Did the solar system begin naturally or supernaturally? The question is as ultimate as how life itself came into being, and science can distinguish between the alternatives. It can point to a supernatural origin by first excluding the possibility of a natural one.
In the past, people have thought about creation on the basis that the chain of causes leading up to the present could have been initiated at any time: creation (if it occurred) produced a solar system with the appearance of a past that did not occur, equivalent to a drama beginning in medias res. in principle, a scientist could retrace the chain of causes back to that moment and not know when he had arrived, before going back into a past that never existed. Essentially, this was the problem that troubled the 19th-century naturalist Philip Gosse: the necessity of creating with apparent age. It seemed to inhere in the very notion of creation.
But the problem is misconceived. Creation is the bringing into existence of that which cannot, of itself, come into existence and therefore cannot be construed as having had a natural origin. It takes place at the point beyond which the chain of causation cannot be pushed back further. Consequently, anything that appears to have a history should be accepted as having a history. While the idea that the solar system had a supernatural origin is not ruled out, it is also not assumed; instead, we test for the possibility by exploring the opposite presumption. Creation theory stakes everything on the prediction that naturalistic attempts to explain the origin of the solar system will prove wanting.
Atoms are incapable of arranging themselves into high states of order. Even so simple an object as a kitchen table will never come together by itself. Supernaturally ordered man-made objects surround us all the time, and we recognise them as such without needing science to tell us; we know from experience that intelligence was required to put them together. The issue for the solar system is whether it could have arisen, of itself, from a state of primeval disorder. If it could, the theory of creation is falsified; if it could not, the theory of a natural origin is falsified.
The standard account of the solar system’s origin is the nebula hypothesis, first proposed in the 18th century by philosopher Immanuel Kant and mathematician-astronomer Pierre-Simon Laplace. Laplace claimed to show that the motions of the planets could be accounted for without having to conclude that the Creator intervened from time to time to restore order. As he famously said to Bonaparte, “I have no need of that hypothesis.” It was a crucial point: in relation to phenomena, scientific explanations were to be based entirely on the properties of nature. (One of Kant’s ideas, by the way, was that Jupiter and Saturn were probably inhabited by creatures more intelligent than human beings.)
Despite the vast amount of new information gathered about the solar system in recent years, the basic picture of how it began has remained the , the basic picture of how it began has remained the same. Initially there was a rotating molecular cloud of dust and gas, similar to the Milky Way’s Orion Nebula. The “dust” was a mixture of silicates, hydrocarbons and ices, the gas mostly hydrogen and helium. Over time gravity flattened the cloud into a disc and matter began to be pulled towards the centre, until most of the cloud had collapsed to form the Sun. The gravitational energy turned into heat, intense enough for nuclear fusion to power the Sun for billions of years. At the same time other matter in the disc contracted into ‘planetesimals’ (bodies up to a few hundred kilometres across) and embryonic planets.
The planets can be divided into three types, corresponding to their distance from the Sun: the terrestrial or rocky planets Mercury, Venus, Earth and Mars, the gaseous planets Jupiter and Saturn (which comprise 92% of the total planetary mass), and the icy planets Uranus and Neptune. In addition there is a belt of asteroids between Mars and Jupiter, smaller groups of asteroids in various other parts of the solar system, and an array of icy ‘trans-Neptunian objects’, principally those that make up the Kuiper Belt on the edge of the solar system. The most famous of the TNOs is Pluto, now demoted to a dwarf planet.
The nebula hypothesis makes sense of this distribution by visualising dust particles clumping together into progressively larger units until they reached the size of present-day moons. At this ‘planetesimal’ stage, their evolution diverged, depending on which side of the ‘snow-line’ they were. Close to the Sun, where it was warmer and solar winds had driven away most of the gas, the rocky cores continued to gather bulk until they exhausted the space around them and they became full-grown terrestrial planets. Further out, where it was colder, their gravity attracted the more volatile elements to form the gaseous and icy planets.
Until recently the assumption was that the asteroid belt and the Kuiper Belt are simply leftovers of that process: dust and gas aggregated into various chunks all much smaller than the planets, which were then prevented from gaining further mass. The idea can be tested by estimating how much material must have existed in these regions if its density was the same as was needed to produce the planets elsewhere.
