Planetary formation - a four stage process #
**Have you read the preceding page: "The formation of our solar system: the sun"?
Perception: a clockwork solar system The reality: a system forged in chaos
When most of us of more mature years were growing up, the solar system seemed reliable and well behaved, and the movement of the planets appeared as if regulated by clockwork - “forever in the past, and forever in the future”[1] – and there was no evidence that planetary orbits had ever changed. So the perception of a clockwork system stuck with us for most of our lives. It was enduringly stable and forever.
We now know that not only the sun exerts a gravitational pull, but that the planets also interact with one another. Their gravitational tuggings are far weaker than those of the sun, but over time they affect the paths of neighbours and the relentless pull of gravity can amplify small deviations until orbits migrate, cross, or otherwise go haywire. Nor was the path of planetary migration, which was both the root cause and the result of this process, necessarily smooth, as once thought. In fact, it was positively chaotic.
Scientists now think that the planets as we now see them about us arose in a four-stage process: coagulation, runaway growth, the era of carnage and the late heavy bombardment.
The first phase is coagulation: Molecules begin to form in the disk, which has the same composition as the molecular cloud: nearly all gas, with some dust. The role of dust is crucial since dust grains are very effective at trapping infrared radiation, and the resultant increase in temperature at the centre eventually leads to the formation of a protostar. Different materials then condense out at different radii. At high temperatures (> 2000K) rocky minerals and metals like iron condense. Below about 270 K water ice condenses, as well as ammonia and methane.
Water is the most abundant of these ices, and condenses at the highest temperature. The distance at which water can freeze out is called the ice line. Beyond that distance there is much more mass available. In the Solar System, this line is at about 5 AU (1 AU (Astronomical Unit) = 148,598,000 kms) which marks the separation between the terrestrial and the Jovian planets. The small rocky terrestrial planets and asteroids, characterised by relatively small size, relatively high density, and a small number of satellites, lie closer to the sun. Beyond 5 AU, the giant planets have the opposite characteristics: large sizes, low densities, and many satellites and ring systems. The condensing materials (again, dust and at greater distances from the centre, ice) stick together by colliding and sticking together using normal chemical forces. They coagulate into larger clumps, eventually forming porous bodies or loose fractal aggregates, best described as fluffy dustballs, leading to the formation of plantesimals (bodies up to about 1 km in size) as the result of accretion[2].
The dust-ball aggregates settle to the nebular midplane of the disk, growing through collisions all the time. Within 10,000 years, the particles have grown to a centimetre or more in size. Friction with the gas makes them spiral towards the Sun, growing as they do so. Now gravity becomes important. The larger planetesimals can sweep up more material, so the biggest bodies grow much faster than smaller ones – a process known as runaway growth. Runaway growth ends when the planetesimal (now called a planetary embryo) has consumed nearly everything within its reach.
The role of dust
Dust has been crucial in all stages of the solar system’s evolution. As the original cloud of material that produced the suns and planets collapsed, dust grains became more effective at trapping infrared radiation; the resulting increase in temperature at the centre of the cloud eventually led to the formation of a protostar. Later, dust (and, at greater distances from the centre, ice) settled toward the nebular midplane and coagulated into larger clumps, eventually forming porous bodies known as planetesimals, ranging in size from a few metres to tens of kilometers. Some of these planetesimals melted. The planets ultimately formed from a diverse suite of such melted and unmelted planetesimals. By studying chondrules (tiny beads of melted material often smaller than a rice grain that formed before asteroids took shape early in the solar system’s history), and the chondrites in which they are found (asteroidal fragments or meteorites that have fallen to earth), scientists have been able to determine where the dust appearing in them originated from before the planets in our solar system actually formed. The densest region of dust appears to have occurred between 2.7 and 4.5 AU, densest at about 3.6 AU. Further out, it declines, and also in the other direction, ending up very sparse in the region of in the region of the future Earth and Mars. This information can tell us something about the formation of our own solar system.[3]
The giant planets appear to have formed by first accreting a core of several Earth masses. This core was more massive than the proto-planets in the inner Solar System because the proto-Jovian planets were beyond the ice line. Once this solid mass had accumulated, the planet starts accreting gas more and more efficiently, in a runaway gas accretion phase. Jupiter and Saturn grew much larger because they formed further in, where the disk was thicker. The giant planets were hot when they were accreted. This expanded their atmospheres to vastly larger dimensions than they have today. Gradually they radiated away this heat and shrank, leaving a disk of gas, ice and dust in orbit: a small-scale analogue of the solar nebula. From these disks emerged the regular satellites and ring systems. Meanwhile, radiation from the young sun started to evaporate the disk. Gas and small dust grains are blown away, leaving only large grains and planetesimals. The gas giants must have completed most of their growth before this happened, though the terrestrial planets continued to grow via collisions.
However, it should be noted that there is another method of giant-planet formation which does not depend upon the ‘bottom up’ method of core accretion. Instead of taking millions of years, a dense, cold clump of gas could collapse in a ‘bottom down’ scenario to form a giant planet in only thousands of years. Such a rapid, efficient collapse would generate and trap intense heat within the newborn planet, giving it a powerful infrared glow for millions of years, much like embers cooling from a campfire. In contrast, under the accretion method, the planet’s core comprised of accumulated grains, rocks, boulders etc gradually melds together flaring in brightness, and then cooling the new planet by radiating away the heat, leaving it cooler and less luminous than a top down disk-instability planet of the same age. The Gemini Space Telescope (GPI) situated high in the remote Andes of central Chile and SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research instrument)at the European Southern Observatory’s Very Large Telescope array are currently engaged in taking the temperature of young giant planets via infrared imaging in order to determine whether most are built from the bottom up or the top down[4].
In any event, once the protoplanets reached the size of the moon or larger, the final stages of planet formation began, where the hundred or so protoplanets were reduced to the current handful. The planetary embryos perturbed each other into crossing orbits, leading to giant impacts: this has been called the Era of Carnage. This last handful of impacts left permanent scars on nearly every member of the solar system. Every old surface bears witness to having been battered by impacts of all sizes.
The formation of the Moon
As the protoplanets perturbed each other’s orbits and collided, they also mixed up, so that planetary embryos which were born far from the sun ended up in the inner solar system - and vice versa[5]. The collisions produced enormous amounts of heat, so the planets would have been molten, with surface temperatures of about 1500K. The surface of the planet melted, with and lighter material floating: the interior differentiates. We think that the Earth’s moon was formed in this last phase of planet formation, as the result of a collision between the proto-Earth and another planet-sized body. Material from the impact was thrown into orbit and coalesced into the moon, with the heavier metals such as iron sinking to the earth’s core. The impacting body must have been about the size of Mars, most of the material left in orbit coming from the impactor, but all the volatiles[6] (nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane) were lost.
However, a more recent study[7] suggests that Earth is actually made up from two planets, the early Earth and an impactor called Theia, which came together in a head-on collision that was so violent it formed the Moon. This is said to have occurred approximately 100 million years after the Earth formed, almost 4.5 billion years ago [8].
The collision was so violent that the two planets effectively melded together to form a new planet, a chunk of which was knocked off to form the Moon. A great deal of debris was also exchanged, leading both the Earth and the moon to have almost identical levels of oxygen isotopes. The researchers studied rocks brought back to Earth by the Apollo 12, 15 and 17 and six volcanic rocks from the Earth's mantle, five from Hawaii and one from Arizona, and found their oxygen isotopes indistinguishable: they contained identical composition of an isotope of oxygen called delta-17. (More than 99.9% of the earth's oxygen is O-16, so called because it has 8 proteins and 8 neutrons. But the re are also small quantities of heavier oxygen isotopes: O-17, which have one neutron, and O-18 which have two).
The study also found that some of the water we find on Earth today may have come from Theia, and that both the early Earth and Theia must have had 'substantial' amounts of water high in delta-17 oxygen either trapped in minerals or as ice. This suggests that much of the water we see on Earth today, which is thought to have been crucial for life forming on our planet, must have come from Theia.
There are also other benefits for Earth deriving from the moon's presence. Without its stabilising gravitational influence, our planet would "wobble like a dying top". It keeps the Earth spinning at just the right speed and angle necessary necessary to foster life both in the short and the longer term. It effectively makes us a binary planet. The bad news is that the moon is receding from the Earth's orbit at about 1.5 inches a year and is expected to move beyond it altogether in 2 billion years time, when stability will once more be an issue, but one of purely academic interest so far as we personally are concerned [9].
