Asteroids, which number in the millions and range from 930 km across to less than a few metres, are classified according to their mineral content (as inferred from their spectra). There are three main classes. C-type asteroids (‘c’ for carbon) comprise about 75% of the total, have very dark surfaces and have similar compositions to carbonaceous chondrites, the most common class of meteorite. S-types (‘s’ for stone) comprise about 15%, contain various silicate compounds and a certain amount of nickel and/or iron and may be the source of ordinary chondrites. The remaining 10% is made up mostly of M-types (‘m’ for metal), having spectra that suggest a predominantly iron content. They are therefore likely to be the source of most iron meteorites, though some M-types are of too low a density to be consistent with this interpretation.

Within the various types, some asteroids can also be grouped into families, each representing a body that broke up into multitudinous smaller pieces. The biggest such group, the Flora family, comprises over 800 asteroids of type S, arising from the break-up of an asteroid around 200 km across. There are around 64 detectable families in the asteroid belt and this number sets constraints on the size of its mass prior to the point when fragmentation began to outpace accretion. Contrary to other estimates, the asteroid belt 4.5 billion years ago may not have been more than 2 times the present mass, with about 50% of the family-forming collisions occurring within the first 500 million years (Marzari et al. 1999).

Vesta, at 530 km in diameter, is a rare example of a large, close-to-extant parent body. It is thought to have spawned the V-type asteroids and the ‘HED’ group of achrondritic meteorites, most if not all deriving from the impact crater, 460 km wide and 13 km deep, near its south pole. Vesta was also large enough to have differentiated into a mantle and core, as internal melting caused light minerals (e.g. silicates) to float towards the surface and heavy minerals (e.g. iron) to sink towards the centre. The source of the heat is uncertain, for although the asteroid formed surprisingly early, only 2-4 million years after CAIs, the heat generated by radioactivity seems to have been insufficient to cause large-scale melting (Kunihiro et al. 2004, Sokol et al. 2007). Possibly Vesta was already in a molten state when it formed, having derived from the explosion of a still larger molten body.

Whatever its origin, Vesta is exceptional. It is the second largest asteroid in the main belt and the only one known to have differentiated. While iron meteorites and M-type asteroids show that there once must have existed other such bodies – large enough to have differentiated to the point of producing a metal core – the majority of asteroids are only small chips, the result of disaggregation rather than aggregation. Some, such as Itokawa, a chondritic S-type, are loosely bound ‘rubble piles’ and are clearly the result of aggregation. It is also clear they accreted from the dust and boulders left from the break-up of older, more coherent bodies; they are not first-generation asteroids and they are not on the way to becoming monolithic planetesimals. Itokawa’s porosity is 40%. Somehow, repeated impacts have failed to tamp it down into anything solid.

Meteorites can be divided into two main categories: those that underwent heating but not melting (chondrites) and those that experienced heating, melting and differentiation (achondrites such as the HED family, stony-irons and irons). The immediate forbears of the chondrites are aggregations of the fine material (condensates, melt droplets, lithic fragments and ash) that issued from the exploded planets. The immediate forbears of the achondrites – though we should bear in mind that Vesta is the only known extant example of such a forbear – were entire globes of expelled molten material. The heterogeneity of Vesta’s bulk composition indicates that the planet from which it originated had not differentiated, or at least had not fully done so. It appears to be somewhat younger than most chondrites because it did not reach a solid state until somewhat later.

Assuming that all terrestrial planets started out with the same composition, metal cores should comprise around 15% of their volume, with the other 85% consisting of olivine-rich mantle and thin plagioclase-rich crust. In principle, therefore, asteroids of mantle silicate composition should be 5-6 times more plentiful than those composed of iron. They are, however, ‘astonishingly rare’ (Haack & McCoy 2005). The problem posed by the scarcity of olivine-rich asteroids compared to the many asteroidal cores sampled by iron meteorites even has a name: ‘the great dunite shortage’ (dunite being a kind of olivine).

The exploded planets hypothesis provides a solution: the parent bodies derived from the cores of the exploded planets themselves, not from the cores of smaller, now vanished, Vesta-like bodies. These parent asteroids were molten globes of metal. As such, they were of the same generation as Vesta, but derived from the core of the former planet.

