Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit (1707.04292v1)

Abstract: The bright M dwarf K2-18 at 34 pc is known to host a transiting super-Earth-sized planet orbiting within the star's habitable zone; K2-18b. Given the superlative nature of this system for studying an exoplanetary atmosphere receiving similar levels of insolation as the Earth, we aim to characterize the planet's mass which is required to interpret atmospheric properties and infer the planet's bulk composition. We obtain precision radial velocity measurements with the HARPS spectrograph and couple those measurements with the K2 photometry to jointly model the observed radial velocity variation with planetary signals and a radial velocity jitter model based on Gaussian process regression. We measure the mass of K2-18b to be $8.0 \pm 1.9$ M$_{\oplus}$ with a bulk density of $3.7 \pm 0.9$ g/cm$^3$ which may correspond to a predominantly rocky planet with a significant gaseous envelope or an ocean planet with a water mass fraction $\gtrsim 50$%. We also find strong evidence for a second, warm super-Earth K2-18c at $\sim 9$ days with a semi-major axis 2.4 times smaller than the transiting K2-18b. After re-analyzing the available light curves of K2-18 we conclude that K2-18c is not detected in transit and therefore likely has an orbit that is non-coplanar with K2-18b. A suite of dynamical integrations with varying simulated orbital eccentricities of the two planets are used to further constrain each planet's eccentricity posterior from which we measure $e_b < 0.43$ and $e_c < 0.47$ at 99% confidence. The discovery of the inner planet K2-18c further emphasizes the prevalence of multi-planet systems around M dwarfs. The characterization of the density of K2-18b reveals that the planet likely has a thick gaseous envelope which along with its proximity to the Solar system makes the K2-18 planetary system an interesting target for the atmospheric study of an exoplanet receiving Earth-like insolation.

• The paper determines precise masses and densities for K2-18b and K2-18c, revealing K2-18b's potential for a voluminous atmosphere.
• It employs high-precision HARPS radial velocity data combined with K2 photometry and Gaussian process regression to isolate planetary signals from stellar activity.
• The discovery of a non-coplanar, warm super-Earth in the system offers new insights into planetary dynamics and formation processes around M dwarf stars.

Characterization of the K2-18 Multi-Planetary System with HARPS

The research conducted by Cloutier et al. provides a comprehensive analysis of the K2-18 planetary system, focusing on the characterization of its planets via high-precision radial velocity (RV) measurements facilitated by the HARPS spectrograph. The system, centered around the bright M2.5 dwarf K2-18, is notable for hosting a super-Earth, K2-18b, located within the star's habitable zone (HZ), as well as a newly detected second super-Earth, K2-18c, on a non-coplanar orbit.

Key Findings and Methodologies

1. Planetary Mass Determination:
   • K2-18b is characterized by a mass of $8.0 \pm 1.9$ M$_{\oplus}$ and a bulk density of $3.7 \pm 0.9$ g/cm$^3$. This suggests the presence of a substantial gaseous envelope or potentially a significant water mass fraction. Such properties make K2-18b an intriguing subject for atmospheric studies due to its Earth-like insolation.
   • The newly discovered K2-18c has a minimum mass of $7.5 \pm 1.3$ M$_{\oplus}$ and orbits with a period of approximately 9 days, featuring a semi-major axis significantly smaller than K2-18b.
2. Radial Velocity and Photometric Analysis:
   • The authors integrated precision RV data from HARPS with K2 space mission photometry, using Gaussian process regression to distinguish planetary signals from stellar activity-induced RV jitter.
   • A robust and comprehensive keplerian model was applied to extract planetary parameters, augmented by dynamical simulations that confirmed the stability of the planetary system across various orbital eccentricities.
3. Detection and Analysis of K2-18c:
   • Despite the indication of a secondary planet through the RV data, K2-18c was not observed in transit via photometric light curves, implying a non-coplanar orbit relative to K2-18b.
   • The detection emphasizes the potential complexity and multi-planarity within M dwarf systems.
4. Dynamical Constraints on Orbital Eccentricities:
   • Dynamical simulations provided enhanced constraints on orbital eccentricities, with $e_b < 0.43$ and $e_c < 0.47$, ensuring system stability consistent with Hill stability criteria across million-year timescales.

Implications and Future Directions

• Atmospheric Characterization: The presence of a substantial atmosphere on K2-18b, potentially retaining volatile molecules such as water or methane, makes it a prime candidate for transmission spectroscopy. Upcoming observations, especially with instruments like the JWST, will likely shed light on its atmospheric composition and the potential for habitability.
• Planet Formation Insights: The confirmed existence of a two-planet system with varying alignments offers a window into studying planet formation dynamics around M dwarf stars. By understanding mutual inclinations and comparative characteristics in M dwarf systems, researchers can derive more nuanced models of planetary system evolution.
• Broader Exoplanetary Studies: Findings from K2-18 contribute to the wider context of small exoplanets, specifically regarding their mass-radius relationships, which point towards a spectrum involving terrestrial, water-world, and Neptune-like structures. This, therefore, aids in classifying and predicting the nature and formation of similar exoplanets in the habitable zone.

Conclusion

The detailed paper of the K2-18 system through HARPS measurements underscores the critical role of precise RV techniques in expanding our knowledge of exoplanetary environments, especially for multi-planetary systems around M dwarfs. With significant implications for atmospheric studies and the understanding of planetary system architecture, K2-18 remains a pivotal focus in the field of exoplanet research, promising further insights with future observational campaigns.