Sunday, July 22, 2012

Solar System Archaeology, Part II

Figure 9. Sailboats in the harbor of Nice, a seaside city at the epicenter of theoretical research
into the evolution of the Solar System.

the grand tack

For a long time, the circular, evenly-spaced orbits of the Solar System were interpreted as relics of a placid evolutionary history. Each planet seemed to have assembled very near its present orbit and maintained long-term dynamical harmony with its neighbors. In addition, the distribution of mass among the eight planets seemed to support a scenario of orderly growth: the smallest, Mercury, travels on the shortest orbit, where planetesimals were least abundant in the primordial nebula; the largest, Jupiter, occupies the nebula’s sweet spot, where solids were plentiful and growth was fastest; the next planet outward, Saturn, is only one-third as massive as Jupiter; and the combined mass of the two outer planets, Uranus and Neptune, is only one-third of Saturn’s.

This simple scenario, as it turns out, has fatal flaws.

  1. Our system’s mass distribution exhibits two striking discontinuities. Considering its position in the planetary line-up, Mars should be a few times the mass of Earth, yet it is only one-ninth as massive (Figure 12); while Neptune, the outermost planet, should be less massive than Uranus, its inner neighbor, yet it is almost 20% heavier.
  2. The skewed configurations of several planetary orbits testify to violent histories. Although Venus and Neptune have orbital eccentricities smaller than 1%, and Earth is under 2%, three adjacent giant planets – Jupiter, Saturn, and Uranus – have eccentricities in the vicinity of 5%, while Mars has 9% and Mercury has more than 20%.
  3. Both Jupiter and Neptune share their orbits with large populations of so-called Trojan asteroids, which occupy the stable Lagrange points located 60 degrees ahead of and behind each planet. A relatively static model of system evolution cannot explain these accumulations of small bodies.
  4. The axis of rotation of Uranus is tilted more than 90 degrees to the plane of the Sun’s equator (Between Earth & Uranus, Part I), a configuration that is often explained as resulting from a collision between Uranus and an otherwise unknown rogue planet in the Solar System’s infancy (Slattery et al. 1992, Parisi et al. 2008).

Taken together, these discontinuities, eccentricities, and irregularities offer incontrovertible proof of a complex evolutionary process.

The Nice model (Figure 9), which has become the most widely endorsed scenario of Solar System evolution, took as its starting point the moment when the Solar nebula had completely evaporated (Gomes et al. 2005, Tsiganis et al. 2005). At that time the four (or five) massive outer planets were assumed to be collected in a much more compact configuration than they are today, orbiting inside a semimajor axis of 15 AU.

However, the original Nice model could not explain how our giants happened to arrive in such a snug relationship. Over the past few years this gap in our understanding has been filled by a series of studies outlining a remarkable sequence of events known as the Grand Tack (Morbidelli et al. 2007, Batygin & Brown 2010, Morbidelli et al. 2011, Pierens & Raymond 2011, Walsh et al. 2011). The “tack” in this nickname has nothing to do with sharp pointy objects. Instead, it refers to a sailing maneuver (Figure 10) in which a ship sails against the wind by zigzagging back and forth (tacking) across her forward path, thereby changing the direction from which the wind hits her sails at the turning point of each zigzag (tack).

Figure 10. 2011 Extreme Sailing Series, Nice, Côte d’Azur

In the Grand Tack model, the rapid accretion of solids in the inner system during the first few million years after the Sun ignited was paralleled by an even more rapid accumulation of rocky and icy particles beyond 3 AU. The problem of narrow feeding zones, which made most inner-system embryos stall somewhere between the masses of the Moon and Mars, did not pose a barrier to growth in this outer region. Two large protoplanets assembled between 3 and 5 AU, with the inner object quickly reaching the phase of runaway growth, during which it accreted a deep hydrogen atmosphere. This object was proto-Jupiter, the first gas giant to emerge from the Sun’s protoplanetary cloud.

Having opened a gap in the dissipating nebula, proto-Jupiter began to spiral toward the Sun, carried along with the flow of accreting gases in a process known as Type II migration. The proto-giant plowed through broad fields of planetesimals orbiting inward of 3 AU, pitching them into the Sun, scattering them on eccentric orbits, or ejecting them from the system altogether.

