The long road to worlds of rock and gas begins:
Image above: Formalhut B, a relatively young planet, [1]orbits amid the left over icy planitesimals at its star systems edge. Image courtesy of NASA and the Hubble Space Telescope (HST)
The Sun shines at last:
Our Sun has formed, from a collapsing cloud of gas and dust. Surrounding it is a pancake shaped disk of gas and dust, known as the [2]protoplanetary disk. Our relatively humble (by stellar standards- see the previous post) has reached the [3]t-tauri phase, still shining by gravitational compression, callous and temperamental, belching out strong stellar winds, [4]unpredictable monster flares, and still carving great gobbets of gas out of the protoplanetary disk with its [5]intense magnetic field. This disk is the remnants of the solar nebula, now weighing in at less than 2% of the mass of the seething t-tauri star at ts center.
Image above: a young (3 million years old) protoplanetary disc around AB Aurigae, a pre-main sequence star. Inset: a region where a giant planet or a brown dwarf may be forming. Image courtesy of The American Museum of Natural History.
Birth by inferno:
This disk is glowing bright pink to begin with: temperatures soar up to 3000 degrees kelvin, but it is about to cool rapidly: the newborn sun gives out a squeal: a stellar wind of incredible intensity, the [6]x-wind. This furious gale of ions drives away much of the gas, chasing it to the depths of interstellar space. Without the extra gas and dust the disk becomes more translucent, and so no longer absorbs as much of its solar masters energy, or retains as much heat. The temperature of the disk drops rapidly, and materials that were previously kept in the gas phase by the intense heat begin to condense into tiny grains of slid matter. These [7]grains are finer tan smoke, but they are the first direct solid matter ancestors of the planets which grace our skies today.
Image above: A section of cometary dust, thought to be primordial matter from the protoplanetary disk. Image courtesy of NASA.
An entrenched mystery:
Here our story becomes less clear: we know that before the planets were fully formed our solar system went through three stages which would have made for bizarrely alien skies:
The protoplanetary disk, full of tiny dust and gas.
Image above: A piece of fluffy cosmic dust, collected by a high altitiude airplane.Image courtesy of NASA.
Then an era of colliding bodies 1000 to 10,000 meters across, known as [8]planetesimals.
Image above: The asteroid Ida, and its tiny moon Dactyl. These bodies makes good approximations of large planitesimals in the protoplanetary disk.
Finally, a solar system containing [9]hundreds of worlds, ranging in size from roughly that of our moon to roughly that of Mars.
Image above: Hubble space telescope images of two surviving protoplanets that never grew to maturity: Ceres (left) and Vesta (right). Ceres has grown sufficiently to qualify as a dwarf planet. Image courtesy of [10]Hubblesite.org
But how things went from dust to planetesimals is a stubborn problem: Dust particles colliding at the kinds of speeds predicted for objects of their size in the protoplanetary disk are difficult to clump together into larger objects. And once things get up to centimeter size the situation gets worse. The disk would be a machine gun hail of projectiles- but moving much faster tan a speeding bullet. Its hard to see how they could come together to make larger objects.
The gas remaining in the disk may provide us with an escape: The gas is not evenly spread throughout the disk, it has [11]turbulence, areas of greater and lesser density, and a process called [12]sedimentation allows the dust in the disk to settle into a layer in the mid-plane of the disk. Where the gas is dense it slows the insane frenzy of collisions, and the particles will tend to linger in that area. As more particles pile up, like a traffic jam caused by a stretch of badly maintained road, the denser areas begin to exert a gravitational pull over other nearby particles. The gentlest forces tame the fiercest.
Another possible route is that of [13]gigantic vortexes; huge hurricanes of gas and dust may have formed in the turbulent structure of the disk, a result of the interactions of [14]rosby waves of pressure. These vortexes would have behaved a bit like whirlpools in the flow, concentrating the minute dust particles at their centres.
It is possible that these vortexes may have been complex and fascinating emergent structure in their own rights; like the [15]great red spot on Jupiter, a vortex ([16]anticyclonic)storm that has survived for hundreds of years (at least!) by consuming other storms like a titanic predator.
Image above: The Great Red Spot, a storm hundreds of years old and bigger than Earth, consumes and absorbs a lesser storm, in a demonstrattion of how relatively simple phenomena like vortexes can show surprisingly complex behavoirs. Image courtesy of NASA.
