TNG STAFF CONTRIBUTED TO THE DISCOVERY AND CHARACTERIZATION of K2-312c, A COLD JUPITER ON A VERY ECCENTRIC ORBIT

The HARPS-N collaboration performed an extended radial-velocity (RV) monitoring of the bright star K2-312 (HD 80653), known to host a rocky ultra-short period (0.72 days only) super-Earth (https://www.tng.iac.es/news/2020/01/23/superEarth/). The long-term trend detected in the first set of HARPS-N RVs was suspected to be due to an outer, highly eccentric giant planet, K2-312c. In this respect the observations collected from March to June 2020 were decisive to detect for the first time the rapid changes in the RV values at the periastron passage.

It is noteworthy to mention that those observations were performed in full service mode by the TNG staff. Due to the COVID pandemic no visiting astronomer could reach the Observatory and consequently the TNG staff entirely took in charge the duty to continue the observing programmes.

With the goal to refine the orbital parameters of K2-312c a second intensive monitoring was carried during the periastron passage occurred in 2025. K2-312c resulted to be a cold Jupiter on a wide orbit (Pc = 871.32 d), with a minimum mass of 5.31 ± 0.21 MJup and an eccentricity of 0.8435 ± 0.0013.

In the exoplanetary scenario, K2-312c is one of the most eccentric cold Jupiters known and the only one both highly eccentric and with a USP planet companion. The simulations performed by the HARPS-N team suggest that planet-planet scattering between two giant planets could have driven K2-312c to its current high eccentricity, ejected the other giant, and still allowed for the survival of K2-312 b. The extended observations baseline secured by the HARPS-N spectra further allowed the identification of both the stellar rotation period and a long activity cycle.

Link to the paper accepted for publication in A&A: Naponiello, L., Poretti E., Rice K., et al. https://arxiv.org/abs/2607.19325