But let's return to the era of carnage. The bombardment of the solar system has not stopped, only reduced in intensity. In July 2004, the Comet Shoemaker-Levy 9 impacted on Jupiter: the very-very-late stages of planetary accretion. The end of the Era of Carnage came when the planets found themselves in stable orbits far enough apart to be stable for billions of years. This naturally led to well-spaced planets, a pattern also seen in the exoplanet systems with at least three planets[10].
The foregoing represents what may be described as the conventional or classical view of solar system formation. However, it left a number of question unanswered, and the realisation that this model was far from classical came about twenty years ago with the discovery of exoplanets in solar systems other than our own. “Why”, it was asked, “is the solar system’s inner region so depleted in mass compared with its interplanetary counterparts, with relatively ‘runty’ rocky worlds instead of the so-called super Earths found in abundance in other solar systems, and no worlds at all inside Mercury’s 88 day orbit?” Why are the orbits of the sun’s giant planets so calm and spread out? And how can one account for Jupiter’s and Saturn’s orbits so far from the sun?”[11]
The Kuiper belt
Before answering these questions, we must consider some comparatively recent discoveries of a number of other players in the field. Firstly the Kuiper belt. The outer parts of the proto-stellar disk never coalesced into planets. Outside the orbits of the planets, the sun is left with a disk of thousands of small icy bodies beyond Neptune’s orbit, some of which are on highly elliptical orbits, periodically visiting the inner solar system as comets – the Kuiper Belt[12]. Just over 20 years ago[13], nobody even knew that the Kuiper belt existed. Since then, beginning in August 1992 more and more of these icy bodies of various dimensions, known as Kuiper belt objects (KBOs) have been discovered until they now number around 1,500. A handful approach and even rival Pluto in size.
In 2014 it was estimated that the Kuiper belt is home to 100,000 objects more than 100 kilometres across and up to 10 billion larger than two kilometres across. So that for every asteroid in the asteroid belt, there are 1000 objects in the Kuiper belt[14] It was reasoned that something must have happened to snuff out the largest Earth-size and beyond members of the Kuiper belt.
Secondly, Pluto. The so-called “oddball” of the solar system dips far above and below the pancake-like plane in which the eight planets travel, its elongated orbit taking it from 30 to 50 times the Earth’s distance from the sun. But the most curious thing about Pluto is its bond with Neptune. For every three times that Neptune orbits the sun, Pluto orbits twice, and in such a way that the bodies never approach each other. This is called a “mean motion resonance” (MMR), and the speculation about how it came to be is that when the solar system was young and full of asteroids and comets, Neptune was closer to the sun[15]. Pluto was originally classified as a planet, but by the 1970s it was clear that Pluto was smaller and much less than the Earth’s moon. What had been discovered was simply the brightest member of the Kuiper belt[16]
And even more recently, in 2016, far beyond the eight planets, the asteroid belt and even the distant Kuiper belt past Neptune, astronomers have seen roughly a dozen objects moving in strange orbits, all taking elongated paths around the sun. They all seem to make their closest approach to the sun around the time they cross the path of the planets, causing scientists to suspect the presence of a large hidden planet known a Planet 9 or Planet X in the far reaches of the solar system whose gravitational pull could account for these bodies’ synchronised behaviour. This body (or bodies, perhaps, “super Earths”, up to ten times more massive than Earth) would be too far and dim to have shown up in any existing telescope, but future observatories may be able to spot them if in fact they are out there[17].
# For the narrative concerning the four stages, I am indebted to Dr Helen Johnston's Continuing Education course, Origins: From the Big Bang to Life, March 2011, Sydney University Physics Department (CCE, Origins). The growth of protoplanets, the processes of planetary accrestion and runaway growth and the role of dust may also be found in Chapter 3 of Caleb Scharf's The Zoomable Universe, Scientific American (2017). Other sources relied upon are mentioned below.
[1] Renu Malhotra in David Jewitt and Edward D Young, “Oceans from the skies”, Scientific American, March 2015.
[2] Accretion is dealt with on the Big History site at: https://www.bighistoryproject.com/chapters/2#the-birth-of-the-sun
[3] Alan E Rubin, “Secrets of Primitive Meteorites”, Scientific American, February 2013, 30 ff.
[4] Lee Billings, “In search of Alien Jupiters”, Scientific American, August 2015, 31-37
[5] In the manner of the comet Wild 2 referred to below.
[6] Volatiles generally constitute a group of chemical elements and chemical compounds with low boiling points associated with a planet's or moon's crust and/or atmosphere.
[7] Published in the journal Science. See report in http://news.abomus.com/en/UK/news/top-novosti/earth-was-created-two-planets-colliding-scientists-conclude . This link appears to be no longer activated so try this: https://www.ancient-code.com/scientists-conclude-that-earth-is-actually-two-planets/
[8] The earth is said to have formed approximately 4.5 billion years ago (sometimes referred to as 4,543 mya (million years ago)): https://en.wikipedia.org/wiki/Age_of_the_Earth The means by which the age of the Earth ultimately came to be calculated (by mass spectograph, a machine capable of detecting and measuring small quantities of uranium and lead locked up in ancient crystals) as utilised by Clair Patterson in 1953 is set out in Bill Bryson's A Short History of Nearly Everything, Broadway Books, 2003. Note that his figure of 4,550 million years ago on page 157 was Clair Patterson's original calculation which still remains remarkably accurate when compared with present day figures.
[9] Bill Bryson, ibid, 248-249.
[10] An extrasolar planet, or planet outside our solar system. There are 539 candidate extra-solar planets that have been identified as at April 8, 2011. See the material on the Kepler space probe under the heading “Technological aids for future research” towards the end of this paper.
[11] Questions posed in Konstantin Batygin, Gregory Laughlin and Alessandro Morbidelli, “Born of Chaos”, Scientific American, May 2016, 21-29.
[12] Named after the Dutch-American astronomer Gerard Kuiper (1905-1973), who in the 1950s proposed that the region just beyond Neptune might once have been filled with icy bodies, but he thought that the gravity of ‘massive’ Pluto would have scattered them away into deep space. He reasoned that that part of the solar system should be mostly empty, an anti-prediction as it turned out. In 1980, the Uruguayan astronomer Julio Fernandez suggested that a swarm of small bodies closer in might explain the frequency and trajectory of short-period comets, and in 1988 Scott Tremaine of the University of Toronto and his colleagues were the first to use the term ‘Kuiper belt’ though the credit should probably have gone to Fernandez: Michael D Lemonick, “Pluto and beyond”, Scientific American, November 2014, 30 at 33.
[13] Writing in November 2014.
[14] Michael D. Lemonick, op cit at 35.
[15] For this and what follows generally, see Robert Irion, National Geographic, “Our Wild Wild Solar System”, July 2013, 42ff.
[16] Michael D Lemonick, “Pluto and beyond”, op cit at 33.
[17] Michael D. Lemonick, “The Search for Planet X”, Scientific American, February 2016, 22-29, and by the same author “Planet 9 from outer space”, Scientific American, May 2016, 28.
The Malhotra hypothesis
In the context of the questions we asked a few paragraphs back, these discoveries have led to a number of competing hypotheses. First we have the hypothesis of the Indian physicist Renu Malhotra in 1993, confirmed by telescopic observations a decade later.
This goes along the lines that if one of those super Earths approached Neptune, the planet’s powerful gravity might either fling the object closer to the sun or out of the solar system entirely – something in the nature of a whip crack. Because action begets reaction, Neptune’s orbit would shift a tiny bit too. There were literally trillions of such interactions, making their precise effects very difficult to calculate, but computer simulations have revealed that on average they would compel Neptune to migrate away from the sun. That led it to “capture” Pluto, which was already farther out, and sweep it into gravitational lockstep. Telescopes have since revealed bunches of Plutinos in the Kuiper belt way beyond Neptune - icy dwarf worlds that have the same two-to-three resonance with Neptune. The Malhotra hypothesis dictates that that this could only have happened if Neptune had advanced toward the Kuiper belt, something “like a gravitational snowplow, piling up dwarf planets into new orbits”[1].
