Although simplicity is appealing, it is not always what Nature has in store for us.

How did the solar system begin – naturally or supernaturally? From a purely scientific point of view, both possibilities deserve consideration. Science cannot just decree what the answer is. On the other hand, you might argue that science can only investigate the natural possibility, on the basis that phenomena must have natural causes, however prejudicial that assumption might be if the solar system did in fact originate supernaturally. What, anyway, would count as evidence of a supernatural origin? When it comes to the bafflingly complex systems encountered in biology, you might argue from the evidence of design. In astronomy the hallmarks are not so clear. The sun, moon and planets do not have the obvious appearance of design that plants and animals have.

In view of these complications, perhaps the best approach is provisionally to accept that science can recognise only natural causes and see whether that limitation works. Without ruling out the possibility that the solar system had a supernatural origin, we can test whether it did by seeing whether it can be accounted for on the opposite presumption. If it can, then we may conclude that, at the very least, the solar system has the appearance of not having been created. In its most absolute and falsifiable form, creation theory proposes that creation gives rise to something that could not have formed by itself. It stakes everything on the prediction that the solar system will be found to have properties that positively require supernatural input. By contrast, the naturalist presumption can experiment with any number of possibilities: many could be, but only one need be, feasible, though of course a scientific hypothesis should be testable as well as feasible.

The question can also be put more neutrally: what are the simplest initial conditions that could have given rise to the present world? If it should prove impossible to avoid starting off with a fully formed sun, moon and planets, that would indicate a supernatural origin (though the proposition would still be open to future refutation). If it proved possible to derive them from something simpler, that would indicate that the solar system had evolved into being, even if the origin of the prior material remained an open question.

These days scientific accounts of the solar system’s origin consist largely of computer simulations, the object being to arrive at the present mass distribution, chemical composition, orbital periods and orbital geometries by a process of cause and effect. Even with the most powerful computers, the calculations can only be performed if simplifications are introduced, and allowances have to be made for what cannot be known with precision or certainty. A model covering an entire disc and the entire growth process along with all the relevant physics is still out of reach, and work done on computers is therefore better described as a ‘numerical experiment’ than a ’simulation’ (de Val-Borro et al. 2006). Such limitations, unavoidable though they are where scientific techniques are applied to an essentially historical question (consider the opposite case where scientific techniques are applied to forecasting the weather!), obviously temper the confidence with which conclusions can be drawn.

The solar nebula hypothesis

The standard account of the solar system’s origin is the nebula hypothesis. Being the most obvious way of conceiving of a natural origin, it was a deduction from philosophy, not scientific data, and was first proposed in the 18th century by philosopher Immanuel Kant and mathematician- astronomer Pierre-Simon Laplace. Data gathered during the past fifteen years have revolutionised our understanding of our solar system, but the basic concept has remained the same. In its modern version the hypothesis starts with a rotating molecular cloud of dust and gas, similar to that of the famous Orion Nebula. The ‘dust’ would have consisted of silicates, hydrocarbons and ices, the gas mostly of hydrogen and helium. Over time gravity caused the cloud to flatten into a disc, matter began to be pulled towards the centre, and eventually most of the cloud collapsed to form the Sun. The gravitational energy turned into heat energy, intense enough for nuclear fusion to power the Sun for billions of years. At the same time other matter in the disc contracted to form the planets.

The planets can be divided into three types, corres- ponding 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 a belt of asteroids lies between Mars and Jupiter, smaller groups of asteroids occupy various other parts of the solar system, and an array of icy ‘trans-Neptunian objects’, principally those that make up the Kuiper Belt, define the edge. The most famous of the TNOs is Pluto, recently demoted to a dwarf planet.

Planets in the nebula hypothesis formed through dust particles clumping into progressively larger units to become planetesimals, bodies the size of present-day moons. Thereafter 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 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.

The missing mass problem

At first sight it might seem that the asteroid belt and the Kuiper Belt can simply be interpreted as leftovers of that process: dust and gas aggregated into chunks all much smaller than the planets, but were then prevented from gaining further mass. This idea can be tested by estimating how much material there must have been in these regions of the nebula if its density was the same as was needed to produce the planets everywhere else.

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). A third question is therefore why collisional erosion of the clumps of matter between Mercury and Mars did not inhibit planetary formation in that region.

In the case of the Kuiper Belt, modelling has to reconcile the belt’s primordial density with the density required for the regions beyond it, namely the partly hypothetical Scattered Disc and the wholly hypothetical Oort Cloud. The Oort Cloud is the presumed source for long-period comets, that is, comets with long elliptical orbits but relatively short life-times whose observed frequency would otherwise point to a much younger age for the solar system 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).

