Gliese 581d is a Stellar Activity False Positive

Periodic Halpha variations in GL 581: Further evidence for an activity origin to GL 581d

Author:

Hatzes

Abstract:

Radial velocity measurements showed evidence that the M dwarf GL 581 might host a planet, GL 581d, in the so-called "habitable zone" of the star. A study of Halpha in GL 581 demonstrated that changes in this activity indicator correlated with radial velocity variations attributed to GL 581d. An exopplanet that was important for studies of planet habitability may be an artifact of stellar activity. Previous investigations analyzing the same activity data have reached different conclusions regarding the existence of GL 581d. We therfore investigated the Halpha variations for GL 581 to assess the nature of the radial velocity variations attributed to the possible planet GL 581d. We performed a Fourier analysis of the published Halpha measurements for GL 581d using a so-called pre-whitening process to isolate the variations at the orbital frequency of GL 581d. The frequency analysis yields five significant frequencies, one of which is associated with the 66.7 d orbital period of the presumed planet Gl 581d. The Halpha variations at this period show sine-like variations that are 180 degrees out-of-phase with the radial velocity variations of GL 581d. This is seen in thefull data set that spans almost 7 years, as well as a subset of the data that had good temporal sampling over 230 days. Furthermore, No significant temporal variations are found in the ratio of the amplitudes of the Halpha index and radial velocity variations. This provides additional evidence that the radial velocity signal attributed to GL 581d is in fact due to stellar activity.

Gliese 581g & Gliese 581d Probably do NOT Exist

Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581

Authors:

Robertson et al

Abstract:

The M dwarf Gliese 581 is believed to host four planets, including one (GJ 581d) near the habitable zone that could possibly support liquid water on its surface if it is a rocky planet. The detection of another habitable-zone planet—GJ 581 g—is disputed, as its significance depends on the eccentricity assumed for d. Analyzing stellar activity using the Hα line, we measure a stellar rotation period of 130 ± 2 days and a correlation for Hα modulation with radial velocity. Correcting for activity greatly diminishes the signal of GJ 581d (to 1.5σ) while significantly boosting the signals of the other known super-Earth planets. GJ 581d does not exist, but is an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g.

N Body Simulation of Gliese 581 Shows 4 Body System Stable in the Long Term

A CONSISTENT ORBITAL STABILITY ANALYSIS FOR THE GJ 581 SYSTEM

Authors:

Joiner et al

Abstract:

We apply a combination of N-body modeling techniques and automated data fitting with Monte Carlo Markov Chain uncertainty analysis of Keplerian orbital models to RV data to determine long-term stability of the planetary system GJ 581. We find that while there are stability concerns with the four-planet model as published by Forveille et al., when uncertainties in the system are accounted for, particularly stellar jitter, the hypothesis that the four-planet model is gravitationally unstable is not statistically significant. Additionally, the system including proposed planet g by Vogt et al. also shows some stability concerns when eccentricities are allowed to float in the orbital fit, yet when uncertainties are included in the analysis, the system including planet g also cannot be proven to be unstable. We present revised reduced χ2 values for Keplerian astrocentric orbital fits assuming four-planet and five-planet models for GJ 581 under the condition that best fits must be stable, and we find no distinguishable difference by including planet g in the model. Additionally, we present revised orbital element estimates for each, assuming uncertainties due to stellar jitter under the constraint of the system being gravitationally stable.