Color difference makes a difference: four planet candidates around tau Ceti (1708.02051v1)

Abstract: The removal of noise typically correlated in time and wavelength is one of the main challenges for using the radial velocity method to detect Earth analogues. We analyze radial velocity data of tau Ceti and find robust evidence for wavelength dependent noise. We find this noise can be modeled by a combination of moving average models and "differential radial velocities". We apply this noise model to various radial velocity data sets for tau Ceti, and find four periodic signals at 20.0, 49.3, 160 and 642 d which we interpret as planets. We identify two new signals with orbital periods of 20.0 and 49.3 d while the other two previously suspected signals around 160 and 600 d are quantified to a higher precision. The 20.0 d candidate is independently detected in KECK data. All planets detected in this work have minimum masses less than 4$M_\oplus$ with the two long period ones located around the inner and outer edges of the habitable zone, respectively. We find that the instrumental noise gives rise to a precision limit of the HARPS around 0.2 m/s. We also find correlation between the HARPS data and the central moments of the spectral line profile at around 0.5 m/s level, although these central moments may contain both noise and signals. The signals detected in this work have semi-amplitudes as low as 0.3 m/s, demonstrating the ability of the radial velocity technique to detect relatively weak signals.

• The paper introduces an innovative noise model combining moving average methods with differential RVs to effectively isolate wavelength-dependent noise.
• The study detects four weak planetary signals, each with a mean semi-amplitude of 0.3 m/s and orbital periods of approximately 20, 49, 160, and 600 days.
• The results highlight the potential of advanced noise modeling techniques to enhance radial velocity analyses and improve Earth-like exoplanet detection.

A Comprehensive Analysis: Detecting Planetary Candidates Around ✰ Ceti Through Enhanced Noise Modeling

This paper presents an intricate analysis of radial velocity (RV) data to identify potential planetary candidates orbiting ✰ Ceti, a Sun-like star. The investigation employs an advanced noise model that transcends traditional RV analysis by incorporating time and wavelength-dependent noise, notably enhancing the precision of detected signals. Focus is placed on identifying and distinguishing between genuine Keplerian signals and those generated or distorted by noise.

Radial Velocity Data: Challenges and Significance

Detecting Earth-analog exoplanets through the RV method is often hindered by noise that correlatively fluctuates with time and wavelength. While extreme precision spectrographs have improved, achieving the necessary precision to detect Earth-like planets remains elusive, especially when instrumental and stellar noise contaminate the RV measurements. The paper addresses these issues by leveraging a new type of noise model, differentiating from standard practice in RV analyses.

Innovative Noise Modeling Approach

The authors propose a novel strategy by integrating moving average models with "differential RVs," which hinge on wavelength variations across different spectral orders. This integration aims to isolate and remove wavelength-dependent noise, effectively untangling planetary signals from the noise that averages over these spectra often fail to eliminate. By comparing various noise models within a Bayesian framework, they ascertain the effectiveness of their model in reducing false positives and improving signal detection fidelity.

Detecting Planetary Candidates

The paper reveals the presence of four distinct signals with periodicities of approximately 20, 49, 160, and 600 days, suggesting these are potential planets. The confirmation of the 600-day signal over its 1000-day alias supports its planetary origin due to lower associated eccentricity. These signals emerge with mean semi-amplitudes around 0.3 m/s, indicative of their weak nature and underscoring the capabilities of the RV method when coupled with advanced noise modeling.

Theoretical and Practical Implications

This work proposes that adopting differential RVs could significantly bolster the fidelity of RV measurements in discerning genuine planetary signals. Stellar activity's wavelength dependency, unveiled by power variations across differential RVs, poses a new dimension for assessing activity influences on RV data. The method offers a roadmap for reanalyzing previously detected sub-m/s planet candidates and highlights the necessity for adopting comprehensive noise models in future observations.

Future Prospects and Research Directions

The conclusions drawn from ✰ Ceti's RV data not only elucidate the potential exoplanetary system surrounding this star but also serve a broader directive for upcoming exoplanet exploration endeavors. As high-precision spectrograph technology advances, the methodological framework laid out within this paper could play a pivotal role in the detection of Earth analogs. Future research should aim at extending these noise modeling techniques to other stars, offering insights into stellar variability and instrumental biases that influence RV data precision.

Conclusion

The paper proposes significant advancements in RV analysis methodologies, notably through the introduction of differential RVs to address wavelength-dependent noise. The enhanced modeling approach substantially improves the capability of detecting weak planetary signals around ✰ Ceti, offering a robust methodology that could have far-reaching impacts on RV-based exoplanet detection, specifically concerning the search for Earth-like planets. Such strategic methodological shifts indicate promising new directions in astrophysical research and planetary exploration.