The iron content of chondrites varies enormously, depending on type. By far the most common group (over 90%) are the ordinary chondrites, which contain abundant chondrules, relatively sparse matrix, few refractory inclusions and 1.5-8 % iron. Relative to the bulk composition of the Earth, most classes of chondrite are therefore depleted in iron – by up to 20% relative to CI chondrites and more than that relative to ordinary chondrites (Palme & O’Neill 2005). By themselves, therefore, chondrites do not provide the recipe for a complete planet. Iron needs to be added, the full recipe being: chondrites (comprising CAIs, chondrules and matrix) + iron meteorites. Conversely, the problem posed by the shortage of olivine-rich asteroids is solved by recognising that olivine and other such silicates are the main ingredients of chondrules. Chondrules make up the bulk of the meteoritic material that falls on Earth, and it is these that represent the primitive, partly differentiated mantle of the former planets. Indeed, recent work has confirmed that the olivine grains within chondrules derive from mantle material (Libourel & Krot 2007). In terms of the nebula hypothesis: ‘Chondrules are not as pristine as conventionally viewed; instead, they consist of nebular and asteroidal materials and must have postdated accretion, thermal metamorphism and differentiation of some early generation planetesimals.’

There are two further predictions from this scenario. The first is that if some or most iron meteorites are to be associated with CAIs and chondrules rather than with the Vesta generation, then they should be of about the same age. It turns out that they are. Tungsten isotope data show that metal-silicate differentiation in the parent or parents of magmatic iron meteorites occurred ‘extremely early’, coeval with the first chondrules (Schersten et al. 2006, Qin et al. 2008). ‘Chondrites do not represent the precursor material from which asteroids accreted and then differentiated’ (Kleine et al. 2005). Instead they ‘derive from asteroids that accreted late … [possibly] by reaccretion of debris produced during collisional disruption of older asteroids’.

The other prediction concerns the size and nature of the bodies from which the iron meteorites arose: the meteorites should have come from predominantly metallic parent bodies rather than ones that, following differentiation, had a large mantle around their cores. Again, this turns out to be the case. We can distinguish between the two kinds of meteorite parent body by reference to their inferred cooling rates, and these are incompatible with cooling in a core insulated by a thick mantle (Yang et al. 2008). They show that the parents were ‘much larger than previously inferred and cooled with little or no mantle’ (Yang et al. 2008). Rather, the cooling rates show that the parents were ‘much larger than previously inferred and cooled with little or no mantle’ (Yang et al. 2007). Protoplanets the size of the Moon or larger were stripped of their mantle, and not by some gradual chipping-away process but in a catastrophic event that destroyed the bodies.

If the initial mass [of the asteroid belt] was ~10³ times higher and many Moon-sized or larger protoplanets were present, collision rates would have been greatly enhanced while the differentiated bodies were still hot. As mantles are not efficiently stripped from cores by catastrophic impacts between asteroid-sized bodies, the IVA metallic body may have formed as a result of a grazing collision between protoplanets that disrupted the core of a Moon-sized body creating a string of metal-rich bodies.

The scenario of collisions leaving strings of metal-rich bodies is as elaborated by Asphaug et al., quoted at the beginning of this article. Colliding protoplanets do not, as is commonly assumed, simply merge. In some cases the result can be explosive, especially for the smaller body. As one author put it in their press release:

“As two massive objects pass near each other, gravitational forces induce dramatic physical changes – decompressing, melting, stripping material away, and even annih¬ilating the smaller object” Williams says. “You can do a lot of physics and chemistry on objects in the Solar System without even touching them.”

A planet exerts enormous pressure on itself through self-gravity, but the gravitational pull of a larger object passing close by can cause that pressure to drop precipitously. The effects of this depressurization can be explosive, Williams says.

“It’s like uncorking the world’s most carbonated beverage” he says. “What happens when a planet gets decompressed by 50% is something we don’t understand very well at this stage, but it can shift the chemistry and physics all over the place, producing a complexity of materials that could very well account for the heterogeneity we see in meteorites.”

Once regarded as the stuff of fringe speculation, exploding planets have now entered the mainstream.