Heavies in the crowd...
Image left: Weird and beautiful, a young star surrounded by a protoplanetary disk (seen edge on) vents its fury in twin beams of ions, seen as orange. Image courtesy of NASA.
We left our solar system under construction.There are no worlds yet, just a mish mash of gas, dust, ices, all busy accreting into weightless mountains of rock and ice, known to scientists as planetesimals. Planetesimals are diverse, cocky, young rowdies, and vary in behavior and character depending on how far they are from the sun. In the crowded lanes of the inner solar system, where the temperature is high, and collisions are frequent they are mainly made of metals or silicate materials. They tumble chaotically, and rumble about each other, seething in a dance of gravitational tag, high speed collision, coalescence, and erosion.
Image right: A planetesimals disk around Beta Pictoris, taken by the Hubble Space Telescope. Image courtesy of NASA.
Further from the adolescent sol they are made of more abundant, but easily evaporated, materials. That mainly means water ice (a very common material in the presolar nebula) but with significant amounts of things like ammonia, and simple (and some not-so-simple) organic molecules, like methane. And the space they move through is not utterly empty: tenuous but abundant, the gasses hydrogen and helium are too volatile to freeze out even in the furthest frigid planetary reaches, and remain as a cloak of gas about the protoplanetary disk.
Image above: A rough diagram of the protoplanetary disk, showing the frost line and the various zones. Image courtesy of the University of Hawaii.
The army of chaotic planetesimals are the bricks, and workforce, which subtle nature will use to make a new kind of creation: worlds that begin to resemble the worlds we see today; the protoplanets.
The party in the inner solar system:
We have already seen (see 'long road to worlds of rock and gas') places where planetesimals, ensnared by denser patches of gas and vortices, are beginning to coalesce into larger bodies. These are planetary seeds, and the beginning of an immense battle; a phase of runaway growth where every planetary seed tries to gobble up all the matter in its gravitational grasp. Put simply; A denser patch of planetesimals pulls more material in, in part through gravitational focusing. This makes it pull new material in even faster. While most of the weight of the disk is still being held as planetesimals the relatively small planetary embryos feel no limits on how much they can accrete, the supply seems endless.
The result is a feeding frenzy- but one that will sate no appetite. Gravity is hunger that only grows when fed.
As the embryonic worlds grow they mature; they become, in some ways, better behaved- wildly eccentric orbits that almost touch the young suns surface then freeze in the out near the frost line became moderated towards well behaved circles. Though many planetesimals still flew between the cool of the disks further reaches and the fires of an irritable young star, over time the largest protoplanets orbited neatly around their young solar master in an almost civilised fashion.
What had happened? Why would the offspring of rowdy crowds of rock and ice decide to follow nice orderly routes and lanes?
A phenomena that occurs on all scales, from gas molecules to growing planets was sitting at the root of it: Dynamical friction. This is analogous to ordinary friction felt by any hefty object (protoplanet) moving through a sea of light and welter weights (planetesimals and planetary seeds). The difference here is that the 'friction' is a result of gravitational interactions, not physical collisions, although there would have been more than enough of those to.
Above: a simulation demonstrating the effects of dynamical friction on two large objects, orbiting each other in a swarm of smaller ones. Over time the smaller objects rob the larger ones of their energy, and they spiral into each other. In the protoplanetary disk, objects that lost all their energy this way would have spiralled into the developing sun! Video courtesy of Idius, via YouTube
The maturing protoplanet transfers momentum to the planetesimals, slowing itself down, and in turn 'heating' them. Heating in this case means increasing their average velocity. This as two important effects: it makes the larger body more likely to find its next meal, as a slower moving object is more likely to interact gravitationally with a planetesimal, and it makes the protoplanets orbit more circular.
Of course not all encounters between protoplanets and planetesimals were polite handshake exchanges of kinetic energy: remember the little worlds consume planetesimals, and occasionally merge with one another - if you can call a collision between two partly molten balls of rock, each a thousand km across, being bombarded with 1 to 100 km wide planetesimals falling like hard raindrops, a 'merger'
Out beyond the frost line:
Image left: Jupiter (left) and Saturn (right), worlds formed by the extreme bloating of what we Earthlings would call atmospheres. Image courtesy of the National astronomical observatory of Japan.
The outer solar system is both a richer and poorer place than the inner: because the sun is much further away the temperature is much lower, but so the wealth of solid matter for a growing protoplanet to glut on is much greater, as many volatile materials like methane, ethane, water and CO2 are available as rock hard ices.
