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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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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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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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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The Origin of Ancient Xenon

I’ve often discussed the origin of various elements and compounds on Earth – most notably the isotopic ratio of water, and what that might tell us about the origin of terrestrial water (1).  Data about this can help provide evidence for the Earth’s early history, and often the data is inconsistent with the general theories of oceanic origin, like the ‘late veneer theory’, for instance, where the bulk of terrestrial waters were supposed to have been supplied by comets.  It turns out that the water was on this planet all along (2,3), raising questions about why the Sun’s heat had not driven this relatively volatile resource away from the primordial Earth during the early history of the solar system.

waterworld

Despite such evidence, the ‘late veneer theory’ continues to hold ground for many scientists, and tends to go unchallenged within the science media.  This is apparent within the following excerpt about a new paper on the mysterious presence of a particular isotope of the noble gas xenon found in ancient terrestrial water encased in rock:

“The scientists have been analysing tiny samples of ancient air trapped in water bubbles found in the mineral, quartz, which dates back more than three billion years. The team found that the air in the rocks is partly made up of an extremely rare form of the chemical element, xenon. It is known as U-Xe and what makes it so rare is that it isn’t usually found on Earth. The component is not present in the Earth’s mantle, nor is it found in meteorites.

“Therefore, the team believe that the U-Xe must have been added to the Earth after a primordial atmosphere had developed. Simply put, comets are the best candidates for carrying the U-Xe to the planet. Co-author, Prof Ray Burgess, from Manchester’s School of Earth and Environmental Sciences explains: “The Earth formed too close to the Sun for volatile elements, such as U-Xe, to easily condense and they would have rapidly boiled off the surface and been lost to space.

“”The reason that oceans and an atmosphere exist at all is because volatiles were still being added after the Earth formed. The puzzle is in identifying where the volatiles came from and what objects carried them to the early Earth. The difficulty is that many of the different volatile ingredients that were originally added have been thoroughly mixed together by geological processes during Earth’s long geological history.”” (4)

asteroid_impact

It turns out that xenon, in general, is mostly absent from the Earth’s atmosphere, particularly compared to other noble gases like argon.  No one knows why.  Perhaps the missing xenon is encapsulated within rocks buried deep within the Earth.  Or perhaps, conversely, it has been driven off the Earth because it is not easily captured by rocks like perovskite (5).  Xenon is missing from Mars, too, which may allude to its propensity for loss from a weak atmosphere.

Read More…

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Red Giants and Planet Formation

This article will explore the potential for life to develop in the outer planetary systems of red giant stars.  It will then discuss the death-throes of red giant stars, and whether the subsequent outward thrust of stellar material might provide another mechanism for free-floating planets in interstellar space.

Exoplanets have already been found orbiting extremely old stars, one some 11 billion years old (1).  This star, named Kepler-444, makes our own Sun, at a mere 4.6 billion years old, seem like an infant in comparison.  The implication of this is that life could readily have got going early on in the history of the universe, long before the birth of our Sun.  Furthermore, if these exoplanets were to benefit from a relatively stable stellar environment during that long timescale, then the chances of life evolving into higher forms are statistically more probable.  Scale this up across trillions of stars, and the possibilities become clear.

Our own Sun has a shorter lifespan than this.  Its main sequence life is expected to last another 5 billion years, by which point it will have burned up all of its hydrogen fuel.  Then it will swell into a red giant star, before collapsing down into a white dwarf.  For Earth, this post-main sequence (post-MS) phase of the Sun’s life will be pretty disastrous.  The Sun’s expansion to a red giant will swallow the Earth up.  However, a less catastrophic outcome might be expected for planets in the outer solar system, beyond, say, Jupiter.  In fact, their climates might significantly improve – for a while, at least.  The habitable zone of the solar system will expand outwards, along with the expanding star.  Saturn’s largest moon Titan, for instance, might benefit greatly from a far milder climate – as long as it can hang onto its balmy atmosphere in the red heat of the dying Sun.

red_dwarf_landscape

The expansion of habitable zones, as late main sequence stars become hydrogen-starved, offers the potential for life to make a new start in previously frigid environments.  The burning question here is how long these outer planets have to get life going before the red giant then withdraws into its cold white shell.  A study published last year by scientists at the Cornell University’s Carl Sagan Institute attempted to answer this question (2), choosing to examine yellow dwarf stars whose sizes range from half that of the Sun, to approximately twice its mass.  They argue that the larger stars along this sequence could well have larger rocky terrestrial planets in their outer planetary systems than our Sun does (at least, insofar as we know it does!)  This is because the density of materials in their initial proto-planetary disks should be that much greater for larger stars (3).  Larger Earth-like planets in outer regions mean more potential for stable atmospheric conditions during the post-MS period under consideration.  In other words, the growing red giant (which is shedding its mass pretty wildly at this point) would not necessarily blast away an outer planet’s atmosphere if that rocky planet had sufficient gravity to hold onto it.

Read More…

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More Dark Stars than Stars in Milky Way

For some time, astrophysicists have argued over how many Dark Stars there might be in the galaxy, with varying opinions.  (Note that astronomers use several different names for these objects: sub-brown dwarfs, Y Dwarfs, ‘planemos’).  In this short article, I argue that new evidence presented about the stellar populations of open star clusters point towards there being more Dark Stars than stars in our galaxy.

