Revisiting Proxima with ESPRESSO

Abstract: We aim to confirm the presence of Proxima b using independent measurements obtained with the new ESPRESSO spectrograph, and refine the planetary parameters taking advantage of its improved precision. We analysed 63 spectroscopic ESPRESSO observations of Proxima taken during 2019. We obtained radial velocity measurements with a typical radial velocity photon noise of 26 cm/s. We ran a joint MCMC analysis on the time series of the radial velocity and full-width half maximum of the cross-correlation function to model the planetary and stellar signals present in the data, applying Gaussian process regression to deal with stellar activity. We confirm the presence of Proxima b independently in the ESPRESSO data. The ESPRESSO data on its own shows Proxima b at a period of 11.218 $\pm$ 0.029 days, with a minimum mass of 1.29 $\pm$ 0.13 Me. In the combined dataset we measure a period of 11.18427 $\pm$ 0.00070 days with a minimum mass of 1.173 $\pm$ 0.086 Me. We find no evidence of stellar activity as a potential cause for the 11.2 days signal. We find some evidence for the presence of a second short-period signal, at 5.15 days with a semi-amplitude of merely 40 cm/s. If caused by a planetary companion, it would correspond to a minimum mass of 0.29 $\pm$ 0.08 Me. We find that the FWHM of the CCF can be used as a proxy for the brightness changes and that its gradient with time can be used to successfully detrend the radial velocity data from part of the influence of stellar activity. The activity-induced radial velocity signal in the ESPRESSO data shows a trend in amplitude towards redder wavelengths. Velocities measured using the red end of the spectrograph are less affected by activity, suggesting that the stellar activity is spot-dominated. The data collected excludes the presence of extra companions with masses above 0.6 Me at periods shorter than 50 days.

• The paper confirms Proxima b’s existence by using 63 high-fidelity ESPRESSO radial velocity measurements to determine an orbital period of about 11.18 days and an Earth-like mass.
• The study leverages ESPRESSO’s superior precision with photon-limited noise (~26–27 cm/s) to significantly improve upon results from previous instruments like HARPS and UVES.
• The analysis effectively distinguishes stellar rotation (87 days) from planetary signals and identifies a potential secondary 5.15-day signal, suggesting the possibility of an additional exoplanet.

Analysis of "Revisiting Proxima with ESPRESSO"

The research paper "Revisiting Proxima with ESPRESSO" concentrates on an in-depth examination of Proxima Centauri utilizing the ESPRESSO spectrograph to assess the presence and characteristics of Proxima b, an exoplanet believed to orbit within its star's habitable zone. The study employs a robust methodology leveraging ESPRESSO's enhanced spectral precision to ascertain more accurate estimates of Proxima b's mass and orbital parameters.

Key Findings

• Radial Velocity Measurements: The investigation involved 63 high-fidelity spectroscopic observations of Proxima Centauri, with radial velocity (RV) data exhibiting photon noise of 26 cm/s. This was a significant enhancement over previous observations, enabling finer resolution of the stellar and planetary factors influencing RV measurements.
• Confirmation of Proxima b: The data confirmed the existence of Proxima b with a period of 11.218 days and an updated minimum mass of 1.29 Earth masses from ESPRESSO data alone. Combining ESPRESSO with HARPS and UVES data refined these estimates to a period of 11.18427 days and a mass of 1.173 Earth masses.
• Stellar Activity and Rotation: Analysis revealed a stellar rotation period of 87 days as derived from photometric and RV data, dismissing stellar activity as the origin of the observed 11.2-day signal. Additionally, some evidence was found for a potential secondary 5.15-day signal, suggestive of another planetary candidate or stellar-related phenomena.
• Instrument Precision and Comparisons: ESPRESSO manifestly surpassed previous generation instruments like HARPS in RV precision, achieving measurement noise close to the photon limit (27 cm/s), meaning the data was primarily limited by the system's photon count rather than instrumental error.

Implications

• Theoretical and Observational Advancements: The results bear significant implications for exoplanetary studies, particularly in confirming Earth's mass planets in the habitable zones of M-dwarfs, which are notoriously challenging due to stellar activity interference. This study demonstrates that with advancements in spectroscopic precision, these challenges can be overcome.
• Potential for Detection of Additional Exoplanets: The mention of another possible planetary signal implies that there might be further planets in the Proxima system that could be distinguished with accumulating data, speculatively intensifying the intrigue in the habitability potential of such systems.
• Techniques for Disentangling Stellar Noise: Techniques involving Gaussian processes for modeling stellar activity and employing FWHM as a brightness proxy highlight methodologies applicable to other systems where stellar noise complicates RV measurements.

Future Direction and Recommendations

• Continued Monitoring and Data Collection: Given the initial indications of another harbored planet, continued observations with ESPRESSO and other global networks are critical. These should focus on enhancing the signal quality and validating the potential new signals.
• Exploitation of Wide Spectral Ranges: The application of RV measurements across a broader spectrum enabled improved activity signal differentiation, suggesting similar techniques could be applied to further disentangle stellar signals across other variable stars.
• Synergistic Use with Space Observatories: Complementary data from space-based observatories such as TESS could enrich spectroscopic findings and provide comprehensive assessments of stellar activities and planetary transits, reinforcing the precision and reliability of RV results.

This study on revisiting Proxima b with ESPRESSO elucidates the significant strides made in exoplanet detection and characterization, advocating ESPRESSO as an exemplary tool for the search of terrestrial planets. The methodologies employed and the instrument's capabilities pave the way for more definitive explorations of nearby exoplanetary systems.