Out here it is quieter, with lower orbital speeds, but the protoplanets form by the same mechanism as in the inner solar system at first. Because of the wider spaces they can clear out broader lanes, becoming bigger. Much bigger: Out in the vastness even a protoplanet is a sphere of ices and rock weighing as much as 15 Earths.
And here the process of planet birth diverges 'twixt the far outer, outer and inner system: The inner planets get as big as they can on solids and then they stop growing.
But where it is dark, in the cold lonely places, there is another way to grow: Guzzling gasses, the two most available of which are hydrogen and helium, inherited from the solar nebula. Having reached the 15 Earth mass point the consumption of solid planetesimals begins to tail off - but while the proto-giant has been gathering ices and rock it has also, slowly, gently, started to pull down the lighter gas. Its great size lets it do this using a gravity well much deeper and fiercer than an inner system world can aspire to.
As the rate of solid accretion tails off the rate of gas accretion accelerates: the protoplanets solid core is quickly lost under an dull, fuzzy bordered, ocean of gas. At this point the planet is not really separate from the protoplanetary disk, it is more like a great gale of gasses blowing inwards to the central point of the core. The mass of the solid core is also increasing, albeit more slowly. Radioactive energy and accretion heat it to the point of melting.
At around the point in time where the mass of the gas slurped up is equal to the mass of the now tacky and molten core, the gas guzzling goes runaway- it outstrips the ability of the protoplanetary disk to bring new gas into the region and the new giant planet quickly develops a well defined boundary, with raw gas falling towards its new surface.
It has left the nebula stage, where it was a part of the greater whole of the protoplanetary disk, and is now in the transition stage. Now it is an entity in its own right, but still feeding from its huge pancake shaped parent. Eventually all the gas in the gravitational reach of the giant is absorbed into its great body.
It has cleared its feeding zone of both solids and gas, and has opened a gap in the protoplanetary disk visible for light years.
Eight million years have passed from the cores formation, and the immense churning world is now in the isolation stage. From here on, by virtue of its immense size, few things can touch it, it will evolve from here along its own course.
Secret palaces of ice:
Image right: Uranus and Neptune, in natural color, and enhanced to bring out details. Image courtesy of JPL, NASA.
Still further out from the still sputtering sun the mechanism of giant building changes, becoming more mysterious. Beyond the orbit of the accumulating mass of Saturn the density of the disk drops, and the seething motion of the planetesimals is even slower. This dictates that cores of solid material out here form more slowly- too slowly in fact, as within a few millions of years, ten tops, the protoplanetary disk will have lost most of its abundant gasses to the innermost gas giants voracious appetites or to stellar winds.
So alternative mechanisms have been fielded, and as the ice giants lie in the far and alien reaches it might be some time before we get solid answers:
1: Uranus and Neptune formed closer to the sun: In this model the two ice giants began their existence on between the orbits of Jupiter and Saturn, first as massive cores then clothing themselves in gasses, but a gravitational battle between the four heavies saw Neptune and Uranus expelled to the farthest reaches, while Jupiter and Saturn occupied the 'sweet spot' closer to the sun.
2: The ice giants formed by an entirely different mechanism, disk instability, where a portion of the disk in that region began to spontaneously collapse under its own gravity. This model has Neptune and Uranus rapidly growing to much bigger than they are today, so some immense event must have occurred to denude them. One possibility comes from the familly of stars that the sun was born a part of: if a blue supergiant star had formed nearby then it could have irradiated the two outer orbs with intense radiation, evaporating their outer layers and expelling them into space.
Whether the answer is one of these, or something we have not imagined yet, only time and further exploration can tell.
Beyond the planets: The Kuiper belt.
Image left; Pluto, the best known world in the Kuiper belt, and its three moons.Image courtesy of hubblesite.org.
Now we are out on a cold limb: out beyond Neptune lie the mysterious and icy worlds of the Kuiper belt. These range in size from dwarf planets, like Pluto and Eris, to tiny chips of ice. It has been speculated that these shivering stragglers are original planetesimals ejected from the main protoplanetary disk, but their exact origin and formation is shrouded in darkness and distance from Earth. In 2015 the New Horizons mission could shed some light on the mysteries when it reaches Pluto. And just last week, at DPS 2010, one idea on where they came from was dealt a blow by new research.
