A candidate short-period sub-Earth orbiting Proxima Centauri

Abstract: Proxima Centauri is the closest star to the Sun. This small, low-mass, mid M dwarf is known to host an Earth-mass exoplanet with an orbital period of 11.2 days within the habitable zone, as well as a long-period planet candidate with an orbital period of close to 5 years. We report on the analysis of a large set of observations taken with the ESPRESSO spectrograph at the VLT aimed at a thorough evaluation of the presence of a third low-mass planetary companion, which started emerging during a previous campaign. Radial velocities (RVs) were calculated using both a cross-correlation function (CCF) and a template matching approach. The RV analysis includes a component to model Proxima's activity using a Gaussian process (GP). We use the CCF's full width at half maximum to help constrain the GP, and we study other simultaneous observables as activity indicators in order to assess the nature of any potential RV signals. We detect a signal at 5.12 $\pm$ 0.04 days with a semi-amplitude of 39 $\pm$ 7 cm/s. The analysis of subsets of the ESPRESSO data, the activity indicators, and chromatic RVs suggest that this signal is not caused by stellar variability but instead by a planetary companion with a minimum mass of 0.26 $\pm$ 0.05 $M_\oplus$ (about twice the mass of Mars) orbiting at 0.029 au from the star. The orbital eccentricity is well constrained and compatible with a circular orbit.

• The paper highlights the detection of a significant 5.12-day radial velocity signal attributed to a sub-Earth planet with a minimum mass of 0.26 Earth masses.
• It employs dual RV extraction techniques and Gaussian processes to distinguish the planetary signal from stellar activity.
• The results demonstrate ESPRESSO's precision in detecting low-mass exoplanets, advancing our understanding of planetary systems around M-dwarf stars.

Sub-Earth Candidate Orbiting Proxima Centauri

Introduction

The paper "A candidate short-period sub-Earth orbiting Proxima Centauri," authored by a multi-institutional collaboration, presents an intricate analysis of radial velocity (RV) data acquired with the ESPRESSO spectrograph to detect planetary companions orbiting Proxima Centauri, the Earth's closest stellar neighbor. Historically, Proxima Centauri has been identified to host Proxima b, an Earth-mass exoplanet located within its habitable zone, and a long-period planet, Proxima c, detected through RV methods. The central focus of the study is to provide evidence for a low-mass planetary body, categorized as a sub-Earth, which unceremoniously emerged during previous observation campaigns.

Methodology

ESPRESSO, known for its high precision in RV measurements, played a pivotal role in this study. Data covering 117 observational sets, conducted from 2019 through 2021, were accumulated and subsequently analyzed. The authors employed two RV extraction techniques. They applied both cross-correlation function (CCF) and template matching (TM) methods to identify and extract potential planetary signals. Additionally, Gaussian processes (GP) were used to model stellar activity, acting as a potential confounding factor in RV measurements.

Results

The analysis unveiled a significant RV signal at 5.12 days. This RV signal displayed a semi-amplitude of 39 cm/s and corresponded to a planetary object with a minimum mass of approximately 0.26 Earth masses (comparable to double the mass of Mars) orbiting at 0.029 astronomical units (AU) from Proxima Centauri. The evidence rules out stellar variability as the cause of the signal, proposing instead that the data suggest a planetary companion, likely in a circular orbit. The robustness of this detection was demonstrated through various techniques, including periodograms and an investigation into the RV's chromatic behavior across multiple spectral wavelengths, which supported the planetary nature of the signal.

Discussion and Implications

This low-mass planetary candidate, informally termed Proxima d, complements the known planetary architecture around Proxima Centauri, alongside Proxima b and c. The detection exemplifies the technological capabilities of ESPRESSO in discerning low-amplitude signals amidst stellar noise, broadening the limits of current RV precision to encompass sub-Earth-mass exoplanets.

Theoretical implications are profound; Proxima d adds to the growing number of detectable planetary bodies around M-dwarfs, enriching the statistical frameworks that assess planetary formation theories and frequency around such stars. For practical pursuits, while its proximity to the star precludes habitability, the potential detection of Proxima d poses intriguing prospects for direct observational campaigns aimed at atmospheric characterization using methods such as transmission spectroscopy, should the planet transit its star.

Conclusion

This study marks an important step in exoplanetary science by identifying a candidate sub-Earth planetary companion via high-precision RV methods. Further corroboration through cross-referenced spectroscopic data or direct imaging could lead to more profound insights into the characteristics and behaviors of planetary systems orbiting our stellar neighbors. This research underscores the importance of continuous improvements in instrumentation sensitivity and the refinement of models accounting for stellar activity in enhancing the capabilities of exoplanet detection methodologies.