The foundations are laid:
Whatever route our solar system took, after a few hundred thousand to tens of millions ( nothing' in cosmic time) of years the nature of the protoplanetary disk had changed. No longer a fairly smooth soup of as and dust, now the immense disk is crammed with irregular pieces of solid matter from 1 to 10 kilometers across. These planetesimals were not all alike. In the inner solar system, where the temperature was still warm- still in the region of thousands of degrees kelvin near the sun ward edge- materials with a low boiling point ( like water, methane, ammonia) never condensed into solid material, and either remained as gas or were driven into the cold depths of the outer solar system .
Image above: Graph of the variation in temperature with distance from the sun, labelled with the condensation points of various solids. Image courtesy of shorstmyer.com.
Destinies diverge:
This had a profound influence on the destiny of the inner solar system. The only solid materials available to build planetesimals from were high temperature ones, like silicates, and metals such as iron, nickel, magnesium and aluminum. These are vanishingly rare in the protoplanetary disk. This put the inner solar system on a cruel starvation diet; While objects in the outer solar system could glut themselves son a ready supply of frozen gasses and water, the inner solar systems growth was hamstrung from the beginning. To make matters worse for the next phase, building protoplanets, the cramped confines of the inner system meant that competition for material was fierce, with little room for a growing object to spread its gravitational net for building material.
Image above: The division of the solar system. The solid matter rich outer disk and the relatively impoverished inner disk are seperated by the 'frost line'. Image courtesy of the University of Hawaii Institute for Astronomy.
In the outer solar system the lower temperatures, and lower [17]orbital velocities, meant growth was easier; the protoplanets out here, in the realm of ices, may have reached as much a s fifteen times the mass of our earth. It is easy to see how the outer solar system became the domain of giants when such large infants fed off such abundant manna. Once the growing core of a giant planet reached a certain size on solid material ( around ten to fifteen Earth masses) its gravity was great enough that it could begin sucking down the tenuous but abundant gasses that had still not frozen out: Hydrogen and helium. These are the most abundant elements in the universe, so the solid cores were sealed away for eternity beneath all consuming oceans of these gasses.
Image above: The ultimate fate of many protoplanets in the early outer solar system lies deep in the core of Jupiter, beneath oceans of ultra high pressure gas. Image courtesy of NASA.
Now, in a break from my normal format, I'm going to present the next few sections in first person. I have used as light a touch of imagination as I can to fill in gaps where the data we have does not clearly tell us what the human expeiance of being inside such a disk would be like. Those who like their facts straight and unadulterated; please forgive me in this, but like the character below, I believe that the human experiance of space exploration is as vital a part of it as the science!
Prelude:
Imagine a scientific project of unprecedented scale and power, a mission to a young star system with one objective: deliver a human to the inner most edge of a planetesimal disk, indistinguishable for all practical purposes from our young suns, with the means to conduct a pilgrimage from the innermost limit of the disk to the outermost, gathering incredible scientific data and images along the way. It is a mission that will take time, but let imagination have free reign, and put yourself in those lonely space boots;
What I did on my Holidays:
It has been a long journey getting here, but I was unaware of it. My 'suit' - more like a skin tight starship- kept me frozen for the decades long journey at two tenths of lightspeed. I began to 'thaw' as we approached this star system, just as the suit re-absorbed the immense loop of superconducting material that acted as a brake, bringing my speed down from the incomprehensible needed for interstellar travel to the merely insane speeds needed to cross interplanetary distances.
I drop, on advanced AI autopilot, towards the central star: A young yellow dwarf still throwing off radiation and flares and guzzling gasses. The autopilot delivers me with machine precision and efficiency to the planetesimal with the lowest stable orbit above the star. By the time the slow process of reanimation has been completed, and I am aware of my surroundings fully, I am less than twenty kilometers from the target planetesimal.
My boots touch the surface, kicking up a fine spray of powder that coats the suits white exterior with a layer of grey.
As I stand upon the innermost chunk the sun is a sea of blinding, searing, white plasma lurking beyond the frighteningly close horizon, and above my suited head is a glittering wall of light reflected off the trillions of other bodies above me- sparser towards the suns poleward horizons, and becoming a churning ribbon directly overhead the equator. These planetesimals have little gravity, so a firm kick, and maybe a little help from the suits intra-system engine (I have a long way to go) on the ground ‘beneath’ me sends me drifting away from the sun.