However, this hypothesis failed to explain two things. Firstly, the wildly eccentric nature of the orbits in the Kuiper belt, which was literally littered with bodies on wildly different orbits, some grouped in a flat disc, some in a doughnut shaped cloud, and others on orbits even more crazily eccentric. Secondly, the fact that some 3.9 billion years ago the earth and the moon underwent a brief but cataclysmic episode of bombardment, known as the late heavy bombardment, causing the impacts which produced the great basins on the moon. During this period, the whole inner solar system was pummeled, and the earth would have been hit by an impact similar to the one that killed the dinosaurs every twenty years. What may have been responsible is a subject we will return to after considering two other possible hypotheses.
The first is the Nice hypothesis: in summary, ‘a yank on the spring linking Jupiter and Saturn prompts some wildly eccentric orbits in the Kuiper belt’
In 2004 a team led by Harold F (Hal) Levison, a planetary scientist specialising in planetary dynamics, used their sabbatical to advance a hypothesis based on computer simulations which came to be known as the Nice hypothesis after the city in France where it was formulated[2].
What appears to have happened is that Jupiter, Saturn, Uranus, and Neptune originally started out much more closely packed together, on nearly circular orbits, with the latter three closer to the sun than they are now. Early on they were embedded in the disk-shaped solar nebula, which was still full of icy and rocky debris. As the planets absorbed those planetesimals or flung them away after close encounters, they cleared gaps in the disk.
Because the planets were also tugging on one another, the whole system was fragile, “almost infinitely chaotic”. Instead of each planet being linked only to the sun by an imaginary brass arm, it appeared as if they were all linked by gravitational springs as well. The most powerful one linked the two biggest bodies, Jupiter and Saturn. A yank on that spring would jolt the whole system.
And that, the team believes, is what happened when the solar system was about 500 million to 700 million years old. As the planets interacted with planetesimals, their own orbits shifted. Jupiter moved slightly inward; Saturn moved slightly outward, as did Uranus and Neptune. Everything happened slowly, until at a certain point Saturn was completing exactly one orbit for every two of Jupiter’s.
Stage 2: Saturn accelerates, hurling Uranus and Neptune violently outwards, causing a violent cascade
This one-to-two resonance wasn’t stable like the one between Neptune and Pluto; it constituted a “brief, vigorous yank on the spring”. As Jupiter and Saturn approached and pulled each other repeatedly at the same point in their orbits, those near-circular orbits were stretched into the ellipses we see today. That soon ended the precise resonance, but not before Saturn had moved close enough to Uranus and Neptune to accelerate them. Those two planets hurtled violently outward. In about half the Nice team’s simulations, they even swapped places.
And now the climax: As Uranus and Neptune plowed through zones of the solar system that were still full of icy planetesimals, they triggered a devastating cascade. Ice balls were catapulted in all directions. The giant planets captured a few as oddly orbiting moons. Many objects were scattered into the Kuiper belt. An untold number, perhaps as many as a trillion, were banished even farther to the Oort cloud[3], a vast cocoon of comets reaching halfway to the next star. A lot of comets were also hurled into the inner solar system, where they crashed into planets bringing their precious water with them and leaving it behind, or fell apart in the heat of the sun.
Another of the objects scattered into the Kuiper belt may have been the recently encountered comet Wild 2, which probably spent nearly all of the past 4.5 billion years in a deep freeze beyond Neptune. However, decades ago Wild 2 somehow got nudged into an orbit that drew it in past Jupiter, where it began to disintegrate in the sun’s heat. In January 2004 a NASA spacecraft called Stardust zipped past Wild 2 and snared thousands of dust specks with a trap made of aerogel. These specks revealed pieces of rock and metals such as tungsten and titanium nitride that could only have been forged near the newborn sun, at temperatures of more than 3000 degrees F, indicating that some violent process must have hurled them into the outer reaches of our solar system during its formative stages[4].
The Batygin, Laughlin and Morbidelli (BLM) hypothesis[5]: Grand Tack, Grand Attack, Snowlough
This also depends on computer simulations modelling the simultaneous evolution of the orbits of Saturn and Jupiter within the sun’s protoplanetary disk. It has some distinct similarities with the Nice hypothesis, but at the same time important nuances (la vérité est dans une nuance, said Renan). The tale goes along the lines that at some point in time the two planets drew closer together and their orbits reached a specific configuration known as a “mean motion resonance” or MMR, remember, in which Jupiter makes three revolutions for evert two made by Saturn - not two-to-one as in the Nice hypothesis – and in so doing, both worlds exerted an “amplified common gravitational influence on each other and their surroundings”. This allowed the planets to collectively “throw they weight against the interplanetary disc, carving a great gap within it, with Jupiter on the inner side and Saturn on the outer”, but Jupiter’s greater mass allowed it to exert a greater gravitational pull on the inner disk than Saturn did on the outer. The resonance ‘torqued the planets’ motion against the disk, slamming the brakes on their inward migration and boomeranging them back to the outer solar system in perhaps half a million years, scattering debris as they went’. This inwards-outwards movement is known as the Grand Tack, drawing an analogy from the term used in sailing.
As Jupiter moved about in this fashion, it gathered up the planetesimals in its path and pushed them ahead like a snowplough. If Jupiter moved as close to the sun as the present orbit of Mars, it could have ferried icy building blocks totalling about 10 times the mass of the Earth into the terrestrial region of the solar system, seeding it with water and other volatiles. This could also account for the stunted growth of Mars. In one fell swoop this can explain the distribution of rocky and icy asteroids as well as the diminutive mass of Mars.
But why are there no planets inward of Mercury, a scenario in complete contrast with the other solar systems we now know are packed with close-in super Earths? The speculation is that Jupiter’s Grand Tack may have unleashed a bona fide Grand Attack on a population of primordial super Earths in our solar system, fragmenting and pulverising them into fragments, which would have locked into resonance with any pre-existing planets in their way, siphoning energy from each. Within hundreds of thousands of years, the swarms would have dragged any super Earths into the sun. Earth and the other terrestrial planets which remain coalesced from the remaining sparse debris over the ensuing hundreds of millions of years, leaving behind “a desolate unpopulated cavity in the solar nebula, extending out to an orbital period of perhaps 100 days. As a result, Jupiter’s glancing swoop through the early solar system produced a relatively narrow ring of rocky debris from which the terrestrial planets neatly coalesced hundreds of million years later” This suggests that small Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos.
In due course Jupiter and Saturn returned to their position in the outer solar system, encountering newly formed Uranus and Neptune and perhaps one other similarly sized body, that mysterious Planet X of Planet 9. Over hundreds of million years, the terrestrial planets formed and the once wild outer worlds settled down to what could have been enduring stability, but for one thing: the final stage of interplanetary violence in our solar system known as the Late Heavy Bombardment, and here the BLM hypothesis coincides with the Nice model to the extent that “where the Grand Tack ends, the Nice model begins”[6].
Before turning to the Late Heavy Bombardment, let’s review the mains distinctions between the Nice and BLM models. Under Nice, Jupiter, Saturn, Uranus, and Neptune originally started out much more closely packed together, on nearly circular orbits, with the latter three closer to the sun than they are now. As the planets interacted with planetesimals, their own orbits shifted. Jupiter moves slightly inward; Saturn moves slightly outward, as did Uranus and Neptune. Everything happened slowly, until at a certain point Saturn is completing exactly one orbit for every two of Jupiter’s. A yank on the spring linking Jupiter and Saturn, and Saturn accelerates, hurling Uranus and Neptune violently outwards, As Uranus and Neptune plow through zones of the solar system still full of icy planetesimals, they trigger a devastating cascade, scattering objects into the Kuiper belt. The giant planets capture a few as oddly orbiting moons.
Under BLM, Jupiter and Saturn draw closer together and their orbits reach an MMR in which Jupiter makes three revolutions for evert two made by Saturn - not two-to-one as in the Nice hypothesis. A Grand Attack by Jupiter clearing out any putative super Earths closer to the sun follows the Grand Tack, the inwards-outwards configuration of the two gas giants. Jupiter and Saturn return to their position in the outer solar system, encountering newly formed Uranus and Neptune and perhaps one other similarly sized body, that mysterious Planet X of Planet 9, leaving the inner solar system largely devoid of planetary bodies.
The Late Heavy Bombardment: the final stage of planetary formation
Meanwhile, the giant-planet migrations also disrupted the rocky asteroid belt between Jupiter and Mars. Scattering asteroids joined comets from farther out to create the Late Heavy Bombardment. A recent NASA moon mission called GRAIL[7] documented how badly our moon suffered then and earlier in its history: Its entire crust was deeply fractured. Earth would have caught even more flak, but shifting tectonic plates have erased the craters. Any early life could only have survived deep underground[8].
