Note: All links are numbered, and listed in order at the end.
Our Sun is the center and lord of our star system in many ways; it is the most massive object, has the most powerful energy source, and dominates the solar system gravitationally. Almost all life depends on the sun for energy, and the surface of our Earth is 'just right' for living things because the sun warms it without scorching it, or battering the surface with [1]gigantic flares.
Image above: The Sun. Imaged in wavelengths corresponding to iron and helium by the Solar Dynamics Observatory. Courtesy of NASA.
But 4.5 billion years ago, as the planets were coalescing out of the [2]pre-solar nebular, the Sun was a very different creature: Not a stable and long lived [3]yellow dwarf, our sun was a temperamental [4]T-tauri star.
Growing pains and angst:
T-tauri stars have developed past the [5]proto-star phase, but have not yet begun the full throated [6]power producing roar of fusion. They are like cocky teenagers: they talk the talk, and from a distance you could think they were full fledged star, but a closer inspection reveals an underdeveloped core, and behavior prone to outbursts, tantrums and occasionally violence.
Image above: The star T-Tauri, first of its kind to be discovered. Image courtesy of astronet.ru.
T-tauri stars shine by [7]converting gravitational energy into heat and light: the star is kept 'inflated' by the heat and energy of the inner layers, and the outer layers emit this heat and light into space. This cools the star, so it contracts. As it contracts it heats up again, and continues to shine. This can continue for millions of years. in face some astrophysicists like to be very clear on the following point: Nuclear fusion didn't make the sun hot and bright- heating from gravitational compression did that- fusion is what took over from compression later in the suns life and keeps it hot and bright today.
At an estimate, just before before nuclear fusion began in its core, our star was contracting at a [8]rate of twenty feet per year. As it began to shine it was a glob of gas [9]the size of the orbit of mercury, around 60,000,000 km across. Today it is around 695,500 km across, so it shrank down a lot getting to its present size.
As it contracts, like a ballerina bringing her arms in as she pirouettes, the Sun spins faster and faster. As the sun is made of plasma, electrically charged gas, this sets up [10]a magnetic field that is colossal compared to todays- up to 1000 [11]gauss. Magnetic field lines like great tree trunk thick tentacles reach out into the protoplanetary disk and [12]scoop down chunks of innocent matter to its searing surface.
Image left: An artists visualisation of a T-tauri star reaching into ots protoplanetary disk with its magnetic field. Image courtesy of NASA/R.Hurt.
A [13]solar wind a million times stronger than today's belches out of the stellar surface, with speeds of several hundred kilometers per second. The protoplanetary disk surrounding the burgeoning sun focuses these winds above the poles, giving ghostly [14]twin pillars of light shooting out of the epicenter of the disk. This T-tauri wind sweeps the lighter as and dust particles out of the protoplanetary disk and into the cold vastness of interstellar space.
The Suns over developed magnetic field has a strange effect on it: Out to around 20 solar radii the magnetic field maintains it s grip on the fleeing star stuff, so the entire wind is forced to rotate at the same rate as the Sun does, like a giant vortex sweeping through the disk. The atmosphere is unstable, and was probably prone to [15]gigantic sunspots, and huge flares that could double the Suns brightness for a brief time. This was not a friendly life giving star, and if todays Sun were to return to its old habits it would [16]devastate our blue marble of a world more thoroughly than any weapon conceived by us tiny humans.
From destroyer of worlds to giver of life:
While the surface has already begun to shine, deep in the core, under [17]incredible pressures, the [18]density and temperature are increasing as the Sun collapses down. The crushing pressures at the bottom of Earths oceans, or the density of the heaviest metal, have nothing on these conditions. The gas here is a a soup of plasma, and as it originally comprised hydrogen this means that the naked proton cores are zipping about and colliding with each other. Very very occasionally, as the density and temperature rise, [19]they will fuse releasing energy. These events are very, very rare. Most protons in a star, even a full fledged main sequence powerhouse, will only go through one fusion in their entire career. But as the density skyrockets the odds of a fusion event happening in any given cubic centimeter of the suns core does too, and as the temperature goes up the protons blast into each other with ever more ferocity making fusion easier and easier to happen.