When I use the term ‘Dark Star’ in my book (1) and internet articles, I’m generally referring to gas giant planets/ultra-cool dwarf stars which are several times more massive than Jupiter, up to perhaps ~13 times as massive (at this point, the gas giant begins to burn deuterium and is reclassified as a brown dwarf).  Most examples of these objects (perhaps more than a few million years old) are essentially dark.  By contrast, very young examples light up more brightly, because they still retain some heat from their formation.  It’s a curious quirk of nature that these sub-brown dwarfs are actually smaller in size than Jupiter, despite being heavier.  Because these objects are so small, and so dim, they are extraordinarily difficult to observe.  Some have been found, but they are usually either extremely young (and therefore still burning brightly), or are exoplanets discovered orbiting parent stars (and so detectable through gravitational ‘wobble’ effects, or other means of finding massive exoplanets).

It has been my contention for some time that the populations of these objects are significantly underestimated.  It is recognised generally that these ultra-cool dwarf stars may be free-floating objects in inter-stellar space, often as a result of having been ejected from young star systems as the fledgling planets in those systems jostle for position.  Opinions about their numbers vary greatly among astrophysicists.  There may be twice as many of these objects as stars, according to studies involving gravitational microlensing surveys of the galactic bulge (2).  Other studies conflict with this conclusion, arguing that there may be as few as 1 object of 5-15 MJup size per 20-50 stars in a cluster (3).  This discrepancy is important because the difference is perhaps as high as two orders of magnitude, and this ultimately affects our understanding of how many free-floating Dark Stars we can expect to find out there.

Their mass, lying between that of Jupiter and the deuterium-burning limit at about 13 MJup (4) seems to single Dark Stars out as rather special objects:

“An abrupt change in the mass function at about a Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.” (2)

Therefore, if the number of free-floating sub-brown dwarfs (also sometimes known as “planemos”) is on the high end of expectation, then it means that there are also likely to be far more of these objects in wide, distant orbits around their parent stars.  This, in turn, increases the likelihood of there being a similar Dark Star object (or more) in our own immediate solar neighbourhood.  Read More…

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Seven Planets Found in Red Dwarf System

NASA made a big announcement this week about new exoplanets found orbiting the dwarf star TRAPPIST-1 some 39 light years away.  I’ve discussed this particular dwarf star system before (1), as it was already known to have three terrestrial planets in attendance orbiting very close to this cool, fairly dim star (2,3).  The dwarf star is approximately one tenth the size of the Sun, and it’s mass places it on the border between a brown dwarf and a red dwarf star.  Unusually for a star this small, TRAPPIST-1 has a high metallicity, which actually exceeds that of the Sun (4).

Now, an international team of astronomers, using the Belgian TRAPPIST telescope in Chile and the Spitzer infra-red space telescope, have released details about a further four terrestrial planets in this mini-star system, three of which (e, f and g) are located within it’s habitable zone, where temperatures favour the presence of liquid water (5):

“Researchers led by Michaël Gillon, of the University of Liège in Belgium, have been studying the infrared light emitted by this miniature star and have detected drops in luminosity characteristic of transits, i.e. the passage of astronomical bodies moving across its face.  As early as 2015, the first three planets (dubbed b, c and d) had been identified.  Tracking the system using TRAPPIST and the space telescope Spitzer, the team was then able to identify four others planets (e, f, g and h) in 2016.  Based on the frequency of these transits and the degree of reduction in luminosity of the star, they have demonstrated that these seven planets are all comparable in size to Earth (to within 15%), and orbit very close to their star.” (6)

Read More…

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Radio Bursts from Space

I recently reviewed a book about Carl Sagan’s interest in ancient aliens, written by Donald Zygutis (1).  Early on in his illustrious career, Sagan expressed scepticism about seeking E.T. life using radio telescopes, instead advocating a search through historical accounts and myths to determine whether our planet had been visited (2).  He argued that in a standard galaxy there are so many stars/planets etc, that all you’d need to do is point the radio receiver at any given galactic source beyond the Milky Way, and alien radio signals should come screaming out at you.

sagan

They generally don’t, of course, which led Sagan to the early logical conclusion that SETI’s search with radio telescopes was bound to fail.  However, this approach became the only game in town, with serious funding at its disposal, and Sagan fell into line behind it – supporting this doomed search for E.T. radio signals ostensibly from stars within out galactic neighbourhood.

vla_nm

Decades on, and SETI has come up with little of any merit.  The odd interesting blip, sure, but nothing demonstrably repetitive, or intelligent.  Other searches have also come up empty-handed, including an extensive search for highly advanced galactic civilisations using infra-red (3), based upon the theories of the physicist Freeman Dyson.  Looking for an infra-red signature from other galaxies seems like a bit of a stretch to me.  Sagan’s initial premise about radio waves emanating from other distant galaxies is more plausible.  By staring at the tiny amount of our sky that any given distant galaxy occupies, radio telescopes can cover a lot of possible stars in a very small space.  If any of them contain radio-emitting alien species, shouting for attention, then we should pick them up one would have thought.  Read More…

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