The rock I’ve just left is a mass of [19]silicate materials: oxides and other compounds of silicon, iron, magnesium, aluminium, nickel, as well as other high temperature materials. It is a dark, irregular black cut out against the blinding glare of the star behind me.
Anything with lower melting point has been driven further up the disk by the suns radiation and the disks own heat. The temperature is infernal; in the region of 1000 degrees Kelvin. My suit-enhanced senses find barely a trace of anything resembling the kind of organic chemistry life on earth is based on- this part of the young star system is truly hostile. the magnetic field is intense, and the t-tauri brat of a star behind my throws off [20]a huge storm of ionised hydrogen and helium even as I ponder it. It can't hurt me in this suit unless I dive to within a few thousand kilometers of the stars surface, but it is an intimidating site. Far smaller storms have done for spacecraft [21]back in near earth space.
Into darkness:
I turn back to journey ahead of me.
Jumping from one rocky mound to the next the suns presence is quickly reduced from a wall of glare to a diffuse glow beneath me, as it is lost behind megatons of spinning boulders and waves of dust.
As I climb higher out of the gravity well the glow of the star is lost completely to my unaided eyes- the only source of light is the occasional flash of light as two planetesimals collide nearby, accompanied by a hail of shrapnel. I navigate by infra- red, radar, and I have to step (hop!) quickly, even with my suits protection.
I am frozen by the sight of something unexpected: a sudden, incredibly intense, flash of blue-white light that sets all the disk around me aglow: My sensors tell me that I have been hit by a huge wave of [22]gamma rays. Somewhere beyond the disk something titanic has happened. My most likely candidates are the collision of two neutron stars, or a [23]hypernova, giving off a [24]gamma ray burst, somewhere within a few hundred light years.
There's no time to ponder the terrible power of this event. Although the material of the disk has shielded me from the worst of the gamma rays, I know from the briefings on protoplanetary disks that he burst will trigger a lesser, but still dangerous and more protracted fury in the disk itself. The disk has been electrically charged by the ionising radiation, and within minutes I am caught in [25]a lightning show as big as a solar system. The bolts are almost a tenth of an [26]AU long, and billions of times more intense than a lightning bolt on earth.
I'm not panicked- yet. The AI's handle on the conditions in the disk is good enough to predict where the worst parts of the lightning storms are, and I'm protected against anything but a direct hit. I need to move though: down and out of the main disk, away from the worst, and as fast as the intra-system drive will allow. Below me, spreading out in all directions, the channels of rock dust and gas are dancing to ten trillion flames of electrical power. I don't even see the sky below me, all I can think about is storm above, and the howling radio noise it is filling my headset with.
This hyper-lightning is very different to its earthbound cousins: Because of the immense distances it spans it does not flash, rather it flows like water droplets sliding down a pane of glass, occasionally broken by flashes of red and ochre as an unlucky planetesimals is shattered into a cloud of molten droplets.
I have no choice.
I wait.
Competition, survival:
Time is hard to measure out here, except by the slow rotation of the disk, but the worst of the storm is over fairly quickly. I plunge back into the disk, aiming for a point a little beyond where I left it, and resume my pilgrimage.
The temperature is dropping, from close to a thousand kelvin to the lower hundreds. The chemical reactions on the planetesimals are getting more complex: although volatile elements like water still won't form solids they are increasingly able to cling to the surfaces of the planetesimals in ultra thin layers, and affect their composition.
I jump towards the 'top' surface of the disk. Dimly through the glare of light reflecting off of stray gas and rocks out of the main plane I can make out that there is something wrong with the stars beyond, but before I can put my finger on what it is my suit alarm chimes: My maneuvers to avoid the hyper lightning have left the suit with little energy reserve, and I am wasting whats left with this diversion. I put the odd sky from my thoughts - after all a gamma ray blast like that could have produced all kinds of weird effects- and must resume my course through the long axis of the disk.
As I climb higher out of the stars gravity well I'm forced to skirt the edge of a region where the rocky planetesimals are merging; the growing [27]protoplanet is already many tens of kilometres across, and its gravity has grown to the point where it is pulling in, by [28]accretion, smaller rocks by itself. It has not yet reached sufficient size to pull itself into a sphere, but its surface is warmed by the impacts and scarred with fresh craters. This huge pock mark faced baby will grow until it has cleared out the remaining planetesimals within its gravitational reach of its orbital track. This is a process that will take only tens of thousands of years- an eye blink in cosmic time. This growing rock is part of a new breed of object: the [29]oligarchic phase.