There are different flavour of fusion: Lithium and deuterium fusion for example need less energy to make work that protium fusion, and [20]brown dwarf psuedo stars, for the brief time the have fusion at all, [21]have only these kinds. But these gasses are much less abundant than normal hydrogen, an are quickly burned up in a full grown star.
So as it first came onto the scene fusion power hardly made an impression on the young Sun. But as the heat got fiercer and the pressure at the core more crushing, it began to supplant compression as the main power source. In a slow and creeping fashion, it began to make compression unneeded, and so the compression slowed. Every little bit of further compression the sun tried meant more fusion, so more power to hold it up, and less compression in the future. Over eight to ten million years as the Sun compressed down the fusion spread. Eventually, like a fungus spreading through the stellar core, the fusion reactions took over completely, and the sun ceased contracting: It had made the leap from T-tauri youngster to main sequence yellow dwarf.
At the same time as this was happening the massive magnetic field was sowing the seeds of its own down fall. Earlier I mentioned that the field was so strong that it was keeping the solar wind connected to the solar rotation- it forced the wind to spin with it. [22]It also connected the sun to the surrounding protoplanetary disk- and this was acting as a brake, taking angular momentum put of the sun and putting it back into the disk.
By the time fusion had taken over in the core the sun had slowed its mad spin, the magnetic field had calmed and the battering force of the T-tauri wind had abated. The [23]Sun was fainter than today, only 70% of its current intensity, but it was recognizably our pleasant and friendly Sun, and it will remain that way for billions of years to come.
How do we know this?
Much of what we know about the young Sun is inferred from what we know of it today- coupled with what we can see of developing stars out in the universe. A recent development in solar studies has been the [24]Solar Dynamics Observatory. Stationed in a geosynchronous orbit this is the most advanced space telescope ever turned on the Sun.
Image left: The Solar Dynamics Observatroy, set for launch. The spacecraft is part of NASAs living with a star program.
T-tauri phase stars are easily picked out by [26]spectroscopy, as hey have yet to burn off there lithium content, and so this gas can be detected in their atmospheres. From looking at these we can see that they are unstable, surrounded by disks of protoplanetary material, and send great pillars of radiation into the empty space above their poles.
Image right: Young stars, sending searing jets of T-tauri wind into space. Known as Herbig-Haro objects. Image courtesy of NASA.
As we cannot see many of these stars directly progress is made in the time honoured fashion of trial and error: Astrophysicists develop a theory, and polish it until it can make predictions of what we should observe. Based on how well the predictions match what we can actually measure about these stars the astrophysicists go back and polish their theories and models some more. A great many observatories, across many frequencies of the [26]electromagnetic spectrum ([27]Radio to [28]infra red to visible to [29]UV to [30]X-ray), are engaged in the study of these young stars, but progress is often frustratingly slow; they are distant and often swathed in the remnants of the [31]bok globules that cocooned them as they went through their early development.
Note: All links are numbered, and the web addresses are listed in order at the end.
Image left: An optical telescope image of the crab nebula. The central pulsar is shown inset, in a sequence of images demonstrating its rhythmic pulse. Image courtesy of the Kitt Peak 4-meter Mayall telescope.
Stranger than fiction:
In science fiction, there is always a way for a major character to [1]come back from the dead. In astrophysics, it seems there are ways for a star system to come back from utter destruction- a process that mirrors the birth of a planetary system in a bizarre way.
Video above: The heart beat of an undead star. The radio 'noise' of a pulsar, as heard by a radio telescope like Aricebo. Visuals are an image generator, but pretty! Video courtesy of IDNmitigatioAPIs via YouTube.
A [10]pulsar is the left over core of a giant star that has gone supernova. The core material of the star is [11]mostly inert iron by the time it hits old age, and [12]iron does not give out energy when it fuses- so there is no energy holding up it weight, and it collapses, and then bounces of itself in an unimaginable explosion. These momentary crucibles of colossal power are [13]responsible for most of the heavy elements in the universe
This leaves the core mostly intact, by transmutes it: The pressure of the explosion, coupled with unmerciful gravity crushes that iron cinder until its atoms start to give in under the strain. If the core weighs [14]more than about 1.4 solar masses, and less than about 3, then it collapses so far individual atoms are smooshed together, positive protons combining with negative electrons, and the dead core becomes a [15]city sized ball of neutrons.
