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)  Read More…

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The Goblin Points to Presence of Planet X

The announcement of the discovery a new object in the outer solar system may bring us a step closer to the elusive Planet X (more recently dubbed Planet Nine).  This new dwarf object, known as 2015 TG387, is a distant member of the mysterious scattered disk of objects beyond the Kuiper Belt.  This particular object can travel so far away from the Sun during its orbit that it moves through the inner Oort cloud of comets, beyond 2000AU:

The newly discovered object is called 2015 TG387, is probably a small dwarf planet at just 300km across, and is incredibly far away. It is currently lying about two and a half times further away from the Sun than Pluto is.  It often reaches much further away. Its orbit takes it to about 2,300 AU — that is 2,300 times as far away from the sun as we are, and vastly more than the already huge 34 AU that the distant Pluto sits at.(1)

The object’s vast orbit is so vast that it takes about 40,000 years to do one circuit around the Sun.  Yet, its orbit is highly eccentric.  It distance from the Sun varies from 64AU at perihelion to 2037AU at aphelion.  Incredibly, then, it skirts both the Kuiper Belt and the inner Oort cloud, transiting between these quite distinct belts of objects.

As more objects are discovered between the Kuiper Belt and the inner Oort cloud (a torus-shaped disk of comets), the classifications of these objects are becoming more complex.  A significant factor is whether these objects have perihelia within 40AU, which might briefly bring them within the influential scope of the planet Neptune.  Extreme scattered disk objects fall into this category.  Significantly, 2015 TG387 is fully detached from this influence at perihelion, and may be considered to be an inner Oort cloud object.  Read More…

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The Moon’s Bombardment and Volcanism Combo

The Earth’s surface is subject to great change over geological time periods, due to the movement of tectonic plates, and volcanic activity, as well as long-term weathering and erosion.  As a result, craters caused by meteorite, asteroid and comet bombardment long ago is gradually eradicated.  We therefore look to the Moon’s cratered surface to provide clearer evidence of the bombardment history of the Earth/Moon binary.  That cratering history is then compared to other planets and objects in the inner solar system, allowing astronomers to discern patterns in cratering over long time periods.  One of the most significant events is the late, heavy bombardment.  Following  a period of relative quiet after the formation of the planets, this mass bombardment was thought to have occurred about 3.9 billion years ago:

“Competing models of meteorite-impact rate for the first 2 billion years (Ga) of Earth and Moon history. Note that Earth is believed to have formed about 4.55 Ga before present. Two hypotheses are shown: exponential decay of impact rate (dashes); and cool early Earth–late heavy bombardment (solid curve).” [see right-hand graph] (1).

More recently however, there has been a gradual realisation that this was not a sudden, dramatic event, but rather a sustained period of impacts by what were some colossal bodies:

“Recent high-resolution orbital data and images, more refined techniques for studying small lunar, terrestrial, and other impact samples and a better understanding of their ages, and improved dynamical models based on orbital and sample data have caused a paradigm shift in how we think about the lunar impact rate … The long-held idea of a “lunar cataclysm” at ~3.9 Ga is being replaced by the idea of an extended lunar bombardment from ~4.2 Ga to 3.5 Ga.” (3)  Read More…

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Proximal Planet Formation

Somehow or other (and it’s by no means clear how), some exoplanet gas giants whizz around their stars at great proximity.  The hottest of these objects so far discovered is an exoplanet named Kelt-9b.  It is a sub-brown dwarf of ~3 Jupiter masses.  It’s so close to its parent star that its rotation is tidally locked, and orbits the star in just 36 hours.  The temperature of its ‘dayside’ is over 4000 degrees C.  This remarkably high temperature is likely due to the immense amount of stellar radiation Kelt-9b is subjected to.  This temperature and stellar irradiation is driving off huge amounts of hydrogen from Kelt-9b’s atmosphere, creating an extended envelope of atomic hydrogen gas (1).  Other similar tailed gas giants have been studied before (2,3).  One can only imagine how spectacular this must look – a gas giant ‘comet’ streaming out a tail from near to or even within its parent star’s extended corona.