Right behind proto-Jupiter came the second proto-giant, the ancestor of Saturn, which initiated runaway accretion not long after its bigger sibling and began to spiral inward even faster. When proto-Jupiter reached a semimajor axis of 1.5 AU (equivalent to the current orbit of Mars) the two objects entered into a mean motion resonance (MMR), such that their orbital periods could be expressed as the ratio of two small integers. Large suites of numerical simulations have specified the likely resonance as 3:2, such that proto-Jupiter completed three orbits for every two of proto-Saturn (Morbidelli et al. 2007, Batygin & Brown 2010, Pierens & Raymond 2011). The 2:1 and 5:3 resonances have also been shown to result in similar outcomes, at least in some iterations of the model (Batygin & Brown 2010).

Figure 11. The Grand Tack. Migration of the four giant planets through the Solar nebula over a period of 600,000 years. Distance is marked in astronomical units (AU). The narrow belt of planetesimals at 1 AU (the present orbit of Earth) later coalesced into the four terrestrial planets.

The establishment of this orbital resonance was in effect a “marriage” between the two proto-giants, such that their dynamical trajectories thereafter became inextricably linked. Their inward migration now halted and abruptly reversed. This striking reversal is the Grand Tack itself, which brought both giants back into the outer system, maintaining their resonant configuration the whole time (Figure 11). Jupiter’s tacking maneuver permitted a considerably diminished belt of planetesimals and planetary embryos to reassemble around 1 AU, the current orbit of the Earth. Meanwhile, the outward passage of Saturn brought the inner of the newly-formed gas dwarfs under its gravitational influence, such that Saturn captured the young dwarf (either Uranus or Neptune) in another MMR (Morbidelli et al. 2007, Batygin & Brown 2010). This gas dwarf, in turn, captured the next dwarf out.

In one promising model developed by Morbidelli and colleagues (2007), Neptune (identified as the outer gas dwarf) participated in a 3:4 MMR with Uranus, Uranus participated in a 2:3 MMR with Saturn, and Saturn participated in another 2:3 MMR with Jupiter. In this model, the ratio of the four orbital periods, from Neptune to Jupiter, is 3:4:6:9.

Having attained their mature masses, Jupiter and Saturn paused in their flight from the Sun. A likely explanation for their journey’s end is the attenuation of the Solar nebula, to the point where it could no longer sustain gas-driven migration: as if an ebbing flood had stranded these two giants in its wake. Jupiter landed near its present semimajor axis of 5.2 AU, Saturn achieved an orbit near 7 AU, while the brood of gas dwarfs – Neptune, Uranus, and perhaps “Amphitrite” – were confined to a region between 9 and 15 AU. Beyond these cold orbits stretched the solid remnants of the protoplanetary disk: a field of rocky and icy objects extending to a radius of about 30 AU or so. This population would eventually contribute to the Kuiper Belt.

Intuitively, the titanic sequence of events involved in the Grand Tack might seem to require many aeons to reach its climax. As a point of reference, the Mesozoic Era on Earth lasted 180 million years, during which the supercontinent Pangaea broke up and the dinosaurs evolved, flourished, and died out. Yet the span of time required for the Grant Tack was actually shorter by a few orders of magnitude. According to recent simulations, only 600,000 years elapsed from the accretion of Jupiter’s atmosphere, through the migration of both gas giants across the Solar nebula, to the establishment of the four-way orbital resonance that provisionally stabilized the outer system (Walsh et al. 2011). Thereafter, according to this model, the four giant planets ran like clockwork for hundreds of millions of years in precisely matched revolutions around the Sun.

Although the duration of the Grand Tack can be estimated, uncertainty surrounds both its absolute chronology (in terms of time since the Sun’s ignition) and its relative chronology (in terms of its temporal relationship with other key system events). For example, Vesta formed about 2 or 3 million years after the Sun, but did its birth occur before, during, or after Jupiter’s migration? The same questions can be asked of Mars, whose accretion was complete about 10 million years after the condensation of the Solar nebula.