Video Above: The tormented Crab nebula around the Crab nebula pulsar seethes, huge waves of matter and energy visible even from 6500 light years away. In this video made by the Chandra x-ray space telescope and the Hubble space telescope the visible light image is on the right, and its x-ray counterpart on the left. the innermost ring is about a light year across, demonstrating how much punch one pulsar packs. Video courtesy of NASA
They do one more thing: They spin, some of them nearly [20]a thousand times a second. As they have intense [21]beams of radiation slicing out of the [22]magnetic poles, and the poles of rotation and magnetism don't align, from a great distance these look like the flashing of a very very fast lighthouse, if you sit in the right spot. Close to..... the business end of 'Vaders death star on high beam is more accurate.
You would not expect something that is born in an explosion which outshines a galaxy, has the mass of a giant star crammed into a space the size of London city center, and gives out scorching twin beams of high energy radiation to have any planets going around it.
But you'd be wrong, and so [23]many astronomers were (not all!). Arecibo showed that something ([24]several somethings) with the right masses to be Earth-sized planets were pulling the spinning neutron star or 'pulsar' [25]PSR 1257 +12 hither and thither, and throwing the timing of its lighthouse flashes off.
Two of these worlds were the mass of Earth, and one was roughly twice the mass of Mars. [26]Later a fourth, dwarf, planet joined the odd little family.
Image above: The PSR 1257+12 system, including orientation to Earth. Image courtesy of extrasolar.net.
Bizarrely and despite having already suffered a horrible, searing, death, this star system had brought itself back!
How does that happen? After all a supernova in the face is about as lethal as lethal gets. Three possibilities jostle for consideration among astrophysicists, and all involve many of the same processes that formed our own solar system in miniature:
Possibility one: A rise from the ashes.
Image above: Supernova remnant Simeis 147. Could part of such a cloud of stellar debris form new planets around its supernovas ticking corpse? Image Courtesy of NASA Astronomy Picture Of the Day.
The supernova left behind more than just the remnant core, it left behind the guts and meat of a giant star, shattered and smeared over that part of space.
In the first scenario not all of that matter stayed smeared over space: some of it came back to its burned home, and [27]collected in orbit about the neutron star, like smoke around a furiously deadly firefly. It formed a protoplanetary disk around the neutron star, and by the same processes that formed our [28]terrestrial (rocky) worlds, (planetesimal accretion, protoplanetary formation, runaway growth Oligarchial growth etc) pulsar PSR 1257 +12 grew new planets.
The protoplanetary disk will have been tiny compared to that which gave rise to us: [29]only ten or so Earth masses. As a result these planets are small, rocky worlds- there isn't enough mass in a whole disk to form one gas giant. And to boot the disk is made from supernova ash, which is going to be [30]loaded with heavy elements, and radioactive isotopes.
This will have a profound effect on the little rocky worlds going around their diminutive and baleful lord. No gas giants, only terrestrial planets, and these are going to be hot, heavy and radioactive. [31]Taking around a hundred thousand years to start to form they will be dense worlds, and even with masses on par with Earth this will make them smaller in radius, and their surface gravity higher. The planets will be infernally hot compared to their counterparts around normal stars, because the abundance of heavy elements will mean that they generate more internal heat, and for longer.
Possibility two: The legacy of stellar murder.
Image left: The site of supernova SN 1987A. Analysis of the debris shows the blast was asymmetric. Image courtesy of the Hubble space telescope.
It is also possible that the supernova blast was [32]seriously lopsided, and threw the neutron star sideways into a companion star. [33]This wouldn't have done the companion much good, but the neutron star would have left the carnage carrying a disk of matter, like cannibalistic interstellar loot. This disk would be similar in composition to the star from which the material was stolen. That allows a lot of scope for the detailed composition to vary, it is possible that these worlds may be chemically very different from the terrestrial worlds we know- [34] very carbon rich for example, depending on the type of star the matter forming them was stolen from.
But on the whole this process of planet formation would be much more like the formation of our own solar system in miniature: there still wouldn't be enough mass to form anything other than crusty hard terrestrial planets, but these would, in density, composition, and internal radioactivity, probably be more like the terrestrial planets in our solar system.
Possibility three: The legacy of a parasitic hunger.
