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) Read More…
One of the pair of Caltech scientists who announced in January that there was a very high probability that a ‘super-Earth’, dubbed ‘Planet Nine’, exists beyond Neptune (1,2), has noted that a newly discovered eccentric Kuiper Belt Object cuts down the possibility that they were wrong still further.
“The object [uo3L91] shares some of the same behavior as the other six Kuiper Belt bodies, suggesting it has also been pushed by a large planet that is between 200 and 1,200 times the distance from the Sun to Earth. The object was discovered by the Canada France Hawaii Telescope, which is conducting the Outer Solar System Origins Survey (OSSOS); information about its movements were presented recently by astronomer Michele Bannister at the SETI Institute.” (3)
A week on from Caltech’s announcement, Dr Mike Brown and Dr Konstantin Batygin, the two astrophysicists proposing the existence of their ‘Planet Nine’, sketched out the range of orbits which their object might be moving through, including its all-important approximate perihelion and aphelion positions. Essentially, Brown and Batygin consider the perihelion position of the Planet Nine body to be in a broad region around the zodiacal constellations Scorpius/Sagittarius (R.A. = ~16hrs), whilst the aphelion positionof Planet Nine is likely in the equally broad Orion/Taurus area (R.A. = ~4hrs) (1,2).
Image credit: Caltech
We seem to be getting very close now to a discovery of a massive Planet X in the outer solar system. I heard this report on the evening BBC news, a slot which indicates the seriousness with which this subject is now being taken by the scientific community:
“American astronomers say they have strong evidence that there is a ninth planet in our Solar System orbiting far beyond even the dwarf world Pluto. The team, from the California Institute of Technology (Caltech), has no direct observations to confirm its presence just yet. Rather, the scientists make the claim based on the way other far-flung objects are seen to move. But if correct, the putative planet would have 10 times the mass of Earth.