Additional planets in Gliese 876 ?

Improved signal detection algorithms for unevenly sampled data. Six signals in the radial velocity data for GJ876
http://arxiv.org/abs/1403.7646

Quote :

The hunt for Earth analogue planets orbiting Sun-like stars has forced the introduction of novel methods to detect signals at, or below, the level of the intrinsic noise of the observations. We present a new global periodogram method that returns more information than the classic Lomb-Scargle periodogram method for radial velocity signal detection. Our method uses the Minimum Mean Squared Error as a framework to determine the optimal number of genuine signals present in a radial velocity timeseries using a global search algorithm, meaning we can discard noise spikes from the data before follow-up analysis. This method also allows us to determine the phase and amplitude of the signals we detect, meaning we can track these quantities as a function of time to test if the signals are stationary or non-stationary. We apply our method to the radial velocity data for GJ876 as a test system to highlight how the phase information can be used to select against non-stationary sources of detected signals in radial velocity data, such as rotational modulation of star spots. Analysis of this system yields two new statistically significant signals in the combined Keck and HARPS velocities with periods of 10 and 15 days. Although a planet with a period of 15 days would relate to a Laplace resonant chain configuration with three of the other planets (8:4:2:1), we stress that follow-up dynamical analyses are needed to test the reliability of such a six planet system.

Hmmmmmm.

I think the key point is as follows:

Quote :

A non-planetary origin might be expected for the 15 day and 10 day signals since both the Correia et al. (2010) and Rivera et al. (2010) analyses failed to spot them when applying a Newtonian integration analysis that considers the dynamical interactions between the planets.

If there's one system that is well known that dynamics plays a major role, Gliese 876 would be it. Conclusions based on Keplerian fitting should be considered suspect, even when they don't arrive on April Fools' Day.

Yeah, that's exactly what I thought. It's an interesting methodology, but that's nullified by the horrifically bad idea of applying it to the most dynamically interactive planetary system known without actually considering said interactions.

Furthermore, Gerlach & Haghighipour found the 15 day orbit unstable.

Quote :

The results of our initial analysis suggested possible stable orbits at regions exterior to the orbit of the outermost planet and also indicated that an island of stability may exist in and around the 15-day orbit. However, examining the long-term stability of an object in that region by direct integration revealed that the 15-day orbit becomes unstable and that the system of GJ 876 is most likely dynamically full

Hara et al. (2017) "Radial Velocity Data Analysis with Compressed Sensing Techniques"
Available on arXiv at https://arxiv.org/abs/1609.01519

In case you were wondering, see section 4.5.3. The 15- and 10-day periods are signatures of the dynamical interactions.