Image right: A simulation of an accretion disk around a neutron star. image courtesy of NASA.
The final possibility for how these worlds formed is less dramatic than the preceding two, but in its own way much more creepy. The pulsar may have [40]fed off a still living companion, using its gravity like a siphon to strip away the living stuff of that luckless star, and accreting it onto its surface. [41]Such feasts can end badly for [42]white dwarfs, but in this case it seems that the pulsar may have slowly bled its companion dry, and left enough material still orbiting itself to coalesce into planets. This scenario is a close sibling to the second- much of the details of the planets formed Will depend upon the composition of the star that donated the material, but it would be a more familiar process than the heavy element rich processes of the first scenario, and its 'depleted uranium shell' worlds.
They will be dangerous, truly unique locals, changing violently on a daily basis. The radiation at the surface of each of these worlds will be intolerable to anything we'd think of as life: even if the death star beams from the pulsar don't actually connect with the planet the pulsar gives out [43]intense x-ray, gamma ray, and particle radiation.
Yet in much the same manner as [44]Io in our own solar system lacks nothing in beauty, majesty, complexity or awe inspiring power (click here for an amazing [45]Io related blog), these little worlds orbiting their wizened little pulsar may well be as fascinating and complicated as anything we ave seen. [46]The moons of Jupiter are proof that planets need not be orbiting a shining sun to have great character and active, complex behaviors.
Image left: Io, innermost moon of Jupiter. A complex, ever changing and fascinating world. Image courtesy of NASA.
The discovery of pulsar planets tipped off planet hunters that the theory of planet hunting by radial velocity measurements was sound, the very fact of their existence tells us how robust the planet forming process can be, and these rare, stunted, star systems give us a unique comparison to our own and its history.
How do we know all this?
[47]Pulsar were found in 1967 by (then student) [48]Jocelyn Bell. The mysterious high speed radio 'ticks' were initially suspected of being signals from an alien intelligence, such was their perfect timing. Over the following decades the study of these incredibly intense objects became a true child of 20th century technology-as most of the radiation they give out is invisible (and incidentally quite deadly at close range) to humans. Mostly their study is the province of space borne x-ray observatories like the [49]Chandra telescope, and gamma ray telescopes such as the [50]Fermi. However the lighthouse flashes do include visible light, and radio waves, allowing nearby ones to be imaged by radio telescopes, and the most powerful optical telescopes.
The pulsar planets are totally enigmatic- they are known only through studying the effect they have on their pulsars timing. They are likely to remain so, given their distance, size and dim central object. Most of what we know about them is , via theory and computer modelling, from their mass, their orbits, and what we understand of how pulsar form. However from the measurements made so far we can make some educated guesses- we know their [51]rough mass and orbits, and we know [52]what angle those orbits make, for two of the planets at least, to the radiation beams coming out of the central pulsar. So we can say that while the environment may be extreme, these worlds are probably spared the full fury of the pulsar, as they orbit well out of the beams path.
Depending on which model of how they form is closest to the truth we can make educated guesses about their composition, and perhaps make a rough guess at their radius,and whether they will have much internal heat. From the [53]age of the pulsar we can guess the systems age. And from the positions and masses of these worlds [54]a stab at the systems history can be made. For now, more details will have to wait future advances.....
Note: All links are numbered with web addresses given at the end, in order.
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, [1]dust, [2]ices, all busy [3]accreting into weightless mountains of rock and ice, known to scientists as [4]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 [5]silicate materials. They [6]tumble chaotically, and rumble about each other, seething in a dance of gravitational tag, high speed [7]collision, coalescence, and erosion.