Two synchronisms seem clear: the Grand Tack finished when the nebula dispersed, and the process of dispersal coincided with the formation of the regular satellites of Jupiter and Saturn in “gas-starved” accretion disks around their parent planets (Canup & Ward 2006). Given what we know about the lifetimes of gas disks, these outcomes transpired no earlier than 3 million years and probably not much later than 6 million years after the Sun’s first light. 

Figure 12. Runts of the Litter. Mercury & Mars are dramatically smaller
and less massive than their cloudy siblings, Venus & Earth.

worlds in collision

The evaporation of the gas nebula ended the tumultuous first act of our Solar System’s origin story. Jupiter’s brief incursion to a perihelion of 1.5 AU had devastated the inner system. All that remained of the earlier population of planetesimals and embryos was a narrow ring of rocky and icy objects orbiting around 1.0 AU, near the present orbits of Venus and Earth. Much of the second act of the drama focuses on events in this inner region, although its finale involves the whole system.

The notion that the four terrestrial planets (Mercury through Mars) originated in a narrow annulus between 0.7 and 1 AU (Hansen 2009), instead of in a much broader field of planetesimals extending from ~0.5 AU to ~3.5 AU, is perfectly congruent with the scenario of the Grand Tack (Brasser 2012, Morbidelli et al. 2012). Yet it was originally advanced by Brad Hansen on the basis of numerical simulations whose sole purpose was to reproduce the current architecture of the inner Solar System (Hansen 2009). In fact, Hansen conceded that, although his model nicely explained the masses, orbits, and formation timescales of the four inner planets, it could not account for the origin of their depleted birth environment or for existence of the current Asteroid Belt.

Serendipitously, the Grand Tack fills both gaps. We have already seen that it explains the annulus as a leftover from Jupiter’s marauding days. The Asteroid Belt then becomes a by-blow of the accretion of the terrestrial planets, a junkyard that collected all the debris ejected from the inner system by gravitational scattering and collisions between planetary embryos.

After Jupiter withdrew from the inner system, the remaining mass of planetesimals accreted into a population of planetary embryos that underwent violent impacts. This process assembled Mars within about 10 million years after the birth of the Sun, and simultaneously ejected the young planet from the main annulus onto a wider and somewhat eccentric orbit (Hansen 2009). Mercury was probably produced on a similar timescale and ejected onto an even more eccentric orbit closer to the Sun, although its formation has not yet been explored in detail. Both of these planets are considerably smaller and less massive than Earth and Venus (Figure 12): Mars has only 0.11 of Earth’s mass (Mea), while Mercury has only 0.06 Mea. Their stunted growth is readily explained by their early expulsion from the “birth ring.” At 0.81 Mea, Venus required the equivalent of five Mercuries and five Marses to attain her current bulk, while Earth needed approximately six of each.

All scenarios agree that Earth is the youngest of the terrestrial planets, and thus the youngest planet in the whole system. According to the most widely accepted models, its formation was not complete until it suffered a glancing collision with a Mars-sized object late in its accretion history. The resulting debris endowed the Earth with its final mass and contributed to a circumplanetary ring from which the Moon rapidly assembled (Canup 2004). Although debate continues over the timing of this Moon-forming event, a date between 30 and 50 million years after the condensation of the Solar nebula has broad support (Canup 2004, Jacobsen 2005, Jacobsen et al. 2009, Hansen 2009).

A remaining problem in inner system evolution is the origin of the Earth’s oceans, sometimes discussed under the heading of  “water delivery.” Numerous studies have argued that proto-Earth was dry, since its birth environment inside the ice line was too hot to sustain freely orbiting volatiles. Water must have arrived from cooler regions of the system, with the Asteroid Belt favored over the Kuiper Belt as its source on the basis of hydrogen isotopes in Earth’s current oceans (Morbidelli et al. 2000). However, the delivery of just the right amount of water at just the right time may have required an implausible degree of fine-tuning. 

Not to worry, because along with so much else, the Grand Tack can also explain the origin of our biogenic oceans (Brasser 2012, Morbidelli et al. 2012). Jupiter’s early migration through the inner system scattered plenty of wet planetesimals from beyond the ice line into the terrestrial birth ring, making them available for accretion by all four protoplanets. In the end, Earth was the only one with the right stuff – mass, temperature, magnetic field, atmosphere – to retain its precious inheritance. The match between the isotopic content of our oceans and that of carbonaceous chondritic meteorites is then explained, not by a bombardment of such objects after Earth formed, but by their presence among the building blocks that originally assembled it.

