Now you see it, now you don't

        Well, actually, now you don't see it, now you still don't. I'm referring to Nibiru, or Tyche, Planet X or Planet Nine. A trans-Neptunian object in a 15,000-year orbit, much loved by pseudo-astronomers including Messrs. Hoagland & Bara. These two half-educated speculators wrote about it in their 2007 book Dark Mission (NOT a NYT best-seller despite its authors' frequent claims). The speculation was, actually, either one massive planet in a posigrade orbit, or two smaller ones going retrograde (p.115, 2nd edn.). It was based on a diagram plotting specific luminosity (W/kg) vs. specific angular momentum (sq cm/sec) that hoagland took from a book called The New Solar System then slapped his own copyright on it. The figures the diagram uses include, in each case, the named body plus anything in orbit around it.

        According to him, the solar system as a whole has less angular momentum than it should, and Planet X (or Planets -y and -z) would bring it into line. I don't think either Hoagland or Bara understand what specific angular momentum means (it means angular momentum per unit mass, so to arrive at the true angular momentum of the solar system, you'd need to multiply that number for 'Sun' in the diagram—4 x 10²⁰ approx—by the mass of the whole kit and caboodle).

        When, in January 2016, Batygin & Brown of CalTech lent their support to Planet X on the basis of the orbital anomalies of six Kuiper Belt objects, Mike Bara did the Toldya dance on his blog. But he got totally confused between Planet X and Nemesis, a hypothetical brown dwarf companion of the Sun that has nothing to do with the Kuiper Belt. Stuart "astroguy" Robbins expressed more than a modicum of skepticism about the announcement from CalTech.

An icy disc
        This week Sefilian and Touma of Cambridge University and the American University of Beirut have come up with a model that explains the orbital anomalies without requiring a massive trans-Neptunian object (TNO).^{note 1}

 "We wanted to see whether there could be another, less dramatic and perhaps more natural, cause for the unusual orbits we see in some TNOs. We thought, rather than allowing for a ninth planet, and then worry about its formation and unusual orbit, why not simply account for the gravity of small objects constituting a disc beyond the orbit of Neptune and see what it does for us?
"If you remove planet nine from the model and instead allow for lots of small objects scattered across a wide area, collective attractions between those objects could just as easily account for the eccentric orbits we see in some TNOs." --Antranik Sefilian

        The hypothesised icy disc probably has a mass equivalent to ten Earths—not very much^{note 2}, but Sefilian and Touma say it's enough. They aren't claiming to have actually observed such a disc, just to have worked out the physical implications assuming it exists. It's important to understand that nobody claims to have actually observed Planet X, or Nibiru, either. Now you don't see it, now you still don't.

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[1] Antranik A. Sefilian, Jihad R. Touma. Shepherding in a Self-Gravitating Disk of Trans-Neptunian Objects. Astronomical Journal, 2019

[2] Neptune is 17 Earths, Jupiter 318