Planet 9 probably doesn't exist, new paper argues

My observation. From the arXiv paper, Introduction "While there is no universally accepted definition for the ETNOs, recent literature has emphasized objects with semimajor axis a >~ 230 au and perihelion q > 30 au. Because ETNOs follow highly elliptical orbits, and their brightness decreases like 1=r^4, they are almost always discovered within a few decades of perihelion. Moreover, telescopic surveys observe a limited area of the sky, at particular times of year, to a limited depth. These effects result in significant selection bias. The six ETNOs considered in the Batygin & Brown (2016) (BB16) analysis were discovered in an assortment of surveys with unknown or unpublished selection functions, making it difficult to establish that the observed angular clustering was indeed of physical origin."

"Table 1. Barycentric orbital elements of the ETNOs used in our analysis." My observation. Page 3 has details of the ETNOs used in the study. Semi-major axis min = 233 au, max = 1559.5 au, e min = 0.69, e max = 0.98. Objects with a ~ 233 au, e ~ 0.9, P ~ 3600 years using Jean Meeus Astronomical Alogrithms for elliptical motion. Apparent magnitudes for these objects in the arXiv paper about 25.2 so extremely faint targets to find and track. The paper wraps up with "Despite other lines of indirect evidence for Planet X/Planet 9, in the absence of clear evidence for clustering of the ETNOs the argument becomes much weaker. Future studies should consider other mechanisms capable of giving the outer solar system its observed structure..."]

Surveying targets in the outer regions of the solar system that take thousands of years to orbit with faint magnitudes in the range of 23 to 25, is very difficult. This new study does indicate Planet Nine may not be real.
 
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