Figure 13. Late Heavy Bombardment. a. At the end of the Grand Tack, two gas giants and two ice giants orbited inside 15 AU (Jupiter = green, Saturn = orange, Neptune = blue, Uranus = turquoise).
b. All four giants suffer orbital instabilities, and Neptune and Uranus switch orbits, scattering asteroids and comets throughout the Solar System. c. The four giants achieve their final orbits amid a dramatically depleted field of leftover planetesimals. Credit: Wikimedia

late heavy bombardment

After the Moon-forming event, the inner Solar System settled into a long period of relative stability. For several hundred million years, asteroid and comet strikes happened intermittently, while the four (or more) outer giants revolved in their clockwork orbits. Trouble was brewing, nevertheless. The presence of a large, encircling field of planetesimals – ancestor of the present-day Kuiper Belt – resulted in a steady enlargement of the orbits of the gas dwarfs (Uranus, Neptune, and maybe “Amphitrite”) located beyond Saturn. Eventually, interactions between Saturn and the next planet out resulted in a scattering event that disrupted the outer system (Morbidelli et al. 2007, 2009a). As Morbidelli and colleagues describe it:

The migration of the giant planets, induced by the interaction with this [planetesimal] disk, increases the ratios of the orbital periods between each pair of planets. Thus, the planets are extracted from their mutual quadruple resonance. Because the system is very compact, new resonances are crossed during the migration. These resonances excite the eccentricities of the planets, triggering a global instability of the system. The orbits of the planets are eventually stabilized by the dynamical friction exerted by the planetesimal disk during its dispersal. (Morbidelli et al. 2007)

During this violent epoch, Uranus and Neptune may have switched orbits, and “Amphitrite” may have collided with Uranus, producing the extreme axial tilt we observe today. Thereafter, any extra gas dwarfs must have been ejected from the system, since we have only two left.  

Regardless of the details, Neptune migrated steadily outward across a distance of 15 AU, to its present semimajor axis of 30 AU. In the process, it scattered planetesimals far and wide and dramatically reduced the overall mass of the Kuiper Belt. The storm of eccentric comets and asteroids launched by Neptune is the Late Heavy Bombardment, whose scars are still visible as craters on the surfaces of the Moon and the larger satellites of Jupiter and Saturn. Earth and the giant planets must also have experienced an intense hail of impactors, but their dynamic local environments have erased all evidence of this cataclysm.

The starting point of the Late Heavy Bombardment has been variously placed between 600 and 800 million years after the birth of the Solar System, while its duration was in the vicinity of 10 to 150 million years (Gomes et al. 2005). By the time the dust settled, the eight major planets had established the orbital architecture we see today, and the second act of our drama of origin had given way to the third.


The Earth as we know it was born in the Moon-forming event, meaning that the Earth and the Moon share the same birthday. The epoch between that date and the Late Heavy Bombardment is known as the Hadean, after the hellish conditions once thought to prevail then. Recent evidence, however, demonstrates that Earth cooled rapidly after the Moon’s formation, so that a global ocean was established within 100 million years (Arndt & Nisbet 2012), or about 4.4 billion years ago. That ocean has persisted to the present day, rising and falling ceaselessly in response to lunar and solar tides. (Update August 2014: for a more recent a far less benevolent picture of the Hadean, see A Billion Years of Hell on Earth.)

Already in the Hadean, then, Earth was a blue planet, with an even smaller ratio of dry land to water than at present. Volcanism was common on land and undersea, but no significant mountain ranges had yet developed (Arndt & Nisbet 2012). The global temperature at that epoch remains a matter of debate, with many researchers favoring a climate cool enough for flotillas of icebergs to populate the seas. Some have argued that the first microbial life may have emerged during the Hadean, perhaps in the vicinity of submerged volcanoes (Arndt & Nisbet 2012).