Levison and his team think that the worst of the Late Heavy Bombardment was over in less than 100 million years, but others think that these ongoing impacts may have disrupted life on this planet for up to two billion more years. The consequences were significant: “When an asteroid slams into Earth, tiny droplets of molten rock are lofted high into the atmosphere, and they later rain out as solid, glassy beads called spherules. Deposits of spherules from the six-mile-wide asteroid that hit the Yucatán some 65 million years ago, wiping out the dinosaurs, have been discovered all over the world. So far at least a dozen comparable spherule beds have been found that date from a series of impacts between 1.8 billion and 3.7 billion years ago”[9].
Computer simulations trace the source of those impacts to a now vanished inner rim of the asteroid belt, which shed asteroids for two billion years after Jupiter disturbed it. According to Bill Bottke of the Southwest Research Institute, a lead researcher in this area, as many as 70 may have struck Earth, each comparable to the one that extinguished the dinosaurs.
[1] This represents the hypothesis of the Indian physicist Renu Malhotra in 1993, confirmed by telescopic observations a decade later; see Robert Irion, National Geographic, “Our Wild Wild Solar System”, July 2013, 42ff. Malhotra’s hypothesis is also elaborated by Michael Lemonick, “Pluto and beyond”, op cit. at 35 ff.
[2] The following resume of the Nice hypothesis is drawn from the same source. Cf the account of this solar mayhem in Michael D. Lemonick, op cit, at 35-37.
[3] Named after the Dutch astronomer Jan Oort (1900-1992), who first predicted the existence of a spherical cloud of proto-comets orbiting as much as a light year from the sun. Occasionally, he hypothesised, one of them would be jostled loose and fall into the inner solar system, where it would burst into life as a comet, thus explaining the existence of long-period comets, which fall in all directions and whose orbital paths take at least 200 years to complete: Michael D. Llemonick, “Pluto and beyond”, Scientific American, November 2014, 30 at 33.
[4] Robert Irion, National Geographic, “Our Wild Wild Solar System”, July 2013, 42ff.
[5] The subject of elaboration in Konstantin Batygin, Gregory Laughlin and Alessandro Morbidelli, “Born of Chaos”, Scientific American, May 2016, 21-29.
[6] Batygin, Laughlin, Morbidelli, “Born of Chaos”, Scientific American, May 2016, op cit 21-29 at 29.
[7] Gravity Laboratory and Interior Laboratory.
[8] The emerging realisation of the existence of tectonic plates is set out in Bill Bryson, A Short History of Nearly Everything, Broadway Books, 2003, pp 178ff. For a grahical depiction of the plates system, see https://www.bighistoryproject.com/chapters/2#tectonic-plates
[9] Robert Irion, National Geographic, “Our Wild Wild Solar System”, July 2013, 42ff. , based Bill Bottke’s work.
Other methods of planetary formation - the exoplanets
There are other methods of giant-planet formation which do not depend upon the ‘bottom up’ method of core accretion. Instead of taking millions of years, a dense, cold clump of gas could collapse in a ‘bottom down’ scenario to form a giant planet in only thousands of years. Such a rapid, efficient collapse would generate and trap intense heat within the newborn planet, giving it a powerful infrared glow for millions of years, much like embers cooling from a campfire. In contrast, under the accretion method, the planet’s core comprised of accumulated grains, rocks, boulders etc gradually melds together flaring in brightness, and then cooling the new planet by radiating away the heat, leaving it cooler and less luminous than a top down disk-instability planet of the same age.
The Gemini Space Telescope (GPI) situated high in the remote Andes of central Chile and SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research instrument) at the European Southern Observatory’s Very Large Telescope array are currently engaged in taking the temperature of young giant planets via infrared imaging in order to determine whether most are built from the bottom up or the top down[1].
Other exoplanetary phenomena can also inform us about planetary formation generally. Take the giant exoplanet 2MASS J2126−8140 for instance. It was recently found in the Octans constellation about 100 light years from earth orbiting a red dwarf star at a distance of more than 1 trillion kilometres - around 6,900 AUs (the distance between the Sun and Earth). Its orbit is 140 times wider than Pluto's. The planet is believed to be a gas giant 12 to 15 times the size of Jupiter, and takes nearly a million Earth years to circle its star. Scientists aren't sure how such a far-flung solar system could have formed, but are of the view that there is no way it formed in the same way as our solar system did, from a large disc of dust and gas. Instead, the research team suspects the star-planet duo were born relatively recently (10 to 45 million years ago, compared to our solar system's birth 4.5 billion years ago), and that they formed from a filament of gas that pushed them together in the same direction. They are so tenuously bound together that any nearby star would have disrupted their orbit completely. It is the largest solar system ever found, and there may also be an as yet undetected more familiar solar system orbiting the host star[2].
The exoplanet J1407b orbiting a star J1407 in the constellation of Centaurus in the Milky Way can also tell us a thing or two about how solar systems form[3]. J1407 is not actually a name, just a code based on the instruments that discovered it and its position in the sky. It originally presented as an otherwise unremarkable star, except that in 2007 with the aid of a planet hunting camera survey named SuperWASP (which has found more than 100 transiting planets by monitoring about 31 million stars) it was noticed that its light curve (its variance in brightness plotted over time) flickered in an unpredictable pattern for many nights, then repeatedly dimmed to near invisibility over a week before returning to its usual brightness. At other times it flickered and faded for months at a time, suggesting it was being eclipsed by an object 180 million kilometres wide.
After dismissing a wide variety of other possible explanations, astronomers came to the view that the star’s flickering must be caused by shadows being cast by a giant ring system some 200 times larger than Saturn’s around an unseen object. Once they had settled on the rings, they then had to go looking for a planet around J1407 which bound the rings in place. This was found in the form of J1407b using advanced instrumentation on two of the largest observatories on Earth: the 10 metre KECK II telescope in Hawaii and the 8.2 metre Very Large Telescope in Chile. Examining the slopes within J1407’s light curve, modelling also suggested the presence of a large gap in the rings, possibly carved out by a newborn Mars-sized moon orbiting J1407b almost every two years, which if confirmed would be the first beyond our solar system suggesting more await discovery. J1407 is a relatively bright star visible from the Southern Hemisphere and is easily observable.
Another exoplanet known as GJ 1132b orbiting a red dwarf star 39 light years from Earth in the constellation Vela has been found to have an atmosphere. While the star is relatively cool, almost half the temperature of our sun, the planet orbits just 2.4 million kms away, making its temperature a steamy 370 degrees, and hence uninhabitable. [4]
And what does all this have to tell us about how planets and moons form around stars? With the aid of discoveries such as J1407, its orbiting planet, rings and moon, we are informed that newborn giant planets such as J1407b can give rise to circumplanetary disks that condense into moons and rings, and it is anticipated that soon more of these systems will be detected with the aid of the shadows they cast across the galaxy.[5]
According to the exoplanet database operated by the Paris Astronomical Data Centre, there are now more than 3,600 confirmed exoplanets, many in multiplanetary systems. [6]
[1] Lee Billings, “In search of Alien Jupiters”, Scientific American, August 2015, 31-37.
[2] "Giant solar system 100 light years wide", SMH, 28 January 2016, p 10. For a list of exoplanets, see https://en.wikipedia.org/wiki/List_of_exoplanets For material on 2MASS J2126−8140, see also https://en.wikipedia.org/wiki/2MASS_J2126-8140 and http://www.popsci.com.au/space/astronomy/the-biggest-solar-system-ever-found-is-very-very-big,414176
[3] For material on star J1407 and planet J1407b see Matthew Kenworthy, "Rings of a Super Saturn"', Scientific American, January 2016, 30-37.
[4] Marcus Strom, "A first for Earth-sized planet outside solar system", Sydney Morning Herald, April 2017. As to the exoplanet GJ 1132b, see https://en.wikipedia.org/wiki/Gliese_1132_b
[5] In quite another context, the erratic dimming of a star called KIC8462852 (Boyajan's star) has been attributed to (among other things) disks of dust and gas, interstellar debris, comet swarms, black holes and even the activities of an advanced cosmic civilisation: Kimberly Cartier and Jason T Wright, "Strange news from another star", Scientific American, May 2017, 30 -35.