New analysis of Kelt-9b’s atmosphere has confirmed the presence of iron and titanium atoms within the planet’s atomic chemical soup (4).  It’s known that brown dwarfs can have cloudy atmospheres containing liquid iron rain, as well as other atmospheric dusts (5).  These dusty, cloudy atmospheres tend to form below 2,500 degrees Celsius, and then clear when the brown dwarf drops its temperature below about 1,500 degrees C.  Read More…

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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)

Read More…

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Planet Nine and the Kuiper belt

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…

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New Simulations Point to Oort Cloud Disturbance in Gemini

The shard-like asteroid from deep space which shot through the solar system last years, known as ‘Oumuamua, set many an astronomer’s heart racing.  The peculiar body was determined to be the first confirmed interstellar asteroid to have been observed (1).  It’s possible, though, that other comets which pursue so-called hyperbolic orbits (moving fast enough to escape the solar system) also have an interstellar origin, rather than having originated from the Oort Cloud.  A team of Spanish astrophysicists, who have more than a passing interest in the topic of Planet X, have performed powerful computer simulations to build up a picture of the trajectories and spatial origins of various hyperbolic comets (2).  The objects they chose to consider have inbound velocities greater than 1km/s

Following adjustment for the Sun’s own movement through space towards the Solar Apex, interstellar visitors would likely have a more or less random distribution to their radiants (the position in the sky from which they came, rather like meteor showers striking the Earth’s atmosphere).  The Spanish team carried out statistical analysis on the emerging sky maps of these radiants, and looked for patterns or clusters of these origin points.  Statistically significant patterns did indeed emerge from the data.  A particularly large source was located in the zodiacal constellation Gemini.  Such a clustering might indicate a number of possibilities, which the astrophysicists explore in their paper.

One possibility is a close flyby of a star in the past which could have disrupted the outer edges of the distant Oort Cloud, sending comets in-bound towards the Sun.  Looking at the tracking of candidate flybys in the (by Cosmic standards) relatively recent past, Carlos de la Fuente Marcos, Raul de la Fuente Marcos & S. J. Aarseth argue that there is a possible correlation between this cluster of hyperbolic orbit radiants in Gemini, and a close flyby of a neighbouring binary red dwarf system known as Scholz’s star some 70,000 years ago (2).  At a current distance of about 20 light years, Scholz’s star may be a close neighbour to the Sun relatively speaking, but even so it took a while for it to be discovered. This was probably because of a combination of factors:  Its proximity to the Galactic plane, its relative dimness, and its slow relative movement across the sky (3).  Its distance was less than a light year 70,000 years ago, and its rapid movement away from us in the intervening time helps to explain why it was difficult to detect as a neighbouring binary star:  Its retreating motion is mostly along our line of sight, making it difficult to differentiate from background stars.  Read More…

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‘Oumuamua’s Many Cousins

The interstellar asteroid 1I/2017 U1 (otherwise known as 1I/’Oumuamua) is fast receding into the distance, towards the constellation of Pegasus (1).  The existence of this rocky visitor from the stars was announced last October (1).  Its trajectory was too fast for it to be a solar system comet – even one from the furthest reaches of the Oort Cloud.  That was an exciting discovery, because that meant that 1I/2017 U1 was the first confirmed observation of an object arriving in the solar system  from deep space.

Although 1I/2017 U1 was initially considered to be an interstellar comet, that thinking changed when it failed to emit any gases as it performed its perihelion transit around the Sun (3).  This barren rock, confirmed as an interstellar asteroid (4), is now speeding away from the Sun.  It spent a relatively short time in the observation zone of professional telescopes, thanks to its great speed, but this was enough to reveal more weirdness (5).  It is an elongated object spinning head over tip, doing cartwheels through the solar system.  Some wondered whether it might be artificial, given the lack of coma as it traversed past the Sun.  But attempts to pick up signals from the object came up blank (6).  Still, its shape is nothing like any known body in our Solar System.  If solar system asteroids resemble rocky potatoes, then 1I/2017 U1 is more like an interstellar carrot, spinning haphazardly through our system.  To remain intact under these conditions, its internal structure must be robust (7).

The colour of our interstellar carrot is neutral with a reddish hue.  The colouration may be patchy across its surface. Solar system minor bodies (asteroids, Kuiper Belt Objects, Trojans) vary in colour, often dependent upon which population group any particular object belongs to.  Continuing my daft vegetable analogy, solar system potatoes come in different varieties.  Many are neutral in colour, some are reddish, others distinctly red.  Like comparing a Maris Piper to a King Edward.  If we compare 1I/’Oumuamua’s colouration to those of various classes of solar system objects, then it seems to most resemble those of the dynamically excited populations of Kuiper Belt Objects.  However, it is less red than the scattered Trans-Neptunian objects whose orbits extend beyond the heliopause (7).

