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

Georgina Dransfield, Amaury H. M. J. Triaud, Tristan Guillot, Djamel Mekarnia, David Nesvorný, Nicolas Crouzet, Lyu Abe, Karim Agabi, Marco Buttu, Juan Cabrera, Davide Gandolfi, Maximilian N. Günther, Florian Rodler, François-Xavier Schmider, Philippe Stee, Olga Suarez, Karen A. Collins, Martín Dévora-Pajares, Steve B. Howell, Elisabeth C. MatthewsMatthew R. Standing, Keivan G. Stassun, Chris Stockdale, Samuel N. Quinn, Carl Ziegler, Ian J. M. Crossfield, Jack J. Lissauer, Andrew W. Mann, Rachel Matson, Joshua Schlieder, George Zhou

Research output: Contribution to journalArticlepeer-review

## Abstract

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 $\rm 56.0067 \pm 0.0003~days$ and $\rm 84.2597_{-0.0008}^{+0.0010}~days$, 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 $\rm 60\,mins$. These two planets were first identified by \tess, and we identified a third planet in the \textcolor{black}{\tess photometry} with a period of $\rm 22.8911 \pm 0.0004~days$. 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 $\rm \sim9\,h$ transit of HD~28109 c from Antarctica. The radii of the three planets are \textcolor{black}{$\rm R_b=2.199_{-0.10}^{+0.098} ~R_{\oplus}$, $\rm R_c=4.23\pm0.11~ R_{\oplus}$ and $\rm R_d=3.25\pm0.11 ~R_{\oplus}$}; we characterise their masses using TTVs and precise radial velocities from ESPRESSO and HARPS, and find them to be $\rm M_b=18.5_{-7.6}^{+9.1}~M_{\oplus}$, $\rm M_c=7.9_{-3.0}^{+4.2}~M_{\oplus}$ and $\rm M_d=5.7_{-2.1}^{+2.7}~M_{\oplus}$, 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.
Original language English Monthly Notices of the Royal Astronomical Society https://doi.org/10.1093/mnras/stac1383 Accepted/In press - 23 May 2022

### Bibliographical note

19 pages, 14 figures

• astro-ph.EP

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