The troublesome giant planets

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, the cores of Uranus and Neptune would have taken longer to accrete (more than 10 million years) than the source of their material – the rotating disc – could have been in existence. Accordingly Alan Boss suggested that ‘gravitational instabilities’ in the disc could have 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, so that the disc instability model is not a particularly attractive option (Armitage 2007). It also fails to account adequately for data gained from recent space probes: much of the inner bulk of Uranus and Neptune, it seems, consists of ice, not rock, Saturn has too small a rocky core and Jupiter’s may be smaller still (Saumon & Guillot 2004). Most of Jupiter’s heavy elements reside in its gaseous envelope rather than its core. Continuing work on the standard model, meanwhile, has come up with postulates that have allowed 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, and the objection is that extreme ultraviolet radiation from the star would have prevented particles from reaching even the much eroded dimensions that we see today.

The migration problem

A further problem with the core accretion model concerns the fact 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 recently 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 experience 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 reached metre-size had on average not much more than 1,000 years to make the leap to planetesimal size.

The disparity in angular momentum

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 stored 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 it has become possible to study 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 time scale). The data reveal a bimodal pattern. Stars with initially high rotation velocities evolve with no appreciable loss of angular momentum during the period, i.e. 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, it appears to be these which 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.

Where did the nebula itself materialise from?

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 in which all elements heavier than helium are synthesised. 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 an 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, mostly between 3.5 and 7.5 kiloparsecs from the galactic centre. That the gas occurs predominantly in the arms is puzzling, since these are rotating 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.

Did they stick? Accounting for terrestrial planet formation

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 inhibited 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.

Star formation does occur, however. Using telescopes that image radiation at non-optical wavelengths, it is possible to pierce the darkness of molecular clouds and actually ’see’ the birth of stars. Many of these stars are surrounded by discs of dust and gas. Other galaxies, which give us insight into conditions further back in time, indicate 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 experi- ments, such as those performed on the International Space Station, confirm that solid millimetre- sized particles will spontaneously coalesce into loosely bound 1-5 centimetre clumps very quickly. Growth to metre size is also thought to be possible. Beyond that, however, growth can be frustrated by even a small amount of turbulence as collision effects turn from sticking to bouncing, or even disaggregation. The process leading to the formation of planetesimals is consequently ‘somewhat murky’ (Armitage 2007). This is not to say that it could not have happened. It is clear from low-density asteroids known as ‘rubble piles’ that aggregation can occur beyond this threshold. They are not direct evidence for the nebula hypothesis, since their bouldery surface shows they coalesced from material produced in a collision, but they are at least evidence of secondary aggregation beyond metre size. One striking example is Sylvia, a lumpy potato-shaped asteroid 380 km across. Mars’s satellite Phobos, pictured above, is also a rubble pile.

In addition there are chronological discrepancies, for the dates yielded by meteorite inclusions – millimetre- to centimetre-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, after which 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 accordingly ‘incompatible with dynamical lifetimes of small particles in the nebula and short timescales for the formation of planetesimals’ (Weidenschilling et al 1998).

The planetary moons

Computer models for the origin of the solar system have not reached the point where they can include the satellites (moons) that orbit planets. There are more than 160 such satellites and their origin too has to be explained. Those with comparatively tight, circular and equatorial orbits round their host planet are termed regular satellites and are believed to have formed out of the same region of the primordial disc as their host. The largest – Titan and Ganymede – are larger even than Mercury, though 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 been captured some time after the formation of the solar system. Some are the products of a collision that shattered a larger moon, which may also be how Saturn’s rings originated. The number of satellites tends to increase with distance from the Sun and with planetary size. Mercury and Venus have no moons, Earth only one, Mars two (both irregular), Jupiter 8 regular and 55 irregular.

As space probes have brought home, the regular moons are surprisingly diverse. With the data being so new, it may be premature to expect robust models for their formation, so no critique of them is offered. The Genesis tradition describes mentions only three classes of body in the solar system: the Sun, the Moon and the ‘stars’, i.e. the luminous bodies that were once referred to as ‘wandering stars’. Creation theory therefore 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 were not created as such, but what about the regular satellites? The answer is probably also not. It may be no coincidence that the Earth is the only terrestrial planet to possess a regular moon. The existence of the other regular moons must have to do with either the nature or the location of the gaseous planets. Thus, if a moon’s composition should be more like its host’s than like that of any other object in the solar system, we may infer an origin from the same condensing gas as the host. If its composition should be similar to that of another body somewhere else, we may infer an origin associated with the genesis of that body. Only if its composition is unique may we infer a supernatural origin. 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 (in 1610) by Galileo. 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 by the opposite pull of moons further out make it volcanically highly active, inducing the eruption of sulfurous gases and so much silicate magma that Io 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. These marks are interpreted as tectonic deformation features. Beneath the shell there is thought to exist a rocky silicate mantle and a small partially molten iron core. The core generates a strong magnetic field. Callisto, the outermost and least dense of the four, consists of about 40% ice and 60% rock, incompletely differentiated. 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.