The Late Heavy Bombardment that ended the era was a catastrophe of literally cosmic proportions, yet it did not evaporate the planetary ocean. Nor was it sufficient to sterilize our planet, if any life already existed (Abramov & Mojzsis 2009). In fact, the first signs of life appear right after this event, leading to some interesting speculations. Perhaps the bombardment triggered the evolution of the first organisms on Earth, or perhaps the impactors themselves brought dormant organisms with them, or at least the amino acids that became their building blocks. In any event, microbial life was abundant on Earth by 3.8 billion years ago, when the Solar System had reached an age of about 800 million years (Arndt & Nisbet 2012). It has survived ever since.

The emergence of multicellular organisms, however, is a far more recent phenomenon. The Ediacaran Period, which was under way just 575 million years ago, when the Earth was already 4 billion years old, marks “the appearance of the first large and architecturally complex organisms in Earth history” (Narbonne 2005). The Ediacaran was immediately followed by the better-known Cambrian Period around 540 million years ago. Modern humans, of course, are no older than about 100,000 years or so. All our works represent a thin veneer on a very ancient world.

Because simple organisms appeared in the Solar System well within the first billion years after its creation, similar life forms may also have emerged elsewhere in the system. For example, the early environments of Venus and Mars may have been much more biophilic than they are at present, even if nothing could survive on either planet today. Still more hopeful speculations have addressed the icy moons of Jupiter and Saturn, some of which – Europa, Ganymede, Enceladus – may support bodies of liquid water beneath their frozen crusts. If they have water today, then maybe they have microbes also. Finally, the unique environment of Titan – Saturn’s largest moon, and the only Solar System object besides Earth that is known to support surface bodies of liquid – may hold still more surprises in store for us. Nothing resembling Earthly organisms could live in such a frigid place, whose rivers run with ethane, and whose lakes are liquid hydrocarbons. Nonetheless, it is conceivable that some as yet unimagined form of life may be drinking that liquid or floating down those streams.

Then there are the hundreds and hundreds of exoplanetary systems waiting for us in the Sun’s back yard. It’s becoming progressively clearer that very few of them resemble our home system (see How Weird Is Our Solar System?). The formation scenarios presented here – Grand Tack, Terrestrial Birth Ring, Late Heavy Bombardment – explain why our system is so unusual. These theoretical constructs detail a highly specific evolutionary history that is unlikely to be repeated with much regularity around other stars. Again, however, the sheer abundance of worlds points to a similar abundance of potential sites for life – life that is weird, unearthly, perhaps even unrecognizable – throughout the Milky Way.

Figure 14. Dickinsonia costata. Fossil imprint of an Ediacaran organism,
dating from about 560 million years ago. Credit: Wikimedia


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Solar System Archaeology, Part I

Figure 1. Lunar craters: Von Karman, Leibniz, and Oppenheimer. Credit: NASA/Michael Benson
The ongoing (and exciting!) Dawn mission to the Asteroid Belt is just the latest in a long line of efforts to understand the nature and origin of our Solar System. The modern era of system exploration began more than half a century ago, back in the 1950s. That’s when C.C. Patterson of Caltech used measurements of lead isotopes in meteorites associated with Meteor Crater, Arizona, to estimate that the Solar System formed 4.55 billion years ago (Patterson 1956). Within error margins, his date still stands today.

Other crucial evidence came from manned and unmanned missions to the Moon, beginning in 1959 with the Luna program of the Soviet Union and quickly followed by the Surveyor and Apollo programs of the United States. These and later efforts mapped the entire Lunar surface, enabling a detailed study of the Moon’s history of cratering (Figure 1). They also brought back rocks from Lunar impact sites that provided a range of dates, none of which were older than 3.9 billion years. This was the first indication of a discrete, system-wide cataclysm now known as the Late Heavy Bombardment.

Since those pioneering space programs, ever larger amounts of data have been accumulating from robotic missions to various parts of the inner and outer Solar System, as well as from ground- and space-based studies of star-forming regions in our neighborhood of the Milky Way. Beginning in the mid-1990s, data on extrasolar planetary systems have contributed strikingly new perspectives, since most exoplanetary systems bear little resemblance to ours, even though they must have formed from similar building blocks.

Figure 2. The eight Solar planets at their approximate relative sizes.