Estimates can be made using the assumption that the nebula contained just enough material of solar composition to form the planets at their current locations and compositions. These model results suggest that the Solar System’s surface density may have varied as r-1 to r-3/2 between Venus and Neptune, where r is heliocentric distance [distance from Sun]. Compared to this prediction, however, the amount of solid material in the main [asteroid] belt zone today is nearly 1000 times lower than our expectations. This is a serious problem.
W. F. Bottke et al, Icarus 175:111-40 (2005)
A similar ‘missing mass problem’ concerns the Kuiper Belt. Its estimated mass is 1,000 times less than expected, a mere one hundredth that of the Earth.
Explanations therefore have to be found for (i) why the objects in these regions failed to accrete despite having a total mass 1,000 times greater than now, and/or (ii) how 99.9% of their present mass might subsequently have been lost. Almost from the start, asteroids and TNOs have been getting smaller and smaller, not larger and larger, as a result of colliding with each other and grinding each other down (Stern & Colwell 1997).
In the case of the Kuiper Belt, its primordial density has to be reconciled with the density required for the partly hypothetical Scattered Disc and wholly hypothetical Oort Cloud beyond the belt. The Oort Cloud is the presumed source for comets with long elliptical orbits but relatively short life-times whose frequency would otherwise suggest that the solar system was much younger than the 4.6 billion years implied by radioisotope dating. Drastic erosion of the Kuiper Belt as a result of objects colliding with each other seems inescapable. On the other hand, the same process that depletes the Kuiper Belt also depletes the Scattered Disk and Oort Cloud and leaves the latter with a population incapable of supplying comets over billions of years (Charnoz & Morbidelli 2007).
These are far from the only problems relating to the distribution of mass in the solar system. In contrast to the main belt asteroids and TNOs, Uranus and Neptune have far too much mass. At their distance from the centre, the disc is thought to have been too thin to generate such bodies, and modelers have to suppose that they formed much nearer the sun and migrated outwards, possibly swapping places in the process. Another problem is how the terrestrial planets acquired their mass. Collisional erosion of the clumps of matter between Mercury and Mars should have inhibited planetary formation, just as erosion inhibited the growth of the asteroids between Mars and Jupiter.
As we have seen, the formation of the gas giants is thought to have taken place in two stages: the accretion of a rocky core first, then the condensation of gases around the core. This is because the refractory elements in the nebula would have condensed out before the volatile elements. However, in that case the cores of Uranus and Neptune would have taken longer to accrete (more than 10 million years) than the disc supplying them with material could have been in existence. Accordingly Alan Boss suggested that ‘gravitational instabilities’ in the disc accelerated the process of dust and gas collapse, producing giant planets in less than 1,000 years. This is uncomfortably quick for many modellers. Moreover, extreme conditions are needed to generate such instabilities, at the same time as minimal turbulence is maintained within the gas. The disc instability model is therefore not an attractive option (Armitage 2007).
How well does what we can deduce about the interiors of these planets accord with the core-accretion model? Data from space probes suggest that much of the inner bulk of Uranus and Neptune consists of ice, not rock, and that Saturn has only a small rocky core (Saumon & Guillot 2004). By contrast, Jupiter, according to a recent computer simulation, could have a rocky core as much as 16 times the mass of the Earth (Militzer et al 2008). If one assumes core-accretion, it appears that the gases making up the bulk of Jupiter may have collected round the core quite quickly. Saturn, Uranus and Neptune, as currently understood, pose more of a problem, notwithstanding that continuing work on the standard model allows the time for core accretion to be halved, down to about 5 million years (e.g. Hubickyj et al 2005).
Boss has also revised his own model, proposing that ultraviolet radiation from a nearby (no longer traceable) star blew away the outer gas of Uranus and Neptune. The drawback of this solution is that radiation disturbance of the outer disc where the Kuiper Belt objects occur would have prevented particles from clumping into larger units. If collision speeds are too fast, objects bounce off each other, or grind each other down into smaller bodies, rather than stick together. As already mentioned, this seems to have been the situation through almost the entire history of the belt. Extreme ultraviolet radiation from the star would have prevented particles from reaching even the much eroded dimensions we see today.