[6] Strom, op cit.
Perception: a clockwork solar system The reality: a system forged in chaos
When most of us of more mature years were growing up, the solar system seemed reliable and well behaved, and the movement of the planets appeared as if regulated by clockwork - “forever in the past, and forever in the future”[1] – and there was no evidence that planetary orbits had ever changed. So the perception of a clockwork system stuck with us for most of our lives. It was enduringly stable and forever.
We now know that not only the sun exerts a gravitational pull, but that the planets also interact with one another. Their gravitational tuggings are far weaker than those of the sun, but over time they affect the paths of neighbours and the relentless pull of gravity can amplify small deviations until orbits migrate, cross, or otherwise go haywire. Nor was the path of planetary migration, which was both the root cause and the result of this process, necessarily smooth, as once thought. In fact, it was positively chaotic.
Scientists now think that the planets as we now see them about us arose in a four-stage process: coagulation, runaway growth, the era of carnage and the late heavy bombardment.
The first phase is coagulation: Molecules begin to form in the disk, which has the same composition as the molecular cloud: nearly all gas, with some dust. The role of dust is crucial since dust grains are very effective at trapping infrared radiation, and the resultant increase in temperature at the centre eventually leads to the formation of a protostar. Different materials then condense out at different radii. At high temperatures (> 2000K) rocky minerals and metals like iron condense. Below about 270 K water ice condenses, as well as ammonia and methane.
Water is the most abundant of these ices, and condenses at the highest temperature. The distance at which water can freeze out is called the ice line. Beyond that distance there is much more mass available. In the Solar System, this line is at about 5 AU (1 AU (Astronomical Unit) = 148,598,000 kms) which marks the separation between the terrestrial and the Jovian planets. The small rocky terrestrial planets and asteroids, characterised by relatively small size, relatively high density, and a small number of satellites, lie closer to the sun. Beyond 5 AU, the giant planets have the opposite characteristics: large sizes, low densities, and many satellites and ring systems. The condensing materials (again, dust and at greater distances from the centre, ice) stick together by colliding and sticking together using normal chemical forces. They coagulate into larger clumps, eventually forming porous bodies or loose fractal aggregates, best described as fluffy dustballs, leading to the formation of plantesimals (bodies up to about 1 km in size) as the result of accretion[2].
The dust-ball aggregates settle to the nebular midplane of the disk, growing through collisions all the time. Within 10,000 years, the particles have grown to a centimetre or more in size. Friction with the gas makes them spiral towards the Sun, growing as they do so. Now gravity becomes important. The larger planetesimals can sweep up more material, so the biggest bodies grow much faster than smaller ones – a process known as runaway growth. Runaway growth ends when the planetesimal (now called a planetary embryo) has consumed nearly everything within its reach.
The role of dust
Dust has been crucial in all stages of the solar system’s evolution. As the original cloud of material that produced the suns and planets collapsed, dust grains became more effective at trapping infrared radiation; the resulting increase in temperature at the centre of the cloud eventually led to the formation of a protostar. Later, dust (and, at greater distances from the centre, ice) settled toward the nebular midplane and coagulated into larger clumps, eventually forming porous bodies known as planetesimals, ranging in size from a few metres to tens of kilometers. Some of these planetesimals melted. The planets ultimately formed from a diverse suite of such melted and unmelted planetesimals. By studying chondrules (tiny beads of melted material often smaller than a rice grain that formed before asteroids took shape early in the solar system’s history), and the chondrites in which they are found (asteroidal fragments or meteorites that have fallen to earth), scientists have been able to determine where the dust appearing in them originated from before the planets in our solar system actually formed. The densest region of dust appears to have occurred between 2.7 and 4.5 AU, densest at about 3.6 AU. Further out, it declines, and also in the other direction, ending up very sparse in the region of in the region of the future Earth and Mars. This information can tell us something about the formation of our own solar system.[3]
The giant planets appear to have formed by first accreting a core of several Earth masses. This core was more massive than the proto-planets in the inner Solar System because the proto-Jovian planets were beyond the ice line. Once this solid mass had accumulated, the planet starts accreting gas more and more efficiently, in a runaway gas accretion phase. Jupiter and Saturn grew much larger because they formed further in, where the disk was thicker. The giant planets were hot when they were accreted. This expanded their atmospheres to vastly larger dimensions than they have today. Gradually they radiated away this heat and shrank, leaving a disk of gas, ice and dust in orbit: a small-scale analogue of the solar nebula. From these disks emerged the regular satellites and ring systems. Meanwhile, radiation from the young sun started to evaporate the disk. Gas and small dust grains are blown away, leaving only large grains and planetesimals. The gas giants must have completed most of their growth before this happened, though the terrestrial planets continued to grow via collisions.
However, it should be noted that there is another method of giant-planet formation which does not depend upon the ‘bottom up’ method of core accretion. Instead of taking millions of years, a dense, cold clump of gas could collapse in a ‘bottom down’ scenario to form a giant planet in only thousands of years. Such a rapid, efficient collapse would generate and trap intense heat within the newborn planet, giving it a powerful infrared glow for millions of years, much like embers cooling from a campfire. In contrast, under the accretion method, the planet’s core comprised of accumulated grains, rocks, boulders etc gradually melds together flaring in brightness, and then cooling the new planet by radiating away the heat, leaving it cooler and less luminous than a top down disk-instability planet of the same age. The Gemini Space Telescope (GPI) situated high in the remote Andes of central Chile and SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research instrument)at the European Southern Observatory’s Very Large Telescope array are currently engaged in taking the temperature of young giant planets via infrared imaging in order to determine whether most are built from the bottom up or the top down[4].
In any event, once the protoplanets reached the size of the moon or larger, the final stages of planet formation began, where the hundred or so protoplanets were reduced to the current handful. The planetary embryos perturbed each other into crossing orbits, leading to giant impacts: this has been called the Era of Carnage. This last handful of impacts left permanent scars on nearly every member of the solar system. Every old surface bears witness to having been battered by impacts of all sizes.
The formation of the Moon
As the protoplanets perturbed each other’s orbits and collided, they also mixed up, so that planetary embryos which were born far from the sun ended up in the inner solar system - and vice versa[5]. The collisions produced enormous amounts of heat, so the planets would have been molten, with surface temperatures of about 1500K. The surface of the planet melted, with and lighter material floating: the interior differentiates. We think that the Earth’s moon was formed in this last phase of planet formation, as the result of a collision between the proto-Earth and another planet-sized body. Material from the impact was thrown into orbit and coalesced into the moon, with the heavier metals such as iron sinking to the earth’s core. The impacting body must have been about the size of Mars, most of the material left in orbit coming from the impactor, but all the volatiles[6] (nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane) were lost.
However, a more recent study[7] suggests that Earth is actually made up from two planets, the early Earth and an impactor called Theia, which came together in a head-on collision that was so violent it formed the Moon. This is said to have occurred approximately 100 million years after the Earth formed, almost 4.5 billion years ago [8].
The collision was so violent that the two planets effectively melded together to form a new planet, a chunk of which was knocked off to form the Moon. A great deal of debris was also exchanged, leading both the Earth and the moon to have almost identical levels of oxygen isotopes. The researchers studied rocks brought back to Earth by the Apollo 12, 15 and 17 and six volcanic rocks from the Earth's mantle, five from Hawaii and one from Arizona, and found their oxygen isotopes indistinguishable: they contained identical composition of an isotope of oxygen called delta-17. (More than 99.9% of the earth's oxygen is O-16, so called because it has 8 proteins and 8 neutrons. But the re are also small quantities of heavier oxygen isotopes: O-17, which have one neutron, and O-18 which have two).
The study also found that some of the water we find on Earth today may have come from Theia, and that both the early Earth and Theia must have had 'substantial' amounts of water high in delta-17 oxygen either trapped in minerals or as ice. This suggests that much of the water we see on Earth today, which is thought to have been crucial for life forming on our planet, must have come from Theia.
There are also other benefits for Earth deriving from the moon's presence. Without its stabilising gravitational influence, our planet would "wobble like a dying top". It keeps the Earth spinning at just the right speed and angle necessary necessary to foster life both in the short and the longer term. It effectively makes us a binary planet. The bad news is that the moon is receding from the Earth's orbit at about 1.5 inches a year and is expected to move beyond it altogether in 2 billion years time, when stability will once more be an issue, but one of purely academic interest so far as we personally are concerned [9].