There are a lot more of these, and they are growing and competing with each other. The great disk is fast becoming a race to gobble up all the remaining matter- and the objects that are the biggest have the most gravity, so the most chance of attracting new matter and the least chance of having an impact blast matter back into the disk. The warm objects in the inner system are cruelly disadvantaged: their home contains less solid material, as high temperature materials are rare in the solar nebula, and the narrow confines mean the orbital tracks which each can claim are close packed and narrow.
Navigating around the gravitational well of the planetary embryo has cost me more fuel reserves. A splinter of worry lodges in my chest, and but I haven't come all this way to give up. Mankind could have sent an unmanned probe- indeed most of the data gathered is being done so by the suits AI- but we thought it important to have a human along for the ride, to give a perspective beyond what cold data can provide
Now, as the suns reflected radiance dims further, I'm seeing more changes in the planetesimals: By the weak light reflected down into the disk by material orbiting outside the main plane I can make out that the rocky silicates are being replaced by ices- not just water ice, but as I climb higher ammonia, methane, and other low temperature materials. The temperature has plummeted from the searing near star heat- and it will be getting colder, perhaps a slow as 100 degrees kelvin (- 173 degrees Celsius in old money). This is why the nature of the stepping stones I'm using has changed, and why they are no less densely packed even out here: I have crossed the [30]frost line, the distance from the sun where water (and other volatile materials) can begin to condense out. There is much, much more low temperature than high temperature material in the disk, so out here there is enough solid material to keep the orbits crammed with planetesimals.
The motion of the planetesimals here is less restlessly seething, as they move in a more stately fashion this far from the sun. This will soon be the realm of the giant planets: with more room and more solid material the planetary embryos I dodge around are already getting much bigger than those in the inner solar system. They will get [31]bigger still- reaching up to 15 earth masses. Their gravity is formidable, and it is more important than ever on my reduced power reserves to avoid them. This means moving against a flow of gas and particles when I pass by one, as some are already acting as gravitational hoovers, drawing in raw materials at a staggering rate.
Treasures beyond the end of the road:
I'm still further out, the suit sending me stern warnings about its reserves, and now the darkness is nearly absolute, and a temperature of cryogenic cold, well below freezing. With less glare the occasional clear spot tantalises me with half glimpses of a starscape that looks oddly wrong still. I turn my suits sensors onto the hodge podge of ices I'm currently standing on:
The [32]organic chemistry in this part of the disk is complex, if mostly slow moving due to the low temperature. There is some evidence that [33]tholins, complex particles of organic matter that are believed to be involved in the origins of life, can form in the cooler regions of the disk. There are some genuine oddities associated with this part of the disk: Microscopic [34]globules of organic matter, that have been shaped into [35]tiny hollow beads the same rough shape as a cell membrane, are preserved in the icy planetesimals. What is more the disk contains [36]abundant water and organic precursors to more complex chemistry. Almost a foreshadowing of things to come...
Image above: Hollow beads of organic compounds recovered from the tagish lake meteorite, viwed by electron microscope. Such material almost certainly rained down upon early Earth, enriching the prebiotic organic chemistry there. Image courtesy of
Now, skirting the areas where nature is beginning to construct protoplanets and lay the foundations for mature planets, weathering storms of shrapnel and monster lightning bolts, I have travelled almost to the edge of the protoplanetary disk. The new born star is scarcely more than a bright speck, even if I had the fuel to hop out of the main plane of the disk and get a clear line of sight.
Many of the planetesimals out here, over 200 AU from the sputtering t-tauri, are already nearing the end of their part in this story; this is the region that will one day be home of countless deep frozen [37]comet nuclei, many of which will never journey inwards to grace skies with their [38]twin tails of ions and dust, and odd, frozen, protoplanets, such as our own solar systems [39]Sedna, [40]Makemake,and Pluto.
Image above: A comet, showing twin tails of dust and ions, spectacularly demonstrates the beauty that can come from even the dullest seeming things. In the outer solar sytem this beautifull object was a tiny dark speck of ices and organic compounds. Image courtesy of ESA.