A further problem with the core accretion model is that the same spiralling in of matter to form the Sun also draws in the growing planetesimals. As has long been known, gravitational interaction between the gaseous disc and the planetary cores causes them to migrate inward, and the time involved, about 100,000 years, is much shorter than the time needed for gas to accrete onto giant gas planets. The growing planets plunge into the Sun before they have had time to form. This was confirmed by Paul Cresswell and Richard Nelson (2006) in more sophisticated simulations. All the simulations show large-scale migration of bodies towards and into the star, with their orbits changing as they approach from strongly elliptical to circular.
The problem is also acute for bodies from 10 cm diameter to a few metres. The key point here is that solid particles orbit in the nebula at speeds substantially greater than the surrounding gas. As they move through it, they become subject to a head wind and lose orbital energy to gas drag, which causes them to migrate radially inwards. At one Earth-distance from the Sun the rate of migration is of the order of only 100 years (Armitage 2007). Particles that reachd metre size have on average not much more than 1,000 years to make the leap to planetesimal size.
Another point that has caused difficulty relates to the Sun’s very low angular momentum. While the Sun has one thousand times as much mass as the planets, the planets have more than 99% of the angular momentum, mostly in the orbits of Saturn and Jupiter. Spin increases as the radius of a spinning body decreases, so the collapse of the nebula should have imparted the Sun with a rotation period hundreds of times greater than its present value. This objection no longer has much force. One explanation is that the Sun’s angular momentum was dissipated by the solar wind (Matt & Pudritz 2006); another involves the concept of disc-locking (Long et al 2005).
In order to assess these solutions astronomers have studied how the spin of stars with a mass comparable to the Sun’s changes in the critical first 1–100 million years of stellar evolution (standard timescale). The data reveal a bimodal pattern. Stars with initially high rotation velocities evolve with no appreciable loss of angular momentum during the period. Continuing contraction actually causes them to spin faster, whereas stars with initially slow rotation velocities lose angular momentum. Since discs have been observed only around the slower-rotating stars, these appear to exert a braking effect (Herbst et al 2007). After 5–6 million years, equivalent to the maximum longevity of the discs, the braking effect drops off. The 1–2 orders-of-magnitude loss of angular momentum between then and the time such stars reach the present age of the Sun (4.6 billion years) is attributed to the torque of stellar winds.
In the Big Bang scenario, galaxies form in similar fashion to individual stars, through giant clouds of gas contracting to form a disc. Much of this gas, predominantly hydrogen and helium, still persists, whether as hot atoms or cold molecules, and it is well established that the heavier elements in the clouds originate from nuclear fusion, such as occurs most intensely when a massive star explodes at the end of its life cycle. Formless matter organises itself into stars, and the exploding stars, the supernovae, are the furnaces which synthesise all elements heavier than helium. In this way the massive stars return to a state of formlessness, enriching the clouds, raw material for new generations of stars, as the galaxy matures. It is as if the universe itself has been endowed with the ability to procreate.
Comprising a mass equal to one billion suns, the molecular gas in our own galaxy is nearly all located in the spiral arms, at some distance from the galactic centre. That the gas occurs predominantly in the arms is puzzling, since the stars in them are rotating round the centre faster relative to the gas and it would take only about 10 million years for the clouds to pass through into the regions between the arms (Williams et al 2000). A simpler, but unorthodox, view would be to understand the spiral arms as ejected from the galactic centre, with the gas being what was left over from star formation in the arms. This would mean that the age of the arms, and hence of the galaxy as a whole, was less than 10 million years, an inference at variance with Big Bang cosmology.
Star formation in giant molecular clouds happens surprisingly slowly. Assuming no inhibiting factors, calculations indicate that the gas should all have condensed into stars within 4 million years, whereas observed star formation occurs at around 1% of this rate. In an effort to solve the problem theorists have suggested that turbulence or strong magnetic fields inhibit the process, or that, contrary to most observational estimates, the clouds are not bound by gravity at all. Whatever the explanation, it appears that clouds must have gone through multiple cycles of condensation (Krumholz & Tan 2006), over a period many times longer than the maximum longevity of the clouds. Thus another problem.