But let's return to the era of carnage. The bombardment of the solar system has not stopped, only reduced in intensity. In July 2004, the Comet Shoemaker-Levy 9 impacted on Jupiter: the very-very-late stages of planetary accretion. The end of the Era of Carnage came when the planets found themselves in stable orbits far enough apart to be stable for billions of years. This naturally led to well-spaced planets, a pattern also seen in the exoplanet systems with at least three planets[10].
The foregoing represents what may be described as the conventional or classical view of solar system formation. However, it left a number of question unanswered, and the realisation that this model was far from classical came about twenty years ago with the discovery of exoplanets in solar systems other than our own. “Why”, it was asked, “is the solar system’s inner region so depleted in mass compared with its interplanetary counterparts, with relatively ‘runty’ rocky worlds instead of the so-called super Earths found in abundance in other solar systems, and no worlds at all inside Mercury’s 88 day orbit?” Why are the orbits of the sun’s giant planets so calm and spread out? And how can one account for Jupiter’s and Saturn’s orbits so far from the sun?”[11]
The Kuiper belt
Before answering these questions, we must consider some comparatively recent discoveries of a number of other players in the field. Firstly the Kuiper belt. The outer parts of the proto-stellar disk never coalesced into planets. Outside the orbits of the planets, the sun is left with a disk of thousands of small icy bodies beyond Neptune’s orbit, some of which are on highly elliptical orbits, periodically visiting the inner solar system as comets – the Kuiper Belt[12]. Just over 20 years ago[13], nobody even knew that the Kuiper belt existed. Since then, beginning in August 1992 more and more of these icy bodies of various dimensions, known as Kuiper belt objects (KBOs) have been discovered until they now number around 1,500. A handful approach and even rival Pluto in size.
In 2014 it was estimated that the Kuiper belt is home to 100,000 objects more than 100 kilometres across and up to 10 billion larger than two kilometres across. So that for every asteroid in the asteroid belt, there are 1000 objects in the Kuiper belt[14] It was reasoned that something must have happened to snuff out the largest Earth-size and beyond members of the Kuiper belt.
Secondly, Pluto. The so-called “oddball” of the solar system dips far above and below the pancake-like plane in which the eight planets travel, its elongated orbit taking it from 30 to 50 times the Earth’s distance from the sun. But the most curious thing about Pluto is its bond with Neptune. For every three times that Neptune orbits the sun, Pluto orbits twice, and in such a way that the bodies never approach each other. This is called a “mean motion resonance” (MMR), and the speculation about how it came to be is that when the solar system was young and full of asteroids and comets, Neptune was closer to the sun[15]. Pluto was originally classified as a planet, but by the 1970s it was clear that Pluto was smaller and much less than the Earth’s moon. What had been discovered was simply the brightest member of the Kuiper belt[16]
And even more recently, in 2016, far beyond the eight planets, the asteroid belt and even the distant Kuiper belt past Neptune, astronomers have seen roughly a dozen objects moving in strange orbits, all taking elongated paths around the sun. They all seem to make their closest approach to the sun around the time they cross the path of the planets, causing scientists to suspect the presence of a large hidden planet known a Planet 9 or Planet X in the far reaches of the solar system whose gravitational pull could account for these bodies’ synchronised behaviour. This body (or bodies, perhaps, “super Earths”, up to ten times more massive than Earth) would be too far and dim to have shown up in any existing telescope, but future observatories may be able to spot them if in fact they are out there[17].
# For the narrative concerning the four stages, I am indebted to Dr Helen Johnston's Continuing Education course, Origins: From the Big Bang to Life, March 2011, Sydney University Physics Department (CCE, Origins). The growth of protoplanets, the processes of planetary accrestion and runaway growth and the role of dust may also be found in Chapter 3 of Caleb Scharf's The Zoomable Universe, Scientific American (2017). Other sources relied upon are mentioned below.
[1] Renu Malhotra in David Jewitt and Edward D Young, “Oceans from the skies”, Scientific American, March 2015.
[2] Accretion is dealt with on the Big History site at: https://www.bighistoryproject.com/chapters/2#the-birth-of-the-sun
[3] Alan E Rubin, “Secrets of Primitive Meteorites”, Scientific American, February 2013, 30 ff.
[4] Lee Billings, “In search of Alien Jupiters”, Scientific American, August 2015, 31-37
[5] In the manner of the comet Wild 2 referred to below.
[6] Volatiles generally constitute a group of chemical elements and chemical compounds with low boiling points associated with a planet's or moon's crust and/or atmosphere.
[7] Published in the journal Science. See report in http://news.abomus.com/en/UK/news/top-novosti/earth-was-created-two-planets-colliding-scientists-conclude . This link appears to be no longer activated so try this: https://www.ancient-code.com/scientists-conclude-that-earth-is-actually-two-planets/
[8] The earth is said to have formed approximately 4.5 billion years ago (sometimes referred to as 4,543 mya (million years ago)): https://en.wikipedia.org/wiki/Age_of_the_Earth The means by which the age of the Earth ultimately came to be calculated (by mass spectograph, a machine capable of detecting and measuring small quantities of uranium and lead locked up in ancient crystals) as utilised by Clair Patterson in 1953 is set out in Bill Bryson's A Short History of Nearly Everything, Broadway Books, 2003. Note that his figure of 4,550 million years ago on page 157 was Clair Patterson's original calculation which still remains remarkably accurate when compared with present day figures.
[9] Bill Bryson, ibid, 248-249.
[10] An extrasolar planet, or planet outside our solar system. There are 539 candidate extra-solar planets that have been identified as at April 8, 2011. See the material on the Kepler space probe under the heading “Technological aids for future research” towards the end of this paper.
[11] Questions posed in Konstantin Batygin, Gregory Laughlin and Alessandro Morbidelli, “Born of Chaos”, Scientific American, May 2016, 21-29.
[12] Named after the Dutch-American astronomer Gerard Kuiper (1905-1973), who in the 1950s proposed that the region just beyond Neptune might once have been filled with icy bodies, but he thought that the gravity of ‘massive’ Pluto would have scattered them away into deep space. He reasoned that that part of the solar system should be mostly empty, an anti-prediction as it turned out. In 1980, the Uruguayan astronomer Julio Fernandez suggested that a swarm of small bodies closer in might explain the frequency and trajectory of short-period comets, and in 1988 Scott Tremaine of the University of Toronto and his colleagues were the first to use the term ‘Kuiper belt’ though the credit should probably have gone to Fernandez: Michael D Lemonick, “Pluto and beyond”, Scientific American, November 2014, 30 at 33.
[13] Writing in November 2014.
[14] Michael D. Lemonick, op cit at 35.
[15] For this and what follows generally, see Robert Irion, National Geographic, “Our Wild Wild Solar System”, July 2013, 42ff.
[16] Michael D Lemonick, “Pluto and beyond”, op cit at 33.
[17] Michael D. Lemonick, “The Search for Planet X”, Scientific American, February 2016, 22-29, and by the same author “Planet 9 from outer space”, Scientific American, May 2016, 28.
The Malhotra hypothesis
In the context of the questions we asked a few paragraphs back, these discoveries have led to a number of competing hypotheses. First we have the hypothesis of the Indian physicist Renu Malhotra in 1993, confirmed by telescopic observations a decade later.
This goes along the lines that if one of those super Earths approached Neptune, the planet’s powerful gravity might either fling the object closer to the sun or out of the solar system entirely – something in the nature of a whip crack. Because action begets reaction, Neptune’s orbit would shift a tiny bit too. There were literally trillions of such interactions, making their precise effects very difficult to calculate, but computer simulations have revealed that on average they would compel Neptune to migrate away from the sun. That led it to “capture” Pluto, which was already farther out, and sweep it into gravitational lockstep. Telescopes have since revealed bunches of Plutinos in the Kuiper belt way beyond Neptune - icy dwarf worlds that have the same two-to-three resonance with Neptune. The Malhotra hypothesis dictates that that this could only have happened if Neptune had advanced toward the Kuiper belt, something “like a gravitational snowplow, piling up dwarf planets into new orbits”[1].