Standing on that last planitesimal, on the edge of the dim cliff before the long cold road to the rest of the galaxy, the central star is all but undetectable. The protoplanetary disk no longer sparkles with reflected light, but is a vast dark chasm beneath me, a counterpoint to the ocean of stellar light that bathed my feet at the beginning of this journey. Things here are quieter in the radio, almost a deathly silence, and the flashes of colliding planitesimals, and residual lightening, that lit the disk before are fewer, farther between, and clouded by dense wreaths of gas and dust.
And as I look outwards from the farthest outpost of the protopanetary disk, my mouth dries with wonder. I know now what it was about the stars that looked odd in the half glimpses during my journey. I should have been expecting this: The sky is not dark, nor is it the sky as seen from Earth, with glittering band of our mily way galaxy stretched across it.
My sky is now ablaze with a display that has never been seen by human eyes: the star behind me was wombed in the darkness of a collapsing nebula fragment, but it was not born alone. Above me are the little stars siblings, birthed by other fragments of the collapsed pre-solar cloud, and closer packed than any stars of our suns neighborhood. Yellows like our sun, [41]red dwarfs with lifespans longer than the age of the universe ahead of them, and perhaps a few blue titans. Tens of thousands swarming in [42]tight packed open cluster, less than a lightyear between them. With the infra red eyes of my suit I can make out still more: dwarfs down to the dimmest that can sustain our suns style fo fusion, and then the heat glows of [43]brown dwarfs: objects no wider than Jupiter and as little as fifteen times as heavy, briefly warming themselves with lithium and deuterium fusion before they settle down into a long maturity of darkenss.
image above: Open star cluster NGC 290, a stellar jewel box. Our sun almost certainly came from a similar cluster, and its skies would have been illuminated by hundreds of stars with less than a lightyear between each other.
This is the new stars familly, showing their colours brighter than any star we can see from Earth, together for one glorious family photo before the tides of momemtum and gravity send them their separate ways.
How do we know all this?
Well, as you can guess from my experiment with the format above, a lot of what we've learned has been from studying [44]protoplanetary disks around other young stars, like newborns and protostars in our old friend the [45]orion nebula. We can study these disks with modern telescopes at resolutions high enough to pick out large planets, and we can see the channels they carve through the dust and gas.
Spectroscopy is once again one of our most powerfull tools here, allowing us to get an idea of what is making up the tiniest dust particles in the disk, even though we can barely discern giant planet sized objects. We can compare these measurments to both computer models and materials preserved from our own ancient times, allowing us to build a detailed picture.
We have ways of sampleing the ancient solar environment directly: [46]Cabanaceous chondrite meteorites are believed to be the condensed remnants of the protoplanetary disk, and contain many structures and isotopic abundances [47]we cannot explain any other way. We can tell by close examination of the mineral structures within them that many of these are essentially unaltered over 4 billion years of history. Very rare icy examples like the [48]Tagish lake meteorite preserve a snapshot of the organic chemistry of that time, which was one of the important processes leading up to life here on earth.
We aso have studies of materials captured from remants of the plantesimal era, most noteably comets, via sample return missions like [49]Stardust, and missions explore comets remotely, such as ESAs [50]Rossetta, [51]Giotto, and NASAs [52]Deep Impact. Some of these are citizen science internet projects that some of you will have been involved in.
We have [53]computer modelling, such as takes place at NASA Advanced Super computing (NAS) division. Computer models can't give us a direct view of the protoplanetary disk, but they can show us how various scenarios would have played out, and point our studies of the real world in more precise directions.
A few specific events in the early solar system we do have more evidence for. The existence of a protoplanetary disk is one example: it is very hard to explain how all the planets orbit in the same plane, in the same direction, without one. And the existence of calcium and aluminum rich [55]chondrules tells us that there had to be some dramatic high temperature events of a specific kind hapening. The amounts of radioactive iron isotopes in meteorites tell us that [56]at some point our solar system was blasted by a supernova explosion- in fact its possible that there were more than one. And we know from observing other star clusters that, after they begin their new glorious lives, stars stay close to each other for a time, forming spectacular open clusters, stellar familys.
And there is one place that we can go to to observe the protoplanetary disk directly, at close range- well almost. The rings of saturn are a very good approximation of many of the processes going on in the protoplanetary disk, includeing [57]ice and dust coalescing into larger bodies under the influence of turbulence.
