Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES (1810.04731v1)

Abstract: In an earlier campaign to characterize the mass of the transiting temperate super-Earth K2-18b with HARPS, a second, non-transiting planet was posited to exist in the system at $\sim 9$ days. Further radial velocity follow-up with the CARMENES spectrograph visible channel revealed a much weaker signal at 9 days which also appeared to vary chromatically and temporally leading to the conclusion that the origin of the 9 day signal was more likely to be related to stellar activity than to being planetary. Here we conduct a detailed re-analysis of all available RV time-series, including a set of 31 previously unpublished HARPS measurements, to investigate the effects of time-sampling and of simultaneous modelling of planetary + activity signals on the existence and origin of the curious 9 day signal. We conclude that the 9 day signal is real and was initially seen to be suppressed in the CARMENES data due to a small number of anomalous measurements, although the exact cause of these anomalies remains unknown. Investigation of the signal's evolution in time, with wavelength, and detailed model comparison reveals that the 9 day signal is most likely planetary in nature. By this analysis, we reconcile the conflicting HARPS and CARMENES results and measure precise and self-consistent planet masses of $m_{p,b} = 8.63 \pm 1.35$ and $m_{p,c}\sin{i_c}=5.62 \pm 0.84$ M$_{\oplus}$. This work, along with the previously published RV papers on the K2-18 planetary system, highlight the importance of understanding one's time-sampling and of simultaneous planet + stochastic activity modelling, particularly when searching for sub-Neptune-sized planets with radial velocities.

Confirmation of Super-Earth K2-18c with HARPS and CARMENES

This paper presents a thorough investigation into the radial velocity (RV) detection of a proposed super-Earth, K2-18c, alongside the previously characterized transiting planet K2-18b. The paper leverages observations from HARPS and CARMENES spectrographs to validate the existence of K2-18c, which has been subject to conflicting interpretations due to weak signals and the interplay of stellar activity.

Key Findings

1. Detection and Confirmation of K2-18c: Utilizing previously unpublished HARPS measurements, the research identifies that the 9-day signal, initially suppressed in CARMENES data due to anomalous measurements, is likely planetary. This conclusion reconciles HARPS and CARMENES discrepancies, emphasizing the significance of understanding time-sampling effects and simultaneous stochastic modeling.
2. Stellar Activity Influence: The paper underscores the importance of distinguishing true planetary signals from stellar activity. It specifically notes the chromatic variability in CARMENES observations, attributing suppressed planetary signals to stellar activity.
3. Chromatic and Temporal Analysis: By analyzing HARPS data independently, the authors provide evidence supporting a planetary origin for the 9-day signal, visible across multiple observing seasons, while highlighting that decreasing stellar activity enhances signal detection.
4. Bayesian Model Comparison: Through Bayesian analysis, the paper robustly demonstrates that the likelihood of K2-18c existing is significantly greater when employing a two-planet model compared to a single-planet scenario. This statistical approach reinforces the hypothesis of K2-18c's planetary nature beyond periodic Lomb-Scargle detections.

Implications

The confirmation of a second super-Earth in the K2-18 system is critical for future exoplanet studies, offering a valuable target for atmospheric characterization, particularly in pre-TESS era discoveries. This research highlights the necessity for meticulous RV data interpretation where stellar activity can obfuscate signals from sub-Neptune-sized exoplanets.

Future Directions

The authors suggest further monitoring and high-precision RV measurements to disentangle potential alias signals, such as those observed near 5.5 days, which may indicate additional planets. Moreover, the work emphasizes pursuing advanced methodologies for simultaneous modeling of planetary and activity signals, ensuring accurate mass and orbital parameter derivations.

In essence, this paper contributes significantly to exoplanetary science through the validation of K2-18c, underscoring the complexities of RV signal interpretation amidst stellar variations. The findings are crucial for refining techniques in RV data analysis and enhancing the robustness of exoplanet detection methodologies.