Star formation does occur, however. With the aid of telescopes sensitive to non-optical wavelengths, it is possible to pierce the darkness of molecular clouds and see stars being born, some of them surrounded by discs of dust and gas. In our own galaxy distances are close enough for us to study individual stars, at a time close to the present. Other galaxies, which give us insight into conditions further back in time, show that in the early universe the rate of star formation must have been much faster than inferred present rates. Essentially the problem is one of chronology and of explaining why the conditions for forming stars have changed.
Planetary core formation poses a rather different problem. Laboratory and microgravity experiments, such as those performed on the International Space Station, confirm that millimetre-size particles will spontaneously coalesce into loosely bound 1–5 centimetre clumps very quickly, and growth can be modeled up to metre size. Sticking is promoted by electrostatic forces, also by frost if present. Beyond that, aggregation can be frustrated by even a small amount of turbulence as collision effects turn from sticking to bouncing, or even disaggregation. Gravity becomes an appreciable force only beyond about 1 km diameter. Because of this ‘metre-size barrier’ the process leading to the formation of planetesimals is ‘somewhat murky’ (Armitage 2007). This is not to say that it could not have happened. It is clear from the low-density asteroids known as ‘rubble piles’ that larger-scale aggregation can occur. They are not evidence for the nebula hypothesis, since their bouldery composition shows they coalesced from collision debris, but they are at least evidence of gravity-aided secondary aggregation. One striking example is Sylvia, a lumpy potato-shaped asteroid 380 km across. Mars’s satellite Phobos, pictured below, is also a rubble pile.
In addition there are chronological discrepancies, for the dates yielded by meteorite inclusions, millimetre-sized condensates known as CAIs and chondrules, suggest that CAIs existed in the nebula separately for up to 3 million years before accreting with the chondrules to form metre-size boulders. Thereafter planetesimal growth stalled at sizes much smaller than several kilometres for a further million years or more (Dominik et al 2006). CAI-chondrule age differences of several million years are ‘incompatible with dynamical lifetimes of small particles in the nebula and short timescales for the formation of planetesimals’ (Weidenschilling et al 1998).
Computer models for the origin of the solar system do not yet include the satellites (moons) that orbit planets. More than 160 such satellites exist. Those with comparatively tight, circular, equatorial orbits round their host planet are termed regular satellites and most are believed to have formed out of the same region of the primordial disc as their host. As space probes have brought home, they are surprisingly diverse. The largest, Titan and Ganymede, are larger even than present-day Mercury, albeit not as massive. The irregular satellites are distinguished by their more elliptical orbits, smaller size and greater distance from the planet. These bodies are thought to have originated elsewhere in the disc and later been captured. Some irregulars may be products of a collision that shattered a larger moon. The number of satellites tends to increase with distance from the Sun and with planetary mass (gravitational attraction). Mercury and Venus have no moons, Earth only one, Mars two (both irregular), Jupiter 8 regular and 55 irregular.
The Genesis tradition mentions only three classes of body in the solar system: the Sun, the Moon and the ‘stars’, i.e. the luminous bodies, once referred to as “wandering stars”. Accordingly, creation theory predicts that the origin of the Earth’s moon will not be susceptible of a satisfactory natural explanation. It makes no predictions about the other moons. Clearly the irregular satellites are not primordial, but what about the regular satellites? The answer is probably also not, though it is always possible that, following a major perturbation, a planet may have become a moon (e.g. Titan, Ganymede). It may be no coincidence that the Earth is the only terrestrial planet to possess a regular moon. Spherical shape is not a sign of creation, since, above a certain mass, gravity will of itself cause a body to assume a spherical shape.
The diversity is well illustrated by Jupiter’s four ‘Galilean satellites’, so-named because they were first observed by Galileo, in 1610. Their greater rockiness closer in is evidence against their having originated in the same process as the planet. Io, the densest and innermost of the four, has a substantial iron core and a solid, mountainous crust. Tidal stresses produced by Jupiter’s gravity and the opposite pull of moons further out make Io volcanically highly active, inducing the eruption of sulfurous gases and so much silicate magma that the moon is now completely resurfaced. By contrast, Europa has a frozen, smooth surface criss-crossed by dark streaks, though tidal flexing maintains an ocean of water up to 100 kilometres thick beneath the ice. Scientists who believe in spontaneous generation speculate that there could be life in this water. Ganymede, the largest moon in the solar system, has an icy shell, with a mix of very old, highly cratered dark regions and somewhat younger lighter regions marked by grooves and ridges. Beneath the shell there is thought to exist a rocky silicate mantle and a small partially molten iron core which generates a strong magnetic field. Callisto, the outermost and least dense of the four, consists of about 40% ice and 60% rock. It is among the most heavily cratered satellites in the solar system, evidence, as with Ganymede, that the moon has been in existence from very early on. Estimated accretion times for the four moons range from 10,000 to 100,000 years.