Above; an animation of the New Horizons mission, courtesy of unbelievablefootage via youtube.
Beyond the Kuiper cliff.....
Next to the bulk of the ice giants, the gas giants, and even the inner rocky worlds, this realm of tiny icelets seems ethereal, scarcely present at all. But it may conceal something rather more heavy weight: The Kuiper belt is rudely terminated at a distance of 48 au from the sun, in one of the solar systems biggest mysteries;one explanation under consideration is the presence of a frozen world perhaps as large as Earth. Even more tantalizing: some have argued that the long period comets, rather than being from random points in the sky, are following a band across it. This could be the signature of a heavy weight companion to our sun, perhaps even an old, cooled, brown dwarf. If either of these were to be confirmed it could be a Rosetta stone in understanding our solar systems history.
Image left: A diagram of the Kuiper belt, Kuiper cliff, and Oort cloud. Image courtesy of finalfrontier.com.
A calm before a renewed storm:
Back in the inner system, after a few tens of thousands of years, the party's over: It was its own success that finally doomed the runaway growth phase- the protoplanets cleared out their own feeding zones, five hill sphere radii on either side, and became so heavy (more than 100 times the weight of a typical planetesimals) that their own gravity began to choke them. Now the largest protoplanets, of which there were around a hundred, had gravity so strong that it stirred the planetesimals into a frenzy of motion, moving them too fast to capture and accrete easily. The stronger the gravity, he harder the stir, and the more the growth slowed. The larger objects put themselves out of the planetesimal market, allowing the smaller ones to catch up.
This phase of growth is known as the Oligarchic phase, and those, mostly long destroyed, worlds that were a part of it are known as the Oligarchs.
Over tens of thousands of years, through a combination of gluttonous planetesimal consumption, and occasionally colliding with and combining with their kith and kin, the Solar Oligarchs eventually settled, hefty and content, into a state of relative calmness: the hundred or so largest worlds, ranging in size between our moon and the planets mars (1/100th to 1/10th of Earths mass), each sat serenely inside their cleared feeding zones, separated by glittering bands of surviving planetesimals. There were still collisions of incredible violence, but compared to the manic times leading up to this point these worlds were fairy-tale stable.
These were miniature planets in their own right, kept warm and differentiated by radionucleide decay, had their own geology, volcanoes, geysers, monutains, craters and even short lived atmospheres and subsurface fluids.
Imagine looking up to a sky filled with the solar system of that time! Not four terrestrial worlds seperated by darkness, but hundreds separated by the glittering bands of surviving planetesimals, and beyond them the world consuming gas oceans of the gas giant planets. Some Oligarchs would even have been co-orbital, essentially sharing the same orbits.The sky would have been a non-stop frenzy of activity by today's standards, with collisions, devastating planetesimal impacts, vast eruptions and lakes of molten materials visible on the nearest neighbours.
But, as with the runaway growth before, and the reign of the planetesimals before that, the good days were numbered. Now over half the mass of the protoplanetary disk was consumed by the Oligarchs, and the density of planetesimals was dropping below a critical point. The process of dynamical friction was no longer strong enough to restrain their orbits: they began to wander, farther and farther afield, their orbits becoming more and more elliptical, at first scattering the remaining planetesimals like fairy dust, and then, perilously, crossing each others orbital tracks.
After 50 million years of building worlds, a demolition derby, involving part-evolved worlds with structures and masses comparable to a modern planet, was about to begin, and there would be few survivors.....
How do we know this?
This is a murky period of solar system history: We have what we think are examples of surviving objects from various points along the time line, and we have observations of other star systems forming. On top of that we have meteorite evidence from various falls around the world, and these give us a chance to examine the rocks of these long obliterated worlds in detail.
Above: An Iron meteorite comes down over Canada, courtesy of Canadian police force, via YouTube.
There are a number of little worlds believed to be survivors from this time: The protoplanets Ceres, Vesta, Pallas and Hygiea are probably among them, and are close enough that the first two are due to be visited by the Dawn spacecraft in 2011 and July 2015 respectively. Observations from Earth, using both ground based telescopes like the Keck observatory, and space based ones like the Hubble, have been able to glean some hard won details from these tiny worlds, and we must fill in the blanks with educated guess work and computer simulations.
Of the four biggest survivors, each has a different character. The solar system of this time was a plethora of unique worlds:
Image above: Ceres. Image courtesy of NASA.