Read More…

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New Infra-red Search for sub-Brown Dwarfs Planned

Brown dwarfs are notoriously hard to find.  It’s not so bad when they are first born: They come into the Universe with a blast, shedding light and heat in an infantile display of vigour.  But within just a few million years, they have burned their available nuclear fuels, and settle down to consume their leaner elemental pickings.  Their visible light dims considerably with time to perhaps just a magenta shimmer.  But they still produce heat, and the older they get, the more likely that a direct detection of a brown dwarf will have to be in the infra-red spectrum.

This doesn’t make them much easier to detect, though, because to catch these faint heat signatures in the night sky, you first need to have a cold night sky.  A very cold night sky.  Worse, water vapour in the atmosphere absorbs infra-red light along multiple stretches of the spectrum.  The warmth and humidity of the Earth’s atmosphere heavily obscures infra-red searches, even in frigid climates, and so astronomers wishing to search in the infra-red either have to build IR telescopes atop desert mountains (like in Chile’s Atacama desert), or else resort to the use of space-based platforms.  The downside of the latter is that the telescopes tend to lose liquid helium supplies rather quickly, shortening their lifespan considerably compared to space-based optical telescopes.

The first major sky search using a space telescope was IRAS, back in the 1980s.  Then came Spitzer at the turn of the century, followed by Herschel, and then WISE about five years ago.  Some infra-red telescopes conduct broad searches across the sky for heat traces, others zoom in on candidate objects for closer inspection.  Each telescope exceeds the last in performance, sometimes by orders of magnitude, which means that faint objects that might have been missed by early searches stand more of a chance of being picked up in the newer searches.

The next big thing in infra-red astronomy is the James Webb Space Telescope (JSWT), due for launch in Spring 2019.  The JSWT should provide the kind of observational power provided by the Hubble Space telescope – but this time in infra-red.  The reason why astronomers want to view the universe in detail using infra-red wavelengths is that very distant objects are red-shifted to such a degree that their light tends to be found in the infra-red spectrum, generally outside Hubble’s operational parameters (1).  Essentially, the JWST will be able to see deeper into space (and, therefore, look for objects sending their light to us from further back in time when the first stars and galaxies emerged).  Read More…

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Return of the Pioneer Anomaly

One of the many pieces of evidence put forward for the existence of Planet X over the last few decades is the so-called ‘Pioneer anomaly’.  The two Pioneer spacecraft were sent on an incredible voyage across the solar system, visiting a number of planets as they went.  They not only imaged these planets, but used the gravity of the planets to accelerate onwards, deeper into the solar system.  This gravity assist is often used to allow spacecrafts to pick up speed.  As the Pioneer probes travelled across the outer planetary zone and on towards the Heliopause beyond in the 1990s, it became apparent that the craft were not moving away from the solar system quite as quickly as the theoretical trajectory projections demanded.  Something was essentially slowing them down.  Additionally, similar effects were noted for the Galileo and Ulysses probes.

Many ideas were put forward, including either gravitational or physical interaction with clouds of interplanetary dust in the Kuiper Belt, or even the added gravitational tug of an undiscovered Planet X body.  One of the lead researchers into the Pioneer anomaly at the Jet Propulsion Laboratory  was John Anderson (1), who, interestingly, also had a longstanding interest in the possible existence of a Planet X body (2).  At one point, puzzled physicists began to wonder whether this marginal but definitive anomaly might require new laws of physics (3).  In the end, it was agreed by technical experts that the anomalous deceleration was a result of radiation pressure caused by non-uniform heat loss from the probes (4,5). Flights of fancy about missing planets and new physics were promptly put to bed.

Despite this, the anomaly seems to persist in the increasingly accurate navigation and telemetry data returning from various spacecraft performing flybys past the Earth (6).  Similarly, the Juno spacecraft, now orbiting fairly closely around Jupiter, is reported to be slightly misplaced from its expected position (7).  This has been determined by looking at the Doppler shift of ranging data from the probe as it circumnavigated the poles of the great gas giant.  Quixotically, Juno did not exhibit the same anomalous behaviour during a previous flyby of Earth.  This suggests that this is not, then, the result of an internal machination of the probe itself, as described for the Pioneer probes.  Instead, there does appear to be an unexplained external effect worth exploring:

Another mystery is that while in some cases the anomaly was clear, in others it was on the threshold of detectability or simply absent – as was the case with Juno‘s flyby of Earth in October of 2013. The absence of any convincing explanation has led to a number of explanations, ranging from the influence or dark matter and tidal effects to extensions of General Relativity and the existence of new physics. However, none of these have produced a substantive explanation that could account for flyby anomalies.” (8)  Read More…

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