The mystery they don't name in the title is Planet Nine.
I checked the original paper[1], and it appears this is either a new paper, distinct from the preprint[2] previously discussed here a couple months ago[3], or the preprint was heavily patched while undergoing peer review, because it appears now they found two candidates, different from the single candidate pinpointed in the preview. Here the two candidades are located at R.A. 42.676 Dec. −15.021 and R.A. 45.297 Dec. −16.711. In the older preprint the single candidate was placed at R.A. 35.18379 Dec. -49.2135. Both final paper and preprint share coauthors, so I think it was likely patched, not a completely different paper.
I checked the original paper[1], and it appears this is either a new paper, distinct from the preprint[2] previously discussed here a couple months ago[3], or the preprint was heavily patched while undergoing peer review, because it appears now they found two candidates, different from the single candidate pinpointed in the preview. Here the two candidades are located at R.A. 42.676 Dec. −15.021 and R.A. 45.297 Dec. −16.711. In the older preprint the single candidate was placed at R.A. 35.18379 Dec. -49.2135. Both final paper and preprint share coauthors, so I think it was likely patched, not a completely different paper.
[1] https://www.cambridge.org/core/services/aop-cambridge-core/c...
[2] https://arxiv.org/pdf/2504.17288
[3] https://news.ycombinator.com/item?id=43874641
Particularly with the way Pluto's classification has changed over time, and the emotional reactions to it.