A mixed world of ice covering a rocky core, and the largest surviving protoplanet in the inner solar system, Ceres is believed to have warmed enough to sustain a subsurface water ocean that might have persisted for billions of years.
Above: Vesta spins in space, in this accelerated footage made using many shots from the Hubble space telescope. Courtesy of NASA and JPL.
A gobstopper of a volcano world, once molten down to its core, now covered in basaltic lava fields, and with its entire south pole apparently removed by a huge impact.
Image above: Pallas as seen from the Hubble. Image courtesy of Hubble space telescope.
Pallas seems the least evolved of the three, with varied surface markings indicating the action of water, and a non spherical shape indicating that its density was just a little to low to pull it into a spherical shape. It is not being visited by Dawn, although an extended mission flyby has been rumoured.
Image above: The shape of Hygeia as calculated from its lightcurve.
Another world which seems to have escaped the ravages of volcanism lightly, leaving a fairly pristine piece of early solar system relic. But hydrated materials on the surface suggest that it may have seen enough heat for liquid water.
Some of the meteorites that have come to Earth have had fascinating tales to tell, and have hinted at geologically active, and varied worlds perhaps with magnetic fields, volcanoes, and histories uniquely their own:
Image above: the Murchison meteorite. The white specks are calcium aluminium inclusions, some of the first solid objects ever to form in the solar system. Courtesy of meteorites.com.
Named after the town where it landed, this small rock has been analysed as much as any we have found. The Murchison meteorite is a carbanaceous chondrite, made of relatively unaltered solar system materials, but these have been changed by water rich fluids passing through them. It is rich in carbon and a bewildering array of molecules believed to play a role in the development of life have been found in it: It contains more amino acids - many more - than life as we know it uses!
Image above: The Orgueil meteorite Image courtesy of the planetary science research division, University of Hawaii.
Another cabanaceous chondrite, which fell over France as around 25 pieces in 1864. Its composition is incredibly similar to that of the sun, aside from the elements H and He, suggesting it is almost pristine solar nebula and presolar material. It may be from a comet. This has made it a rich source of knowledge on the early years and formation of the solar system. It was once involved in a hoax designed to make it appear that it carried life forms.
HED (Howardite-Eucarite-Diogenite) Meteorites:
Image above: A HED meteorite. Image courtesy of saharamet.com.
These come from parent bodies that experienced a lot of internal heating, melting and processing these rocks in the same fashion that Earths igneous rocks are processed beneath our feet. For this reason the most likely origin for these rocks is the protoplanet Vesta, which is covered in frozen lava flows, suggesting a volcanic history, and which suffered a huge impact sending large amounts of it into space.
 GRA 06128:
Image above: A section of the GRA 06129 meteorite.Image courtesy of NASA.
An andesite rich meteorite, which fell in Antarctica. The andesite is rich in feldspars, and the composition of the rock is startlingly similar to that of Earths continental crust- although analysis of the oxygen isotope ratios within it confirm that it is from off world. The best guess is that the little traveller came from a world over 100km across- lage enough for partial melting of the innards, and for vents to heap and andesite crust on the surface, but too small for full differentiation- or even probably becoming round as 21 Lutetia shows below:
Image above: Am angrite found in northwest Africa. Image courtesty of meteoritestudies.com.
These also seem to have come from a differentiated body, and are basaltic rocks, mostly made of the augite with some alivine and troilite. Angrite studies have revealed a new aspect to the protoplanets: Fossil magnetic fields in three angrites suggest that some protoplanets able to generate their own magnetic field, an amazing feat for such small objects.
Nickel -Iron meteorites:
Image above: A cut and etched nickel-iron meteorite, showing the triangular pattern only seen in this type. Image courtesy of arizonaskiesmeteorites.com
As the name suggests these little 'ard nuts are made mainly of nickel and iron, heavy elements suggesting that hey came from the cores of worlds that were warm enough to differentiate and then got pulverised by a blast big enough to splatter their cores across space. These are testaments to the incredible violence of the early solar system. These meteorites are also notable for being very tough- most meteorites explode in the atmosphere, these will tend to punch right through!
The Kaidun meteorite:
Image above: A section of the Kaidun meteorite. Image courtesy of NASA.
On the 3rd of December 1980 this strange rock came down near a soviet army base. It contains over 60 different minerals, some of which have been found nowhere else in nature. Its origins remain a mystery, although the Martian moon Phobos has been suggested.
Next in this series: The wars of worlds.
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