However, this hypothesis failed to explain two things. Firstly, the wildly eccentric nature of the orbits in the Kuiper belt, which was literally littered with bodies on wildly different orbits, some grouped in a flat disc, some in a doughnut shaped cloud, and others on orbits even more crazily eccentric. Secondly, the fact that some 3.9 billion years ago the earth and the moon underwent a brief but cataclysmic episode of bombardment, known as the late heavy bombardment, causing the impacts which produced the great basins on the moon. During this period, the whole inner solar system was pummeled, and the earth would have been hit by an impact similar to the one that killed the dinosaurs every twenty years. What may have been responsible is a subject we will return to after considering two other possible hypotheses.
The first is the Nice hypothesis: in summary, ‘a yank on the spring linking Jupiter and Saturn prompts some wildly eccentric orbits in the Kuiper belt’
In 2004 a team led by Harold F (Hal) Levison, a planetary scientist specialising in planetary dynamics, used their sabbatical to advance a hypothesis based on computer simulations which came to be known as the Nice hypothesis after the city in France where it was formulated[2].
What appears to have happened is that Jupiter, Saturn, Uranus, and Neptune originally started out much more closely packed together, on nearly circular orbits, with the latter three closer to the sun than they are now. Early on they were embedded in the disk-shaped solar nebula, which was still full of icy and rocky debris. As the planets absorbed those planetesimals or flung them away after close encounters, they cleared gaps in the disk.
Because the planets were also tugging on one another, the whole system was fragile, “almost infinitely chaotic”. Instead of each planet being linked only to the sun by an imaginary brass arm, it appeared as if they were all linked by gravitational springs as well. The most powerful one linked the two biggest bodies, Jupiter and Saturn. A yank on that spring would jolt the whole system.
And that, the team believes, is what happened when the solar system was about 500 million to 700 million years old. As the planets interacted with planetesimals, their own orbits shifted. Jupiter moved slightly inward; Saturn moved slightly outward, as did Uranus and Neptune. Everything happened slowly, until at a certain point Saturn was completing exactly one orbit for every two of Jupiter’s.
Stage 2: Saturn accelerates, hurling Uranus and Neptune violently outwards, causing a violent cascade
This one-to-two resonance wasn’t stable like the one between Neptune and Pluto; it constituted a “brief, vigorous yank on the spring”. As Jupiter and Saturn approached and pulled each other repeatedly at the same point in their orbits, those near-circular orbits were stretched into the ellipses we see today. That soon ended the precise resonance, but not before Saturn had moved close enough to Uranus and Neptune to accelerate them. Those two planets hurtled violently outward. In about half the Nice team’s simulations, they even swapped places.
And now the climax: As Uranus and Neptune plowed through zones of the solar system that were still full of icy planetesimals, they triggered a devastating cascade. Ice balls were catapulted in all directions. The giant planets captured a few as oddly orbiting moons. Many objects were scattered into the Kuiper belt. An untold number, perhaps as many as a trillion, were banished even farther to the Oort cloud[3], a vast cocoon of comets reaching halfway to the next star. A lot of comets were also hurled into the inner solar system, where they crashed into planets bringing their precious water with them and leaving it behind, or fell apart in the heat of the sun.
Another of the objects scattered into the Kuiper belt may have been the recently encountered comet Wild 2, which probably spent nearly all of the past 4.5 billion years in a deep freeze beyond Neptune. However, decades ago Wild 2 somehow got nudged into an orbit that drew it in past Jupiter, where it began to disintegrate in the sun’s heat. In January 2004 a NASA spacecraft called Stardust zipped past Wild 2 and snared thousands of dust specks with a trap made of aerogel. These specks revealed pieces of rock and metals such as tungsten and titanium nitride that could only have been forged near the newborn sun, at temperatures of more than 3000 degrees F, indicating that some violent process must have hurled them into the outer reaches of our solar system during its formative stages[4].
The Batygin, Laughlin and Morbidelli (BLM) hypothesis[5]: Grand Tack, Grand Attack, Snowlough
This also depends on computer simulations modelling the simultaneous evolution of the orbits of Saturn and Jupiter within the sun’s protoplanetary disk. It has some distinct similarities with the Nice hypothesis, but at the same time important nuances (la vérité est dans une nuance, said Renan). The tale goes along the lines that at some point in time the two planets drew closer together and their orbits reached a specific configuration known as a “mean motion resonance” or MMR, remember, in which Jupiter makes three revolutions for evert two made by Saturn - not two-to-one as in the Nice hypothesis – and in so doing, both worlds exerted an “amplified common gravitational influence on each other and their surroundings”. This allowed the planets to collectively “throw they weight against the interplanetary disc, carving a great gap within it, with Jupiter on the inner side and Saturn on the outer”, but Jupiter’s greater mass allowed it to exert a greater gravitational pull on the inner disk than Saturn did on the outer. The resonance ‘torqued the planets’ motion against the disk, slamming the brakes on their inward migration and boomeranging them back to the outer solar system in perhaps half a million years, scattering debris as they went’. This inwards-outwards movement is known as the Grand Tack, drawing an analogy from the term used in sailing.
As Jupiter moved about in this fashion, it gathered up the planetesimals in its path and pushed them ahead like a snowplough. If Jupiter moved as close to the sun as the present orbit of Mars, it could have ferried icy building blocks totalling about 10 times the mass of the Earth into the terrestrial region of the solar system, seeding it with water and other volatiles. This could also account for the stunted growth of Mars. In one fell swoop this can explain the distribution of rocky and icy asteroids as well as the diminutive mass of Mars.
But why are there no planets inward of Mercury, a scenario in complete contrast with the other solar systems we now know are packed with close-in super Earths? The speculation is that Jupiter’s Grand Tack may have unleashed a bona fide Grand Attack on a population of primordial super Earths in our solar system, fragmenting and pulverising them into fragments, which would have locked into resonance with any pre-existing planets in their way, siphoning energy from each. Within hundreds of thousands of years, the swarms would have dragged any super Earths into the sun. Earth and the other terrestrial planets which remain coalesced from the remaining sparse debris over the ensuing hundreds of millions of years, leaving behind “a desolate unpopulated cavity in the solar nebula, extending out to an orbital period of perhaps 100 days. As a result, Jupiter’s glancing swoop through the early solar system produced a relatively narrow ring of rocky debris from which the terrestrial planets neatly coalesced hundreds of million years later” This suggests that small Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos.
In due course Jupiter and Saturn returned to their position in the outer solar system, encountering newly formed Uranus and Neptune and perhaps one other similarly sized body, that mysterious Planet X of Planet 9. Over hundreds of million years, the terrestrial planets formed and the once wild outer worlds settled down to what could have been enduring stability, but for one thing: the final stage of interplanetary violence in our solar system known as the Late Heavy Bombardment, and here the BLM hypothesis coincides with the Nice model to the extent that “where the Grand Tack ends, the Nice model begins”[6].
Before turning to the Late Heavy Bombardment, let’s review the mains distinctions between the Nice and BLM models. Under Nice, Jupiter, Saturn, Uranus, and Neptune originally started out much more closely packed together, on nearly circular orbits, with the latter three closer to the sun than they are now. As the planets interacted with planetesimals, their own orbits shifted. Jupiter moves slightly inward; Saturn moves slightly outward, as did Uranus and Neptune. Everything happened slowly, until at a certain point Saturn is completing exactly one orbit for every two of Jupiter’s. A yank on the spring linking Jupiter and Saturn, and Saturn accelerates, hurling Uranus and Neptune violently outwards, As Uranus and Neptune plow through zones of the solar system still full of icy planetesimals, they trigger a devastating cascade, scattering objects into the Kuiper belt. The giant planets capture a few as oddly orbiting moons.
Under BLM, Jupiter and Saturn draw closer together and their orbits reach an MMR in which Jupiter makes three revolutions for evert two made by Saturn - not two-to-one as in the Nice hypothesis. A Grand Attack by Jupiter clearing out any putative super Earths closer to the sun follows the Grand Tack, the inwards-outwards configuration of the two gas giants. Jupiter and Saturn return to their position in the outer solar system, encountering newly formed Uranus and Neptune and perhaps one other similarly sized body, that mysterious Planet X of Planet 9, leaving the inner solar system largely devoid of planetary bodies.
The Late Heavy Bombardment: the final stage of planetary formation
Meanwhile, the giant-planet migrations also disrupted the rocky asteroid belt between Jupiter and Mars. Scattering asteroids joined comets from farther out to create the Late Heavy Bombardment. A recent NASA moon mission called GRAIL[7] documented how badly our moon suffered then and earlier in its history: Its entire crust was deeply fractured. Earth would have caught even more flak, but shifting tectonic plates have erased the craters. Any early life could only have survived deep underground[8].