In the case of the Earth’s Moon, its composition is so distinct that it cannot be explained as condensing from the same part of the nebula as the Earth’s place of origin. The Moon has a smaller proportion of volatiles and iron and is 40% less dense. In contrast to the scenarios proposed for other regular satellites, the Moon’s origin is attributed to a planet-destroying collision. An exotic planet the size of Mars crashed into the Earth and vaporised, producing an enormous ring of debris that was part exotic, part Earth-derived, and the Moon then condensed out of that. This is known as the ‘giant impact hypothesis’. Although extreme, the hypothesis was regarded as the most plausible of the naturalist explanations available, one of which, it is presumed, must be true. Believing it now requires a lot of faith. As one researcher put it, writing in the journal Nature, ‘Following almost three decades of some certainty over how the Moon was formed, new geochemical measurements have thrown the planetary science community back into doubt. We are either modelling the wrong process, or modelling the process wrong’ (Elkins-Tanton 2013). Scientists who are certain are not necessarily right.
Asteroids (‘star-like’ bodies, from the Greek word aster) are irregularly distributed across the solar system, ranging in size from several hundred km in diameter to mere pebbles. By far the largest is Ceres, now classified as a dwarf planet. At 900–1,000 km in diameter it comprises about a third of the total mass of solar system asteroids, with a rocky core and a water-ice mantle. Some of the largest asteroids have undergone sufficient thermal evolution for their interiors to differentiate. For example, the second most massive asteroid, Vesta, has a crust, mantle and core. Differentiation of this kind, whether in an asteroid, satellite or planet, including Earth, must have occurred naturally. Indeed it is clear from the fragments reaching us as meteorites that all asteroids have a complex history.
Most asteroids are located in the main belt between Mars and Jupiter, where they consist of silicates (rock), with some iron and other elements. There are also icy, irregularly orbiting bodies called centaurs between Jupiter and Neptune and a host of Trans-Neptunian Objects between Neptune and the edge of the solar system. It is widely believed that centaurs are TNOs that have been disturbed from their Kuiper Belt orbits to interact gravitationally with Neptune, with some eventually drifting further across towards the orbit of Jupiter. This appears to be the explanation for the arrival of comets in the inner part of the solar system: they are TNOs drawn in from the Kuiper Belt and beyond.
In the nebula story asteroids are remnants of the protoplanetary disc that failed to accrete further because of perturbations caused by Jupiter’s gravity. Nearer the Sun the rocky bodies accreted to form planetesimals and eventually full-sized planets, the Earth-Moon system reaching its present mass in about 100 million years (or maybe 40 million years). By this stage the terrestrial planets had stopped growing: the inner region of the solar system was now vacated of bodies that might have crashed into them and added to their mass. But how is it then that Mercury, Mars and the Moon are pockmarked with craters produced by impacting asteroids apparently hundreds of millions of years after the accretion process finished? As space.com writer Leonard David puts it, commenting on the lunar craters:
Things should have been settling down, according to solar system creation experts. Having chunks of stuff come zipping along some hundreds of millions of years later out of nowhere and create a lunar late heavy bombardment is a puzzler.
In fact, dense impact craters are visible on every solid surface which has not been subsequently resurfaced. Explosions and/or collisions were frequent throughout the solar system.
The main asteroid belt between Mars and Jupiter could be the remains of one or more such explosions, as would the rocky core of Jupiter itself. Less probably, some of the Kuiper Belt objects, including the largest, such as Pluto and recently discovered Eris, might be the remains of one or more explosions in the outer solar system. One of the peculiarities of these objects is that their orbits are both more inclined and more elliptical than those of the major planets. In the words of Bernstein et al. (2004), ‘the TNO population only vaguely resembles the preconception of a dynamically pristine planetesimal disk.’
