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)