In conjunction with a scientist from the University of Michigan, the Caltech team who originally coined the term Planet Nine in 2016 have written a new paper about its formation, and the subsequent layout of the outer solar system. Having set out the evidence for this proposed object in the paper (1), they note three possible scenarios for its formation:
1) The planet’s capture from the retinue of a passing star; or, alternatively, the capture of a free-floating interstellar planet
2) The planet’s semi-ejection from the inner solar system and subsequent gradual drift outwards
3) The planet’s formation in situ.
All three of these scenarios require certain conditions for them to work, which means that no single formation theory stands out as particularly probable. The capture and scattering models depend upon the interjection of outside bodies (passing stars or brown dwarfs, or objects in the Sun’s birth cluster). The in situ formation of a planet so far from the Sun implies that the Sun’s protoplanetary disk was significantly larger than generally accepted. The formation of Planet Nine in its calculated position thus remains problematic, based upon standard models of planetary and solar system formation (e.g. the Nice model). Further, whatever processes which placed it in its proposed current position would have significantly affected the layout of the Kuiper belt within its overarching orbit. This factor is what the current investigation described by this paper aims to solve.
This paper then describes computer simulations of the early Kuiper belt, and how the shape and extent of the fledgling belt may have affected the complex interplay between it, Planet Nine, and the objects in the extended scattered disk (1). The research team modelled two distinct scenarios for the early Kuiper belt, each of which matches one or more formation scenarios for Planet Nine. The first is a ‘narrow’ disk, similar to that observed: The Kuiper disk appears to be truncated around 50AU, with objects found beyond this zone likely having been scattered outwards by processes which remain contentious. The second scenario is a ‘broad’ disk, where objects in the Kuiper belt would have routinely populated the space between Neptune and the proposed orbit of Planet Nine, hundreds of astronomical units out. This would match a formation scenario involving an extensive protoplanetary disk. Read More…
Last month, scientists working on the Outer Solar System Origins Survey (OSSOS) published a large dataset of new Kuiper Belt Objects, including several new extended scattered disk objects discovered way beyond the main belt (1). These four new distant objects seemed to have a more random set of properties, when compared to the rather more neat array of objects which had previously been constituted the Planet Nine cluster. This led to scepticism among the OSSOS scientific team that there was any real evidence for Planet Nine. Instead, they argued, the perceived patterns of these distant objects might be a function of observational bias (2).
Whilst reporting on these new discoveries and their potential implications, I predicted that the debate was about to hot up, bringing forth a new series of Planet X-related articles and papers (3). Indeed, leading outer solar system scientists were publishing related materials in quick succession (4,5), each finding new correlations and patterns which might indicate the presence of an unseen perturbing influence.
Now, Caltech’s Konstantin Batygin has published an article analysing the impact of the discovery of these new extended scattered disk objects on the potential for a Planet Nine body. The short conclusion he draws is that although the objects are, on the face of it, randomly distributed, their property set is largely consistent with Caltech’s original thesis (6). They are either anti-aligned to the purported Planet Nine body (as the original cluster is thought to be), or aligned with it in a meta-stable array.