My yelp of delight upon hearing about this on the radio this morning was joyous. Mrs DarkStar commented that few homes in the land would have met such a story in this way. True, but few householders have written 98% of a book about missing planets and novel forms of planetary formation, and just need one more jigsaw piece to finish it. And here it is: Space grease! Admittedly, this does not sound too exciting. But I have faced a problem figuring out just what can stick interstellar protoplanets together, given a lack of gas pressure in interstellar space (this gas pressure, apparent in the early solar system’s pre-solar nebula and subsequent protoplanetary disk, likely plays a part in granular accretion). What better way to accrete than space grease. There’s masses of it out there (10 billion trillion trillion tonnes in the Milky Way), created in stars, and distributed across space:
“Prof Tim Schmidt, a chemist at the University of New South Wales, Sydney and co-author of the study, said that the windscreen of a future spaceship travelling through interstellar space might be expected to get a sticky coating. “Amongst other stuff it’ll run into is interstellar dust, which is partly grease, partly soot and partly silicates like sand,” he said, adding that the grease is swept away within our own solar system by the solar wind.” (1)
Material moving through interstellar space encounters this grease routinely, then. It will stick to surfaces. Over billions of years of such interactions, major accumulations of this type of gloop will build up on objects, like interstellar comets, and free-floating asteroids and planets.
The first interstellar object to be directly observed moving through our solar system was 1I/’Oumuamua, a tumbling, shard-shaped object which was detected last autumn (2,3). Such objects were expected to behave like comets, and outgas as they approach the Sun. However, this object did not spray the solar system with its internal gases, leading astronomers to conclude that this object had originally been an asteroid which had been ejected from another star system. However, recent observations and work on 1I/’Oumuamua’s trajectory indicate that its motion is being affected by another factor beyond gravitational interactions – it is moving faster than it should (4). This is thought to be due to outgassing after all, leading to the conclusion that this object is an interstellar comet after all (5).
“Such outgassing is a behaviour typical for comets and contradicts the previous classification of `Oumuamua as an interstellar asteroid. “We think this is a tiny, weird comet,” commented Marco Micheli. “We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets.”
““We did not see any dust, coma, or tail, which is unusual,” explained co-author Karen Meech of the University of Hawaii, USA. Meech led the discovery team’s characterisation of `Oumuamua in 2017. “We think that ‘Oumuamua may vent unusually large, coarse dust grains.”” (5)
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…