In the case of the Earth’s Moon, the Genesis tradition states that it was created. Consistent with this, its composition has been found to differ so markedly from the Earth’s that it cannot be explained as condensing from the same region of the solar nebula. The Moon has a smaller proportion of volatiles and iron and is 40% less dense than the Earth. In contrast to the scenarios proposed for other regular satellites, the Moon is therefore thought to have sprung from a collision. An exotic planet the size of Mars crashed into the Earth, produced an enormous ring of vaporised debris, and the Moon condensed out of that. This is known as the ‘giant impact hypothesis’. Although extreme, the hypothesis is seen as the most plausible of the explanations available. Since they appear to exhaust the naturalist possibilities, one of them, it is presumed, must be true.

The problem of too many drifting rocky bodies too late

Asteroids (’star-like’ bodies, from the Greek aster) are distributed irregularly across the solar system, ranging in size from several hundred km in diameter to the size of pebbles. By far the largest is 1 Ceres, now classified as a dwarf planet, like Pluto. At 900-1,000 km in diameter it comprises about a third of the total mass of solar system asteroids. Some of the largest asteroids have undergone sufficient thermal evolution for their interiors to differentiate. For example, the second most massive asteroid, 4 Vesta, has a crust, mantle and core. Differentiation of this kind, whether in an asteroid, satellite or planet, including Earth, is evidence that the process of separation occurred naturally. Indeed it is clear from the fragments that reach 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 to cross 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 that have been drawn in from the Kuiper Belt and beyond.

In the nebula hypothesis asteroids are remnants of the protoplanetary disc that failed to accrete further because of perturbations brought on 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 (some evidence suggests 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 further mass. But how is it then that Mercury, Mars and the Moon are all 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, equally dense impact craters are visible on every solid surface which has not been subsequently resurfaced, throughout the solar system. The enrichment of the atmospheres of Uranus and Neptune in carbon and possibly nitrogen also points to a high number of impacts.

Such phenomena imply that the chunks producing the impacts were fragments from explosive planetary collisions. The main asteroid belt between Mars and Jupiter would be the remains of one or more such explosions, and at least some of the Kuiper Belt objects, including the largest, such as Pluto and recently discovered Eris, would be the products of other collisions in the outer solar system. This would explain why their orbits are both more inclined and more elliptical than those of the major planets – a peculiarity which is yet another problem for the nebula hypothesis. As Bernstein et al. put it (2004), ‘the TNO population only vaguely resembles the preconception of a dynamically pristine planetesimal disk.’

Other planetary systems

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. However, creation theory does not exclude the natural formation of stars other than the Sun, whether in our own galaxy or other parts of the universe; nor does it exclude the possibility that planets formed 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 the other planetary systems are similar enough to our own to corroborate the solar nebula hypothesis, given the solar system’s apparent uniqueness in the universe as the only place hosting life. The answer is currently no. As Beer et al note (2004), ‘it has yet to be established that the solar system is a typical planetary system’. Of the 150 or so now detected none resembles our own. The extrasolar planets observed so far (there may be others beyond the present detection threshold) are nearly all gas giants, with most being much closer to their parent stars than Jupiter is to the Sun – about 40% are closer even than Mercury. They therefore refute the idea that such bodies could have formed only in the outer regions of the disc, albeit many have migrated inward to their observed positions. Another difference is that, contrary to theories of planetary formation in the solar system, 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, its uniqueness (to date) is a significant finding, especially as it is a conclusion based on observation of the real universe rather than the uncertain simulations of computer models. The prediction, nonetheless, is 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). Creation theory predicts that solar systems such as our own will not be found in other parts of the galaxy, just as it predicts that life will not be found.

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 that they are migrating inwards on the way to being totally consumed. The maximum plausible time for this inward migration is 100,000 years. Also, the fact that some of the stars are dwarfs supports the general case that planets formed over short timescales, since the core accretion model is known not to work in such a situation (Boss 2006). Some discs, however, are reckoned to be several million years old.

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. That said, the only data we ever have on which to draw a conclusion, however tentative, is 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.

See also:
Worlds in Collision – a false start
The Creation
References