How Dust Clumps Together in Space
One of the essential ingredients of planet-building is the clumping of dust in space. Planets can build up through the gravitational attraction of objects in space which are already about 1000km across. The problem is how do these proto-planetessimals get built? The mechanism for how dust clumps together has not been well understood. After all, when materials moving at speed through space collide, they may break apart in the force of the impact, showering down collisional cascades of ever small materials – the exact opposite of planetessimal-building. Somehow, dust must clump together into grains, which then join forces to create space pebbles, then boulders, then mountains, etc.
For these materials to adhere together, an inherent stickiness may be needed, aided by the presence of greasy organic compounds (in the form of aliphatic carbon). While it is recognised that this greasy component is more readily available in interstellar space than previously suspected (1), does that adhesive property extend down to space dust? If not, what mechanism could be bringing together ever larger clumps of plain old granular dust in space?
New research work suggests that dust and gas are not happy bedfellows within a magnetic field. So, rather like oil in water, dust particles seem to come together within gas as the mixture traverses the galactic tides. Indeed, any force brought to bear on dust moving through gas seems to create this clumping effect:
“… it was previously assumed that dust was stable in gas, meaning the dust grains would ride along with gas without much happening, or they would settle out of the gas if the particles were big enough, as is the case with soot from a fire. “…dust and gas trying to move with one another is unstable and causes dust grains to come together,” says [Phil] Hopkins [Professor of theoretical astrophysics at Caltech]...These gas-dust instabilities are at play anywhere in the universe that a force pushes dust through gas, whether the forces are stellar winds, gravity, magnetism, or an electrical field.” The team’s simulations show material swirling together, with clumps of dust growing bigger and bigger.” (2)
Computer simulations looking at how dust moves through magnetized gas seems to show this clumping effect as a general mechanism. The dust grains are like boulders in a fast moving and turbulent river (the gas within a moving stream of magnetized material). As the flows wrap around these grains and pull them back and forth, the grains have a tendency to coalesce, forming ever larger clumps. This is not just applicable to planet formation in proto-planetary disks, but may also extend to interstellar space:
“As examples, we introduce several new instabilities, which could see application across a variety of physical systems from atmospheres to protoplanetary disks, the interstellar medium, and galactic outflows.” (3)
Well, well. Regular readers of the Dark Star blog will recognise that this is a theme that I have been exploring for a few years, in an effort to explain how planets might form in interstellar space. I have wondered for some time how a substantial Planet X body might have formed beyond the extent of the Sun’s early proto-planetary disk. I realised that there must be a missing mechanism for planetary formation – that it needn’t just be taking place in proto-planetary disks (which it clearly is), but that a slower mechanism for planet-building might also be taking place outside these zones. This was not self-evident, because the Sun clears out the zone around it of dust and gas through the action of the solar wind. The solar wind impedes slow planet-building mechanisms within the planetary zone. Because the planetary zone is what we can observe, we are none-the-wiser as to what’s going on beyond it.
Beyond the heliopause, the mechanism for dust extraction is not taking place in the same way. In interstellar space, particularly within large gas clouds, there should be scope for gradual planet-building. That could extend, intermittently, to the outer solar system, particularly when the solar system moves through gigantic molecular clouds, nebulae, and the like. I wrote this up in a paper in 2016 (4), and have included broad discussions about it in my new book ‘Darker Stars’, which will be published soon. In 2016, I submitted my paper for publication in MNRAS, but it was rejected on the grounds that it lacked scientific rigour (fair enough – I don’t have super-computers at my disposal, after all) and wasn’t adding anything new (this reason really puzzled me – I have not come across the concept of planets forming in interstellar space anywhere else in the literature).
Now, thanks to Professor Hopkins et al., we have some science around this clumping mechanism which could significantly reinforce my arguments. Happily, this new work moves this argument forward significantly. I would suggest that there are significant implications leading on from this. I’ll set them out briefly (much more detail in my forthcoming book ‘Darker Stars’ (5)):
- Planet X-type bodies can grow slowly over time because, beyond the heliopause, they reside in interstellar space where dust may clump, free of the action of the solar wind.
- Because this is a general principle, any older star system may have one or more wide-orbit planets beyond its heliopause, perhaps a number of them.
- This mechanism could lead to the formation of an abundant supply of dark, free-floating planets in interstellar space. These need not have been ejected from young planetary systems, but could be native denizens of interstellar space.
- Continued dust clumping in the vicinity of substantial Planet X bodies will be aided by the juxtaposition of moving interstellar medium and their powerful planetary magnetic fields.
- This will lead to the accumulation of shrouds of dust around substantial Planet X bodies (6). The same would apply to free-floating planets.
- These localised shroud-like nebulae will impede efforts to observe the planets in infrared. They will also lead to misidentification of these phenomena during surveys in visible light (7)
- This would then explain why the presence of a Planet X object can be strongly inferred from indirect evidence, yet continue to evade direct detection.
This argument seems logical and coherent to me. It seems that science is now providing the mechanisms whereby it could be substantiated. It also seems something of a coincidence that this work is emerging from Caltech, now so famous for the Planet Nine hypothesis (8). Is it part of a similar push to understand how Planet Nine could be? I would argue that this helps beyond providing the mechanism for its formation (as important as that is). It will also explain why we can’t seem to find it.
Written by Andy Lloyd, 12th October 2018
1) Andy Lloyd “Space Grease and Interstellar Objects” 30th June 2018, http://www.andylloyd.org/darkstarblog63.htm
2) Whitney Clavin “How the seeds of planets take shape” 10 October 2018, https://phys.org/news/2018-10-seeds-planets.html with thanks to John
3) J. Squire et al. “Resonant Drag Instability of Grains Streaming in Fluids”, The Astrophysical Journal, 23 March 2018, 856:1
4) 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, https://www.researchgate.net/publication/293488683_The_Cumulative_Effect_of_Intermittent_Interstellar_Medium_Inundation_Upon_Objects_In_The_Outer_Solar_System
5) Andy Lloyd “Darker Stars” 2018, Timeless Voyager Press; in press
6) Andy Lloyd “The Shroud Hypothesis ” 12 January 2015, http://www.andylloyd.org/darkstarblog22.htm
7) Andy Lloyd “The Shroud Hypothesis as part of a Dark Star Solution” 9th February 2016, http://andy-lloyd.com/the-shroud-hypothesis-as-part-of-a-dark-star-solution/
8) K. Batygin & M. Brown “Evidence for a Distant Giant Planet in the Solar System” 20 January 2016, The Astronomical Journal, 151(2) http://iopscience.iop.org/article/10.3847/0004-6256/151/2/22