As a result of all this cosmic archaeology over the past six decades by some thousands of researchers, it’s now possible for a geeky layperson (like myself) to sketch a surprisingly detailed history of our Solar System (Figure 2). This review of current findings is principally informed by the work of Alessandro Morbidelli and the growing number of astronomers and planetary scientists who have collaborated on successive iterations of the so-called Nice model of system evolution. (As a theoretical framework, this model very nice indeed, but its name actually derives from the French city (Nice) that houses the institution (l’Observatoire de la Côte d’Azur) with which Dr. Morbidelli is affiliated.)

The story proceeds in seven sections:
  • primordial chaos: Birth environment of the Solar System
  • structure emerges: Structure and early evolution of the Sun’s protoplanetary disk
  • accretion everywhere: Assembly of planetesimals and planetary cores
  • the grand tack: Migration of the four giant planets through the primordial gas nebula
  • worlds in collision: Formation of the four terrestrial planets by giant impacts
  • late heavy bombardment: Asteroids and comets scatter throughout the system
  • life: Biogenesis on Earth and potentially elsewhere in our system

primordial chaos

All stars form in clusters. Our Sun was no exception. All clusters originate in huge dark nebulae (Figure 3) composed of molecular hydrogen swirling in supersonic turbulence (McKee & Ostriker 2007). Such a vast dark cloud, extending for tens of light years, was the primordial chaos out of which you and I and everything else on Earth originated.

Figure 3. Dark molecular clouds in Taurus.
Credit: European Southern Observatory/Davide de Martine

Fragmentation and clumping within the birth nebula produced hydrogen “cores” that collapsed into protostars. Evidence from short-lived radioactive isotopes in the present-day Solar System indicate that our native cloud produced at least one extremely massive O-type star (~25 times Solar mass), which enriched its surroundings either by exploding as a supernova or by producing fierce stellar winds. A star so massive can form only in a relatively large cluster with more than 1,000 members (Adams 2010).

Accordingly, various analyses have suggested an initial membership of 1,000 to 5,000 stars for the Solar System’s birth cluster. This is an unusually large population, since the median membership for birth clusters in the Milky Way is only about 300 protostars (Lada & Lada 2003, Adams et al. 2006). The powerful gravitational influence of member stars would have kept our home cluster bound together for tens or even hundreds of millions of years – a span of ages comparable to those of the Pleiades (100 million; see Figure 7) and Hyades Cluster (625 million). Our huge cohort of sibling stars was originally confined to a volume only a few parsecs in diameter (~10 light years), but the cluster steadily expanded over time as individual stars began to spill out of its tidal radius.

Stars and substellar objects are currently classified into 10 types according to their spectra. They range from the very brightest, hottest, and bluest objects (types O and B, which include the three stars of Orion’s Belt), through bright white stars (type A, represented by such familiar backyard stars as Sirius, Vega, and Altair), through the yellowish Sun-like stars (types F, G, and K, represented by the nearby planet-hosting stars Upsilon Andromeda, our Sun, and Epsilon Eridani, respectively), through the red dwarfs (type M, which includes GJ 581, another nearby planetary host), down to the dim brown dwarfs (types L, T, and now Y), which are not massive enough to sustain nuclear fusion, and thus are not stars.

Remarkably, star clusters tend to produce objects at the full range of spectral types (except for type O, which as we have seen is confined to larger clusters). True stars are born in numbers that correspond inversely to their mass and temperature, so that red dwarfs are the most numerous and O stars are the rarest. This correlation does not extend into the substellar regime, however, as brown dwarfs are apparently much less common than red dwarfs (Kirkpatrick et al. 2012).

Figure 4. The Iris Nebula (NGC 7023), a stellar nursery undergoing
dispersal. Credit: Wikimedia/Hunter Wilson

Within the depths of our primordial cloud, star formation proceeded in waves – sometimes called starburst episodes – extending over thousands or even millions of years. As soon as the brightest stars ignited, their intense radiation and stellar winds began to disperse the parent nebula (Figure 4). Complete dispersal of the gas envelope required 1 to 10 million years (Lada & Lada 2003, Throop & Bally 2008), whereupon the cluster of newborn stars was unveiled for the universe to see. By this time the process of star formation had ended and cluster evaporation had already begun.

