5. Asteroids, comets and moons

Asteroids in the main belt tend to orbit at a much more inclined angle to the ecliptic than the planets, the majority within 20°, a few up to 40°. The average inclination for objects larger than 100 km is 11°.

If we ignore the question of origins and assume that the asteroids began within a degree or two of the ecliptic, lower-angle inclinations can be explained by collisions, which certainly occurred. Impact scars are common even on quite small asteroids. Higher-angle inclinations are more difficult to explain and point to an explosive origin. Fragments that shot out in roughly the same direction as the former planet’s orbit, at whatever angle, would have remained at approximately that distance. Other fragments would sooner or later have crashed into the Sun or other planets, or been captured by other planets to become satellites. Some became locked into stable synchrony with Jupiter’s orbit. Known as Trojans, they mostly have a D-type spectrum, like the asteroids nearest them in the main belt.

Some of the material presumed missing from the asteroid belt was swallowed up by Jupiter, which thereby acquired a diffuse rocky core, equal to about 10–25 Earths. (This sounds enormous, but a planet with 25 times Earth’s mass would have had a diameter just 3 times greater, and Jupiter would still have been many times more massive.) In the other direction, much of the material would have ended up in the Sun.

The only direct way of probing the Sun’s composition is through analysing its photosphere. If the Sun condensed from gas that was already impregnated with metals, its composition should be fairly uniform and, apart from helium and from the major volatiles carbon, nitrogen and oxygen, similar to that of chondrites. Which it is. Nonetheless, astronomy struggles to model the Sun’s interior on the basis of chondritic proportions (Amarsi et al. 2021). According to creation theory, most of the Sun’s carbon, nitrogen and oxygen was generated by nuclear fusion within the star rather than inherited. The presence of heavier elements is due to planetary debris plunging into it. In that case too the proportions should be chondritic, but the photosphere relative to the interior will be enriched.

The explosion of a large planet between Saturn and Uranus would similarly account for Saturn’s core. Distances in the solar system are measured in astronomical units or AU, one unit being the mean distance between the Sun and the Earth. The orbits of Saturn and Uranus are 9.6 AU apart, almost exactly the distance between Saturn and the Sun, so there is plenty of space for another planet. Beyond Jupiter are the centaurs, possibly numbering several million and mostly orbiting between Saturn and Uranus. With Uranus’s axis tilted 98 degrees it certainly seems as if something catastrophic occurred in this region.

As a population, the existence of centaurs was not known about until the 1990s. They were thought to be half comet, half asteroid (hence their name – in mythology centaurs were half man, half horse). Hidalgo, the first to be discovered, has an optical spectrum characteristic of D-type asteroids, orbits the Sun at a distance of 1.9–9.5 AU (i.e. the orbit is highly eccentric) and has an inclination of 42°. Damocles orbits at a distance of 1.6–22 AU and has an inclination of 62°. The largest confirmed centaur, the 260 km Chariklo, is encircled by two rings. Many of the main belt asteroids are thought to have been centaurs drawn sunward by the gravity of Jupiter.


Comet 67P photographed by RosettaAs with every other class of solar system body, ideas about comets have changed over recent years. Once assumed to be unaltered leftovers of the cold outer nebula, they have proved to be compositionally much the same as asteroids, only richer in volatiles (frozen frozen H2O, CO2, CO, CH4, NH4). We know most about the short-period comets, those with orbits of less than 200 years. Some consist primarily of volatiles; some have solid nuclei, dented by craters; others are low-density rubble piles. They manifest as comets when their elliptical orbits bring them close to the Sun. Each year several asteroids are reclassified as comets on being found to exhibit a coma and/or tail.

The data sent back by the Deep Impact and Stardust missions, investigating the short-period comets Tempel 1 and Wild 2, were revelatory. Contrary to the expectation that the silicate particles in them would be amorphous, like the dust in interstellar space, the particles turned out to be crystalline, indicating crystallisation from melts at temperatures up to 1700° C (Grossman 2010). Analysis of one such silicate in Wild 2 indicated a cooling rate of 10–100° per hour, in close agreement with the cooling rates of chondrules or asteroidal lava (Leroux et al. 2008). Carbonates, iron, CAIs and chondrules were also detected.

How is one to explain the existence of high-temperature and low-temperature materials in the selfsame body? Since the core must have preceded the volatiles round the core, its rocky composition suggests an origin in the inner solar system. On the other hand, both their orbits and their abundance of volatiles suggest that comets originated in the outer solar system. Did rocky planets also exist there?

‘Long-period’ comets have orbits that take more than 200 years to complete. The orbits are highly elliptical and eccentric – attributed ad hoc to the gravitational pull of passing stars – and extend far beyond the observable solar system. Like the short-period comets, they do not differ fundamentally from asteroids in composition. They can orbit the Sun at any inclination and are as likely to have a retrograde orbit (moving in the opposite direction to the planets) as a prograde one. Not unreasonably, the solar system is thought to be enclosed by a vast replenishing reservoir of such comets known as the Oort Cloud. Unfortunately the reservoir is too far away to be directly observable.

The difference in orbital period between the two types is not absolute. Long-period comets can become short-period if the gravity of the giant planets draws them into the inner solar system. Most short-period comets orbit between Mars and Jupiter and are thought to have drifted in from the Kuiper Belt.


TritonWith the exception of Earth’s moon, probably none of the planetary satellites formed in situ. Mercury and Venus have no satellites. Mars has just two, both clearly asteroids. Jupiter has around 92 and Saturn 145, the majority less than 10 km in diameter. Uranus has 27, Neptune 14. Once captured, the different types frequently crashed into each other. Some collisions resulted in complete fragmentation. Occasionally the fragments turned into rings, or re-assembled to form new moons. Other collisions left the moons cratered and scarred. Spherical shape is not evidence of creation, since above a certain mass self-gravity will naturally pull a body into a sphere.

Astronomers classify the moons into regular satellites, which trace close, circular orbits about the host’s equator, and irregular satellites, which trace more distant, elliptical orbits at an angle to the equator. It is possible to model the capture of both types (Astakhov et al. 2003). Usually the regular satellites orbit in the same direction as the planet’s rotation, and tend to be bigger. They are supposed to have formed from the same material and at the same time as their hosts. However, all the moons are predominantly rocky whereas, apart from Earth and Mars, the hosts are gaseous.

At 2700 km diameter, Triton is by far the biggest of Neptune’s satellites, has an inclined but circular retrograde orbit, and lies between a group of inner prograde satellites and several outer satellites with both prograde and retrograde orbits. No other large moon in the solar system has a retrograde orbit. Density calculations suggest it consists 30–45% of water ice and 55–70% of rocky material. On the surface frozen nitrogen, carbon dioxide and methane (CH4) have also been detected.

The present solar system offers no mechanism by which moons could be captured, so the thinking is that captures must have taken place early in its history, when there was a much greater likelihood of collisions and near misses (Jewitt et al. 2006).