Levison and his team think that the worst of the Late Heavy Bombardment was over in less than 100 million years, but others think that these ongoing impacts may have disrupted life on this planet for up to two billion more years. The consequences were significant: “When an asteroid slams into Earth, tiny droplets of molten rock are lofted high into the atmosphere, and they later rain out as solid, glassy beads called spherules. Deposits of spherules from the six-mile-wide asteroid that hit the Yucatán some 65 million years ago, wiping out the dinosaurs, have been discovered all over the world. So far at least a dozen comparable spherule beds have been found that date from a series of impacts between 1.8 billion and 3.7 billion years ago”[9].
Computer simulations trace the source of those impacts to a now vanished inner rim of the asteroid belt, which shed asteroids for two billion years after Jupiter disturbed it. According to Bill Bottke of the Southwest Research Institute, a lead researcher in this area, as many as 70 may have struck Earth, each comparable to the one that extinguished the dinosaurs.
[1] This represents the hypothesis of the Indian physicist Renu Malhotra in 1993, confirmed by telescopic observations a decade later; see Robert Irion, National Geographic, “Our Wild Wild Solar System”, July 2013, 42ff. Malhotra’s hypothesis is also elaborated by Michael Lemonick, “Pluto and beyond”, op cit. at 35 ff.
[2] The following resume of the Nice hypothesis is drawn from the same source. Cf the account of this solar mayhem in Michael D. Lemonick, op cit, at 35-37.
[3] Named after the Dutch astronomer Jan Oort (1900-1992), who first predicted the existence of a spherical cloud of proto-comets orbiting as much as a light year from the sun. Occasionally, he hypothesised, one of them would be jostled loose and fall into the inner solar system, where it would burst into life as a comet, thus explaining the existence of long-period comets, which fall in all directions and whose orbital paths take at least 200 years to complete: Michael D. Llemonick, “Pluto and beyond”, Scientific American, November 2014, 30 at 33.
[4] Robert Irion, National Geographic, “Our Wild Wild Solar System”, July 2013, 42ff.
[5] The subject of elaboration in Konstantin Batygin, Gregory Laughlin and Alessandro Morbidelli, “Born of Chaos”, Scientific American, May 2016, 21-29.
[6] Batygin, Laughlin, Morbidelli, “Born of Chaos”, Scientific American, May 2016, op cit 21-29 at 29.
[7] Gravity Laboratory and Interior Laboratory.
[8] The emerging realisation of the existence of tectonic plates is set out in Bill Bryson, A Short History of Nearly Everything, Broadway Books, 2003, pp 178ff. For a grahical depiction of the plates system, see https://www.bighistoryproject.com/chapters/2#tectonic-plates
[9] Robert Irion, National Geographic, “Our Wild Wild Solar System”, July 2013, 42ff. , based Bill Bottke’s work.
Other methods of planetary formation - the exoplanets
There are other methods of giant-planet formation which do not depend upon the ‘bottom up’ method of core accretion. Instead of taking millions of years, a dense, cold clump of gas could collapse in a ‘bottom down’ scenario to form a giant planet in only thousands of years. Such a rapid, efficient collapse would generate and trap intense heat within the newborn planet, giving it a powerful infrared glow for millions of years, much like embers cooling from a campfire. In contrast, under the accretion method, the planet’s core comprised of accumulated grains, rocks, boulders etc gradually melds together flaring in brightness, and then cooling the new planet by radiating away the heat, leaving it cooler and less luminous than a top down disk-instability planet of the same age.
The Gemini Space Telescope (GPI) situated high in the remote Andes of central Chile and SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research instrument) at the European Southern Observatory’s Very Large Telescope array are currently engaged in taking the temperature of young giant planets via infrared imaging in order to determine whether most are built from the bottom up or the top down[1].
Other exoplanetary phenomena can also inform us about planetary formation generally. Take the giant exoplanet 2MASS J2126−8140 for instance. It was recently found in the Octans constellation about 100 light years from earth orbiting a red dwarf star at a distance of more than 1 trillion kilometres - around 6,900 AUs (the distance between the Sun and Earth). Its orbit is 140 times wider than Pluto's. The planet is believed to be a gas giant 12 to 15 times the size of Jupiter, and takes nearly a million Earth years to circle its star. Scientists aren't sure how such a far-flung solar system could have formed, but are of the view that there is no way it formed in the same way as our solar system did, from a large disc of dust and gas. Instead, the research team suspects the star-planet duo were born relatively recently (10 to 45 million years ago, compared to our solar system's birth 4.5 billion years ago), and that they formed from a filament of gas that pushed them together in the same direction. They are so tenuously bound together that any nearby star would have disrupted their orbit completely. It is the largest solar system ever found, and there may also be an as yet undetected more familiar solar system orbiting the host star[2].
The exoplanet J1407b orbiting a star J1407 in the constellation of Centaurus in the Milky Way can also tell us a thing or two about how solar systems form[3]. J1407 is not actually a name, just a code based on the instruments that discovered it and its position in the sky. It originally presented as an otherwise unremarkable star, except that in 2007 with the aid of a planet hunting camera survey named SuperWASP (which has found more than 100 transiting planets by monitoring about 31 million stars) it was noticed that its light curve (its variance in brightness plotted over time) flickered in an unpredictable pattern for many nights, then repeatedly dimmed to near invisibility over a week before returning to its usual brightness. At other times it flickered and faded for months at a time, suggesting it was being eclipsed by an object 180 million kilometres wide.
After dismissing a wide variety of other possible explanations, astronomers came to the view that the star’s flickering must be caused by shadows being cast by a giant ring system some 200 times larger than Saturn’s around an unseen object. Once they had settled on the rings, they then had to go looking for a planet around J1407 which bound the rings in place. This was found in the form of J1407b using advanced instrumentation on two of the largest observatories on Earth: the 10 metre KECK II telescope in Hawaii and the 8.2 metre Very Large Telescope in Chile. Examining the slopes within J1407’s light curve, modelling also suggested the presence of a large gap in the rings, possibly carved out by a newborn Mars-sized moon orbiting J1407b almost every two years, which if confirmed would be the first beyond our solar system suggesting more await discovery. J1407 is a relatively bright star visible from the Southern Hemisphere and is easily observable.
Another exoplanet known as GJ 1132b orbiting a red dwarf star 39 light years from Earth in the constellation Vela has been found to have an atmosphere. While the star is relatively cool, almost half the temperature of our sun, the planet orbits just 2.4 million kms away, making its temperature a steamy 370 degrees, and hence uninhabitable. [4]
And what does all this have to tell us about how planets and moons form around stars? With the aid of discoveries such as J1407, its orbiting planet, rings and moon, we are informed that newborn giant planets such as J1407b can give rise to circumplanetary disks that condense into moons and rings, and it is anticipated that soon more of these systems will be detected with the aid of the shadows they cast across the galaxy.[5]
According to the exoplanet database operated by the Paris Astronomical Data Centre, there are now more than 3,600 confirmed exoplanets, many in multiplanetary systems. [6]
[1] Lee Billings, “In search of Alien Jupiters”, Scientific American, August 2015, 31-37.
[2] "Giant solar system 100 light years wide", SMH, 28 January 2016, p 10. For a list of exoplanets, see https://en.wikipedia.org/wiki/List_of_exoplanets For material on 2MASS J2126−8140, see also https://en.wikipedia.org/wiki/2MASS_J2126-8140 and http://www.popsci.com.au/space/astronomy/the-biggest-solar-system-ever-found-is-very-very-big,414176
[3] For material on star J1407 and planet J1407b see Matthew Kenworthy, "Rings of a Super Saturn"', Scientific American, January 2016, 30-37.
[4] Marcus Strom, "A first for Earth-sized planet outside solar system", Sydney Morning Herald, April 2017. As to the exoplanet GJ 1132b, see https://en.wikipedia.org/wiki/Gliese_1132_b
[5] In quite another context, the erratic dimming of a star called KIC8462852 (Boyajan's star) has been attributed to (among other things) disks of dust and gas, interstellar debris, comet swarms, black holes and even the activities of an advanced cosmic civilisation: Kimberly Cartier and Jason T Wright, "Strange news from another star", Scientific American, May 2017, 30 -35.
[6] Strom, op cit.