Image right: A planetesimals disk around Beta Pictoris, taken by the [8]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 ([9]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 [10]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 [11]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 [12]vortices, are beginning to coalesce into larger bodies. These are planetary seeds, and the beginning of an immense battle; a phase of [13]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 [14]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- [15]wildly eccentric orbits that almost touch the young suns surface then freeze in the out near [16]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 [17]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: [18]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 [19]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 [20]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 [21]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 [22]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 [23]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: [24]Guzzling gasses, the two most available of which are hydrogen and helium, [25]inherited from the solar nebula. Having reached the 15 Earth mass point the [26]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 [27]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. [28]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 [29]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- [30]too slowly in fact, as within a few millions of years, ten tops, the protoplanetary disk will have [31]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 [32]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, [33]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 [34]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 [35]Pluto and [36]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 [37]DPS 2010, one idea on where they came from was dealt a blow by [38]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 [39]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 [40]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, [41]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 [42]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 [43]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 [44]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 [45]occasionally colliding with and combining with their kith and kin, the [46]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 [47]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 [48]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 [49]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. [50]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, [51]a demolition derby, involving part-evolved worlds with structures and masses comparable to a modern planet, [52]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 [53]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 [54]little worlds believed to be survivors from this time: The [55]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 [56]Dawn spacecraft in 2011 and July 2015 respectively. Observations from Earth, using both ground based telescopes like the [57]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: Pallas as seen from the Hubble. Image courtesy of Hubble space telescope.
Pallas seems the least evolved of the three, with varied [65]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 [67]lightcurve.
Another world which seems to have escaped the ravages of volcanism lightly, leaving a fairly pristine piece of early solar system relic. But [68]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 [70]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 [71]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 [73]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 [74]involved in a hoax designed to make it appear that it carried life forms.
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.
Image above: A section of the GRA 06129 meteorite.Image courtesy of NASA.
An [77]andesite rich meteorite, which fell in Antarctica. The andesite is rich in [78]feldspars, and the composition of the rock is startlingly similar to that of Earths [79]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: [81]Fossil magnetic fields in three angrites suggest that some protoplanets able to generate their own magnetic field, an amazing feat for such small objects.
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 [83]punch right through!
Note: All links are numbered and the web addresses listed in order at the end
Before I launch into this weeks post (which will be up Sunday night and is on neutron star planetary systems and how they form) I've got three notes, all courtesy of the DPS 2010 conference [1]:
First: A new piece of research [2] on how the Kuiper belt [3] should look if certain models of how the ice giants[4] formed are correct, and how it doesn't in fact look that way. We should get a close look at some of those distant and shy Kuiper belt objects when the New Horizons [5] space probe gets out there in 2015.
Video above: New Horizons launch, just because I love this kind of stuff. Video courtesy of Kieran Griffith via YouTube.
Video above: A lecture given by the staff at the John J Mcarthy observatory, on the Kuiper belt. Video Courtesy of Nedski 42yt on YouTube.
As how those two mysterious ice giants, Uranus and Neptune, came to be is a big section in it, I'm delaying 'Rise of the Oligarchs' until the middle of next week. Until last night it was ready to go, now I'd like to try and work this new research into it. Damn you, hardworking astrophysicists!
Second: water ice and organic molecules have been found [6] on the surface of the 290 km wide asteroid 65 Cybele[7]. This comes soon after the discovery of the same thing on 24 Themis [8]. This suggests that the frost line (the point where ice could form) was closer in to the young sun than thought in the early solar system. I have read values for the frost/snow line ranging from 5 AU to 2 AU, and as Themis and Cybele have maximum distances from the sun of 3.5 AU and 3 AU respectively it seems we can narrow the range of possible values now. This also prompts some facsinating and provocative thoughts on the origin of life's chemical precursors, as both asteroids are large enough to have been briefly warmed[9] by short lived isotope decay during the earliest days- perhaps enough for subsurface liquid water?
Video Above: an animation of what 24 Themis might look like up close. Courtesy of Universesum2010 on youtube.
Finally; A call out for anyone who's actually at DPS 2010. I've been slavering over the abstracts presented for the sessions yesterday on the results from the Rosetta [10] flyby of 21 Lutetia, but precious little has been said by anyone- were the results really that uninteresting (which I doubt) or just not finished enough to be worth much reporting?
List of links:
[1]http://dps.aas.org/meetings/2010/
[2]http://arxiv.org/abs/1009.3495
[3]http://nineplanets.org/kboc.html
[4]http://www.lpi.usra.edu/opag/outer_planets.pdf
[5]http://www.nasa.gov/mission_pages/newhorizons/main/index.html
[6]http://www.sciencedaily.com/releases/2010/10/101007114114.htm
[7]http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=65
[8]http://www.wired.com/wiredscience/tag/24-themis/
[9]http://iopscience.iop.org/1538-4357/632/1/L41/19781.fg2.html
[10]http://www.esa.int/export/SPECIALS/Rosetta/index.html
All links are numbered, and listed in order at the end.