Next: The rise of the Oligarchs.
[1]http://www.nasa.gov/mission_pages/hubble/science/fomalhaut.html
[2]http://www.daviddarling.info/encyclopedia/P/protoplandisk.html
[3]http://www.universetoday.com/24802/t-tauri-star/
[4]http://research-repository.st-andrews.ac.uk/bitstream/10023/336/1/thesis.pdf
[5]http://www.iaus243.org/IMG/pdf/JohnsKrull_IAUS243.pdf
[6]http://www.space.com/scienceastronomy/solarsystem/chondrites_010305.html
[7]http://www.sciencedaily.com/releases/2009/04/090421080504.htm
[8]http://www.universetoday.com/35974/planetesimals/
[9]http://www.nova.org/~sol/solcom/stars/sol-sum.htm
[10]http://hubblesite.org/
[11]http://iopscience.iop.org/0004-637X/662/1/613/pdf/0004-637X_662_1_613.pdf
[12]http://iopscience.iop.org/0004-637X/643/2/1219/pdf/0004-637X_643_2_1219.pdf
[13]http://ctr.stanford.edu/Summer00/barranco2.pdf
[14]http://www.noc.soton.ac.uk/JRD/SAT/Rossby/Rossbyintro.html
[15]http://www.accessscience.com/IOW/iow.aspx?iowID=31
[16]http://www.ecn.ac.uk/Education/anticyclones.htm
[17]http://www.sjsu.edu/faculty/watkins/orbital.htm
[19]http://www.globalspec.com/LearnMore/Materials_Chemicals_Adhesives/Ceramics_Glass_Materials/Silica_Quartz_Silicate_Materials
[20]http://adsabs.harvard.edu/full/1981ApJ...244..520W
[21]http://www.msnbc.msn.com/id/38242512/
[22]http://science.hq.nasa.gov/kids/imagers/ems/gamma.html
[23]http://imagine.gsfc.nasa.gov/docs/features/news/20may99.html
[24]http://imagine.gsfc.nasa.gov/docs/science/know_l1/bursts.html
[25]http://www.astronomy.com/asy/default.aspx?c=a&id=2853
[26]http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980122b.html
[27]http://www.universetoday.com/37053/protoplanets/
[28]http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/001106a.html
[29]http://www.gps.caltech.edu/classes/ge133/reading/oligarchy.pdf
[30]http://www.universetoday.com/63580/astronomy-without-a-telescope-%E2%80%93-the-nice-way-to-build-a-solar-system/
[31]http://www.maths.qmul.ac.uk/~rpn/preprints/Accreting_planets.pdf
[32]http://astrobiology.gsfc.nasa.gov/IAU-Moore-Hudson.pdf
[33]http://www.centauri-dreams.org/?p=1658
[34]http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1130.pdf
[35]http://www.psrd.hawaii.edu/Jan07/organicGlobules.html
[36]http://www.nasa.gov/mission_pages/spitzer/news/spitzer-20080313.html
[37]http://www.solarviews.com/eng/comet/whatis.htm
[38]http://www.universetoday.com/40820/comet-tail/
[39]http://www.nasa.gov/vision/universe/solarsystem/planet_like_body.html
[40]http://apod.nasa.gov/apod/ap080716.html
[41]http://www.universetoday.com/24670/red-dwarf-stars/
[42]http://seds.org/messier/open.html
[43]http://www.daviddarling.info/encyclopedia/B/browndwarf.html
[44]http://www.solarviews.com/cap/ds/orion4.htm
[45]http://hubblesite.org/gallery/tours/tour-orion/
[46]http://www.daviddarling.info/encyclopedia/C/carbchon.html
[47]http://discovermagazine.com/2001/mar/featbland
[48]http://www.astrobiology.com/news/viewpr.html?pid=21384
[49]http://www.nasa.gov/mission_pages/stardust/main/index.html
[50]http://www.esa.int/esaMI/Rosetta/index.html
[51]http://sci.esa.int/science-e/www/area/index.cfm?fareaid=15
[52]http://www.nasa.gov/mission_pages/deepimpact/main/
[53]http://www.nas.nasa.gov/
[55]http://www.daviddarling.info/encyclopedia/C/chondrule.html
[56]http://www.scientificamerican.com/article.cfm?id=solar-system-trigger-sn
[57]http://www.planetary.org/blog/article/00002594/
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