially given their position in the CVZ of JWST. 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.
19 pages, 14 figures
- planets and satellites: detection
- planets and satellites: dynamical evolution and stability
- planets and satellites: fundamental parameters