Space Grease and Interstellar Objects

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)

This idea rests upon the erosion of the comet through interstellar space – the exact opposite of what I’m advocating.  From our local perspective, interstellar space begins beyond the heliopause, which means that all solar system objects lying outside the Sun’s heliosphere are in interstellar space, including any potential Planet X body.  A heavy coating of space gloop on 1I/’Oumuamua would readily explain the lack of cometary tail: The main body of the body is so covered by a thick coating of grease, which would have toasted nicely during perihelion, that the volatiles were locked in.  The dark red surface of 1I/’Oumuamua shows variation in tome and colour (6), which is probably attributable to thick coverings of organic materials irradiated in interstellar space.  Such a covering could seal in the comet’s volatiles, largely preventing the expected spectacular comet display at perihelion.  Rather like tarring a wooden boat to keep it watertight.  This comet/asteroid has been colliding with ‘space grease’ and other materials for as long as it has been tumbling through interstellar space, the amount of which seem significantly higher than previously thought:

“Until now there has been uncertainty over how much carbon is drifting between the stars. About half is expected to be found in its pure form. The rest is chemically bound with hydrogen in either a grease-like form, known as aliphatic carbon, or as a gaseous version of naphthalene, the main chemical component of mothballs.

“Helen Fraser, a senior lecturer in astronomy at the Open University, said: “It remains a major question in astronomy how dust forms, evolves and is destroyed.” Fraser said the work suggests that are many more “grease-like” molecules in space than previously thought. “The consequence could be important in how such dust grains stick and form planets, or even ‘seed’ planetary surfaces with the ingredients for the origins of life,” she added.” (1)

This raises the prospect of continuous accumulation of materials in interstellar space, as I have been arguing for a while (7).  Protoplanets ejected from young, chaotic planetary systems can continue to accumulate mass while floating freely in interstellar space, I argue.  This mops up interstellar materials, condensing their distribution into a more granular, i.e. dark planets.  We can’t see them, because they are not illuminated by local stars.  I argue that such objects are more pervasive through the galaxy than previously though, adding to it overall mass.  This explains where all this space grease is, which remains a mystery in terms of direct observation:

“The resultant aliphatic carbon [a hydrocarbon with no aromatic ring] column densities are least five times higher than some values reported previously. Using the two ISDA [interstellar dust analogues] integrated absorption coefficients, we obtained an abundance range between ~ 54−135 ppm for aliphatic carbon in the ISM [Interstellar Medium]. This leaves a substantial proportion of the dust-bound carbon to be found in aromatic or olefinic structures.” (8)

This space grease is surely an incredibly important component in planet-building, helping to explain why colliding materials don’t simply blast away from each other at the kinds of speeds they are travelling relative to one another.  Instead, they stick together.


Written by Andy Lloyd,  28-30th June 2018


1)   Hannah Devlin “Space is full of dirty, toxic grease, scientists reveal” 27 June 2018

2)   Andy Lloyd “‘Oumuamua’s Many Cousins” 21 January 2018,

3)  Elizabeth Landau “Chasing ‘Oumuamua” 27 June 2018

4)  ESO Pres release 1820 “ESO’s VLT Sees `Oumuamua Getting a Boost: New results indicate interstellar nomad `Oumuamua is a comet” 27 June 2018,

5)  Marco Micheli et al. “Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua)”. Nature, 27 June 2018;

6)  Meech, K. J. et al. A brief visit from a red and extremely elongated interstellar asteroid. Nature 2017, 552: pp 378–381

7)  Andy Lloyd “The Cumulative Effect of Intermittent Interstellar Medium Inundation Upon Objects In The Outer Solar System” 02/2016, DOI: 10.13140/RG.2.1.5112.5526,

8) B. Günay et al. “Aliphatic Hydrocarbon Content of Interstellar Dust” Monthly Notices of the Royal Astronomical Society, 13 June 2018,

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