Generations of newborn stars arrived in cocoons of hydrogen and dust known as protoplanetary nebulae or protoplanetary disks (“proplyds”). Like the vast, encompassing parent cloud, these smaller clouds also dispersed within a few million years, with dissipation proceeding faster around hotter, more massive stars and slower around smaller, cooler stars. Typical lifetimes of protoplanetary disks fall in the same range as the lifetimes of molecular clouds – i.e., 1-10 million years – with a median age between 2 and 3 million years (Williams & Cieza 2011). Stars older than 6 million years rarely preserve their primordial disks (Williams 2010).

structure emerges

Despite the Copernican principle of mediocrity, which asserts that there is nothing special about Earth, many factors indicate that we humans occupy a favored piece of real estate. First, we originated in an unusually large birth cluster with a substantial cohort of very bright stars. Second, our native star belongs to spectral class G2, meaning that it is hotter and more massive than 90% of the stars in the Milky Way. Notably, stars of higher mass (such as our Sun) are more likely to host planets than stars of lower mass (Johnson et al. 2010). Third, our Sun has a higher than average enrichment in elements heavier than hydrogen (known to astronomers as metals). All members of our birth cluster must have shared this endowment, which like high mass encourages the evolution of planets, especially gas giants like Jupiter and Saturn (Johnson & Li 2012). Thus, our native environment showed clear promise of future greatness.

 Figure 5. Proplyds in the Orion Nebula.
Credit: NASA/ESA/Luca Ricci

The protoplanetary nebula surrounding the newly-formed Sun was probably fluffy and irregular at first, like many of the proplyds in Figure 5. In short order, nevertheless, the angular momentum of our rapidly spinning parent star caused this puffy cloud to flatten into a thin disk of gas and dust, typically known as the Solar nebula. The disk midplane was aligned with the Sun’s equator, and its spin naturally followed the Sun’s rotation (Figure 6).

The ratio of gas to solids in the Solar nebula was originally about 100 to 1 (Andrews & Williams 2005), and its overall mass must have been about 1% to 3% of the Sun’s (Williams 2010, Adams 2010), corresponding to 10-35 Jupiter masses (Mjup; the Sun is 1048 Mjup). Since the total mass of all surviving planets, dwarf planets, moons, asteroids, and comets in the present-day Solar System is less than 2 Mjup, the evolutionary process was markedly inefficient: 80% to 95% of our original patrimony of gas and dust has vanished.

Considerable evidence indicates that the primordial disk of solids in our system had a sharp outer edge at a radius of 30-35 astronomical units (AU; Levison et al. 2003, Tsiganis et al. 2005, Adams 2010). This is notably smaller than the observed dimensions of many nearby dust disks, suggesting that our native nebula was truncated in some way, perhaps by sibling stars in the birth cluster. A likely culprit would be a nearby O star, since observations of the Orion Nebula demonstrate that such massive luminaries can burn off the outer layers of the protoplanetary nebulae of smaller stars that venture within a few light years of their highly energetic photospheres (Williams & Cieza 2011).

Figure 6. Artist’s impression of a young protoplanetary disk.
Credit: NASA/JPL-CalTech

Our own Sun’s magnetospheric activity created an inner cavity in the disk at an approximate radius of 0.05-0.10 AU (Laine et al. 2008). Between this inner edge and the outer limit at ~30 AU, the temperature, surface density, and composition of the disk varied systematically. At small radii, close to the blazing Sun, only the refractory particles (metals and silicates) suspended in the gas cloud could survive, while icy particles (water, methane, ammonia) dominated the mix on wider orbits.

The boundary between the two regimes of ice and rock is known as the ice line or snow line – the distance from the Sun where free-floating molecules of water and other volatiles condense into ice (Ida & Lin 2004, Kennedy & Kenyon 2008). This occurs at a temperature of 170 K, which in the contemporary Solar System corresponds to a radius of 2.7 AU. At primordial times, however, the situation was different. On the one hand, our Sun was about 30% cooler than it is now, so it added less heat to the circumstellar environment; on the other hand, the viscous flow of gases through the Solar nebula created a powerful alternative source of heat. The net result is that our system’s ice line may have been located around 4 or 5 AU when the Sun was ~100,000 years old, shrinking to 2 AU by the time the Sun reached its one-millionth birthday (Kennedy & Kenyon 2008).

