Pluto may have been demoted from planet status back in 2006 but astronomers are still trying to explain the orbits of a number of other Kuiper Belt objects which are in highly elliptical orbits.
Whilst searches for a conventional “Planet Nine” have proved unsuccessful so far a report in Physics World, dated 19th May, ( Link ) looks at the proposal by Edwar Witten, Princeton, to evaluate the suggestion, first made in 2019 by Scholtz, Durham, and Unwin, Chicago, that a small black hole, around 10 times the mass of the Earth, could be stabilising these orbits. This is very small, a black hole the mass of the sun only has a radius of around 3 km (the Schwarzschild radius), the distance from Abergavenny to Govilon.
Witten’s proposal is to launch a fleet of lightweight probes, 100g, in the direction of the presumed Black Hole. To quote the article “His proposal is a more modest version of the Breakthrough Starshot project ( Link ) which aims to send ultra-light probes on a 20 year journey to the nearby star Alpha Centauri using an Earth bound laser array”. Witten’s proposal would involve a 10 year journey to 500 AU, well beyond the Kuiper Belt, which is thought to extend to 50 AU and towards the hypothetical Oort Cloud which may start at 2,000 AU.
However, Witten does qualify his proposal, “It is far from clear that this approach is practical…”
Mike Brown, Caltech, a Planet 9 searcher says “We’re still looking hard. If we don’t find Planet Nine in any of the dedicated searches, I suspect it will turn up pretty quickly in LSST [the Large Synoptic Survey Telescope”. Read Brown’s Planet 9 hypothesis at ( Link )