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From Wikipedia Planet Nine#Citizen science:

In April 2017, using data from the SkyMapper telescope at Siding Spring Observatory, citizen scientists on the Zooniverse platform reported four candidates for Planet Nine. These candidates will be followed up on by astronomers to determine their viability. The project, which started on 28 March 2017, completed their goals in less than three days with around five million classifications by more than 60,000 individuals.

But it has not been updated for nearly 3 years. Is there any follow up study that confirm what these 4 candidates actually are?

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A frequently cited study about Planet Nine is arXiv:1601.05438: Evidence for a Distant Giant Planet in the Solar System, but I can confirm that after 2016 there are not too many studies on the subject, at least I could not find many. There are two interesting studies I would like to share, but none of them spoke of four candidates.

  1. Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project. The abstracts reads

    We present Spitzer follow-up imaging of 95 candidate extremely cold brown dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project, which uses visually perceived motion in multiepoch Wide-field Infrared Survey Explorer (WISE) images to identify previously unrecognized substellar neighbors to the Sun. [...]

  2. Even more interesting is What If Planet 9 Is a Primordial Black Hole? by Jakub Scholtz and James Unwin from 2020:

    We highlight that the anomalous orbits of trans-Neptunian objects (TNOs) and an excess in microlensing events in the 5-year Optical Gravitational Lensing Experiment data set can be simultaneously explained by a new population of astrophysical bodies with mass several times that of the Earth (M⊕). We take these objects to be primordial black holes (PBHs) and point out the orbits of TNOs would be altered if one of these PBHs was captured by the Solar System, inline with the Planet 9 hypothesis. Capture of a free floating planet is a leading explanation for the origin of Planet 9, and we show that the probability of capturing a PBH instead is comparable. The observational constraints on a PBH in the outer Solar System significantly differ from the case of a new ninth planet. This scenario could be confirmed through annihilation signals from the dark matter microhalo around the PBH.

I hope that this is helpful for you.

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