Another key structural feature of the Solar nebula was its flared profile, such that its vertical dimension increased along with its radius. This configuration is typical of protoplanetary disks (Williams & Cieza 2011), and is visible in the proplyd at the very center of Figure 5.

Figure 7. The Pleiades star cluster. Image by Antonio Fernández Sánchez.
Credit: NASA/courtesy of

accretion everywhere

The Solar System’s primordial ice line has been compared to a standing blizzard (Lin 2008). Just beyond this limit, the presence of ice particles increases the density of solid materials by a factor of 3 or 4 (Kennedy & Kenyon 2008). This “bump” in density and solid mass has crucial implications for the evolution of planetary systems.

The abundance of icy particles, along with the favorable collisional velocities obtaining on long, chilly orbits, combined to make this region the “sweet spot” of planet formation (Mordasini et al. 2012). Conditions for the coagulation of solids were more favorable here than anywhere else in the disk, so it is no surprise that our system’s most massive planet, Jupiter, assembled just beyond this boundary.

The formation of gas giant planets has been described as a threshold phenomenon, as it requires a specific set of initial conditions – sufficient density of solids in the protoplanetary nebula to assemble large planetary cores, sufficient persistence of nebular hydrogen to support runaway gas accretion – that apparently obtain around less than half of all stars (see How Weird Is Our Solar System?). As Andrew Mann and colleagues have argued, “a substantial fraction, and probably the majority of stars do not host [gas] giant planets” (Mann et al. 2010). Low-mass stars are especially likely to be giant-free (Laughlin et al. 2004).

The Solar nebula was moderately massive, with a modest enhancement of metals. These initial conditions enabled it to generate two relatively small gas giants. Although Jupiter has triple the mass of Saturn, it is still a relative lightweight in the exoplanetary population. The median mass of extrasolar gas giants is 1.6 Mjup, and many systems host giants of 5 or even 10 Mjup. Regardless of their mass, however, gas giants seem more likely to come in pairs than in triplets or quadruplets; only four systems (47 Ursae Majoris, HD 37124, HIP 14810, Mu Arae) are known to contain three, and only Upsilon Andromedae has so far been proposed as the home of four.

Yet proto-Jupiter and proto-Saturn were not alone on the cold orbits of the outer nebula. At least two and possibly three or more objects in the range of the gas dwarfs formed on the far side of the sweet spot – Uranus and Neptune for sure, and perhaps a smaller gas dwarf or Super Earth nicknamed “Amphitrite” (Desch 2007, Desch & Porter 2010). Their cores assembled somewhere between 5 and 10 AU.

Figure 8. The South Pole of Vesta, oldest rocky sphere in the Solar System.

A single favored region of the Solar nebula, then, produced all of the most massive planets in our system. Nevertheless, accretion – the process of growth by gradual accumulation – was active throughout the nebula as soon as it flattened into a disk. Accretion is the route by which primordial dust grains conglomerated into planetesimals, the building blocks of planets. Current theories propose that a steady, stepwise process of growth was punctuated by the much more rapid collapse of refractory particles into full-sized planetesimals, with an approximate range in diameter of 1 to 100 kilometers (Johansen et al. 2007, Morbidelli et al. 2009b).

In the inner Solar System, accretion proceeded quickly because of the relatively high velocities of orbiting solids. The result was most likely a large population of so-called planetary embryos traveling in concentric orbits. The presence of nebular gas damped the eccentricity of their orbits – i.e., prevented any deviations from circularity – so that their growth stopped once they exhausted their local “feeding zones.” Isolation masses for these rocky embryos have been estimated at 1%-10% of Earth mass, equivalent to a population of Moon- to Mars-sized objects (Morbidelli et al. 2012).

Some sense of the time required for embryo gestation is provided by the rocky asteroid Vesta (Figure 8), which evidently attained its mature mass and spherical shape just 2 to 3 million years after the condensation of the Solar nebula (Russell et al. 2012). Vesta must have had numerous siblings of roughly similar mass and composition in the young Solar System, though very few survive as isolated objects today.

. . . . continued in next blog post, with list of references . . . .