Bayesian search for low-mass planets around nearby M dwarfs. Estimates for occurrence rate based on global detectability statistics (1403.0430v1)

Abstract: Due to their higher planet-star mass-ratios, M dwarfs are the easiest targets for detection of low-mass planets orbiting nearby stars using Doppler spectroscopy. Furthermore, because of their low masses and luminosities, Doppler measurements enable the detection of low-mass planets in their habitable zones that correspond to closer orbits than for Solar-type stars. We re-analyse literature UVES radial velocities of 41 nearby M dwarfs in a combination with new velocities obtained from publicly available spectra from the HARPS-ESO spectrograph of these stars in an attempt to constrain any low-amplitude Keplerian signals. We apply Bayesian signal detection criteria, together with posterior sampling techniques, in combination with noise models that take into account correlations in the data and obtain estimates for the number of planet candidates in the sample. More generally, we use the estimated detection probability function to calculate the occurrence rate of low-mass planets around nearby M dwarfs. We report eight new planet candidates in the sample (orbiting GJ 27.1, GJ 160.2, GJ 180, GJ 229, GJ 422, and GJ 682), including two new multiplanet systems, and confirm two previously known candidates in the GJ 433 system based on detections of Keplerian signals in the combined UVES and HARPS radial velocity data that cannot be explained by periodic and/or quasiperiodic phenomena related to stellar activities. Finally, we use the estimated detection probability function to calculate the occurrence rate of low-mass planets around nearby M dwarfs. According to our results, M dwarfs are hosts to an abundance of low-mass planets and the occurrence rate of planets less massive than 10 M$_{\oplus}$ is of the order of one planet per star, possibly even greater. ...

Bayesian Search for Low-Mass Planets Around Nearby M Dwarfs: An Expert Analysis

The paper "Bayesian search for low-mass planets around nearby M dwarfs" by M. Tuomi et al. provides a detailed analysis aimed at detecting low-mass exoplanets orbiting nearby M dwarf stars using radial velocity (RV) data. By employing Bayesian statistical methods, the authors reevaluate the UVES RV data of 41 M dwarfs complemented by HARPS-ESO spectra, aiming to identify periodic signals indicative of planetary companions. This research seeks to refine estimates on the occurrence rates of low-mass planets, shedding light on their frequency and distribution in the proximity of M dwarf stars.

The rationale for focusing on M dwarfs is well-founded. Due to their lower masses, M dwarfs exhibit higher planet-star mass ratios, making the detection of smaller planetary signals more feasible than with Sun-like stars. Furthermore, the reduced luminosity of M dwarfs places their habitable zones closer to the star, thus shortening the orbital periods of habitable planets, which enhances detectability through RV methods.

The authors employ Bayesian signal detection techniques alongside noise models that account for data correlations, addressing issues such as stellar activity that could confound planet detection. Their analysis reveals eight new planet candidates within the sample, highlighting the potential abundance of low-mass planets around M dwarfs. They identify several multi-planet systems and confirm two previously suggested planets around GJ 433 using combined UVES and HARPS data.

Strong numerical results reported include the detection of planets less massive than 10 M$_{\oplus}$ at a rate close to one per star, with a marked occurrence of planets between 3 and 10 M$_{\oplus}$ in the habitable zones. The Bayesian framework enables the estimation of a favorable detection probability function, enhancing the reliability of their occurrence rate estimates. The paper indicates that the occurrence rate of planets within the habitable zones of M dwarfs for those with masses of 3 to 10 M$_{\oplus}$ stands at approximately 0.21$^{+0.03}_{-0.05}$ planets per star.

The implications of this research are significant both practically and theoretically. The detection of low-mass planets in close proximity to their host stars could inform future astrobiological studies on the potential for life around such stars. From a theoretical standpoint, the abundance of such planets supports models of planetary formation that predict prolific super-Earth and sub-Neptunian populations, influencing the development of star and planet formation theories.

Looking to the future, the refinement of RV techniques and Bayesian models is likely to enhance our understanding of planetary populations around low-mass stars. The paper underscores the importance of combining datasets from different observatories to increase detection confidence and suggests that further analyses with upcoming precision instruments could tease out additional low-mass planets from stellar RV signals.

Overall, the research conducted by Tuomi et al. contributes substantially to our understanding of the prevalence and properties of planetary systems around M dwarfs, offering robust statistical tools and methodologies that could be pivotal in the ongoing exploration of extrasolar planetary environments.