Above: A map of the Carina nebula. Image courtesy of NASA/ESA/European Southern Observatory.
I'm a sucker for a beautiful nebula. That said, I will try to keep this brief. Study of [1]star forming regions is one way by which we learn more about our own solar systems history, and studying the range of stellar and planetary systems out there gives our solar system context.
The [2]Carina nebula lies 7500 light years away in the direction of the constellation Carina. For a mind breaking zoomable version of the image above, [3]follow this link. At 10 parsecs across it is a tumultuous ocean of sculpted gas, dust and ice particles. One of the most active star forming regions locally, it is home to the trunks of '[4]pillars of creation', sculpted by hard radiation from massive stars, wider than our solar system and light years tall.......
Image above: a light year long pillar of gas and dust, with a ferocious young star attempting to break through the tip, sending out beams of high energy particles. Image courtesy of NASA.
......[5]dark Bok globules, cocooning nascent stars and their attended disks of unformed matter.....
Image above: A large Bok Globule, a cocoon for emerging star systems, named with odd aptness 'the caterpillar' by astronomers. Image courtesy of NASA.
......[6]Wolf Ryet stars, hurling out stellar winds so strong they make the edge of the star fuzzy, and hard radiation by the bucket....
Image above, scattered amongst their better behaved companions, hard radiation and particles from Wolf-rayet stars excite the gases of the nebula. Image courtesy of the European Southern Observatory
.....[7]open clusters, of young stars still huddled together against the interstellar darkness........
Image above: An open cluster of young stars set against within the Carina nebula. Image courtesy of ESA.
Image above: The Keyhole nebula, cold dust and gas set against the brighter, vaster Carina nebula Image courtesy of NASA, JPL.
........the collosal [9]hypergiant star HD93129A, less than a million years old, brighter than five million suns, and a stunning 52000 degrees kelvin in temperature.......
Image Left: The relative sizes of our sun and the nightmareish intensity of hypergiant HD93129A. No contest.
Image above: The enigmatic Homunculus Nebula, birthed in incredible violence. Image courtesy of NASA.
.......by its denizens and parents the [13]Eta Carina multiple star system- not just a series of giant furnace amongst giant furnaces, but an unstable, gibbering wreck of a system to boot. Exactly what is going on within the expanding shockwaves is hard to discerne, but the 1840s event was close to being a supernova- the warring Eta Carina familly is literally tearing itself apart as we watch.
Whatever the event that birthed the homunculus nebula was, in 1841 Eta Carina became the second brightest star in the sky- despite being 7491 lighteyears further from Earth than [14]Sirius, the brightest.
How do we know these things, and get these beautifull images?
A big source of information is the good ol' [15]Hubble Space Observatory, along with occasional help from other space telescopes, followed closely by the [16]European Sourthern Observatory which has produced a lot of spectacular nebula images. Many of the stunning images are false-colour interpretations of frequencies of light our mere human eyes can't see. The detailed story of what is going on comes from years of patient research and observation, coupled with masses of high end computer modelling, and even crosses over into areas of particle physics.
And all of these stunners are the end product of many many hours of telescope exposure, image processing, and planning, all at the mercy of one softtware glitch or hardware breakdown. So enjoy these, and spare a thought for the astronomers whose fascination with the universe brings them down to us!
List of links:
[1]http://www.ipac.caltech.edu/Outreach/Edu/sform.html
[2]http://hubblesite.org/newscenter/archive/releases/2007/16/
[3]http://hubblesite.org/newscenter/archive/releases/2007/16/image/a/format/zoom/
[4]http://www.spacetelescope.org/images/opo9544a/
[5]http://www.physorg.com/news194877369.html
[6]http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980603a.html
[7]http://apod.nasa.gov/apod/ap090831.html
[8]http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/molecular_clouds.html
[9]http://www.universetoday.com/25176/hypergiant-stars/
[10]http://blogs.discovermagazine.com/80beats/2008/09/11/mysterious-stellar-blast-in-the-1840s-was-a-supernova-imposter/
[11]http://www.sciencedaily.com/releases/2008/09/080910133659.htm
[12]http://www.grantchronicles.com/astro17.htm