Undoubtedly the most significant development in recent years has been the observation of planets orbiting other stars in the Milky Way. Gaseous discs have also been observed, accompanying, it appears, only very young stars (5 million years or younger). At first sight this provides a great boost for the nebula hypothesis, for it is now evident that entire solar systems do form by condensation from rotating gas clouds. It could be that most stars have planets orbiting round them. However, creation theory does not exclude the natural formation of stars other than the Sun, nor the possibility that planets form in the process. The evidence that these things have occurred is neutral as regards the formation of our own solar system.
The crucial question is whether other planetary systems are similar enough to our own to corroborate the solar nebula hypothesis, given the solar system’s apparent uniqueness as the only place hosting life. The answer is no. As Beer et al note (2004), ‘it has yet to be established that the solar system is a typical planetary system’. Among the many thousand detected no system resembles our own. A large proportion of the extrasolar planets observed are gas giants, with most being much closer to their parent stars than Jupiter is to the Sun – about 40% are closer even than Mercury. Either it is not true that such bodies can form only in the outer regions of the disc, as the nebula hypothesis presumes, or they must have migrated inward to their observed positions, again unlike those in our own solar system. The idea that nearly all these giants migrated to positions nearer their star has to be reckoned implausible. Another difference is that, contrary to theories of planetary formation round the Sun, the orbits of the planets further out from the star are highly elliptical.
Given that naturalistic theories expect our own solar system to be like most others (in line with the ‘Copernican principle’), its exceptional character, even uniqueness, is a significant finding, one based on observation rather than the uncertain simulations of computer models. Naturalistic theory predicts that more than a third of the systems with giant planets will turn out to harbour terrestrial planets, including water-rich planets on stable orbits in the habitable zone (Raymond et al. 2006). With the development of instruments capable of detecting planets as small as Earth, this prediction has also fallen flat. ‘Super-Earths’, to be sure, with orbits much faster than our own, are now reckoned to be the most common type of exoplanet. In 2011 a ‘rocky’ planet, named Kepler 10b, was discovered with a radius just 1.4 times that of Earth’s. One academic hailed it as “among the most profound scientific discoveries in human history”. However, it was too close to its star – twenty times closer than Mercury from the Sun – to be in any habitable zone: its surface temperature exceeded 1300° C.Then in 2013 astronomers found a planet with the same mass as Earth – but again, too warm for liquid water on its surface, and much less dense than Earth: it had the right mass but it was not rocky. Because of the possibility that such planets might support life (in the view of scientists who, contrary to all attempts to replicate it, believe that life is an accident just waiting to happen) such discoveries are repeatedly hyped up in the top journals. But while absence of evidence is not evidence of absence, it is not evidence of existence either. By contrast, creation theory predicts that an Earth like our own will not be found in other parts of the galaxy, just as it predicts that life will not be found – either there or elsewhere in our own solar system.
The ‘hot Jupiters’ orbiting so close to their parent stars suggest that naturally formed planetary systems have short life-spans. In agreement with the simulations of Creswell and Nelson, their orbits are circular and their nearness to the stars suggests they are migrating inwards on the way to being totally consumed. One such planet, WASP-18b, has ten times the mass of Jupiter, orbits its star in less than a day, and as a result of inward migration is so close to the star that it must be less than 1 million years old, possibly much less. Wasp-18b’s parent is thought to be around 1 billion years old – compelling evidence, surely, that there is something wrong with stellar chronologies. The fact that some of the stars are dwarfs supports the general case that planets formed over short timescales, since the core accretion model does not work in such a situation (Boss 2006). Yet some discs are reckoned on the basis of normal dating methods to be up to 25 million years old (Hartmann et al 2005).
To sum up, at present, ‘the origin of the Solar System represents one of the oldest unsolved problems in science’ (Taylor 2004). The problem resists a naturalistic solution and, by the same token, suggests a theistic solution, even though we do not see the solar system in its originally created state. The book is of course not yet closed. One may always, in the face of a perceived problem or an unpalatable solution, hope that the key insight still awaits discovery. So far as science is concerned, conclusions must remain provisional. That said, the only data one ever has are the data available now. We have one short life in which to make up our minds and, ultimately, it is the data that should determine our outlook on reality, not the other way round. The belief that the solar system came into being supernaturally is well supported.