8. How many exploded planets this side of Neptune?

In the discussion about the diffuse rocky cores of Jupiter and Saturn it was suggested that there were two ex-planets: one between Mars and Jupiter and the other between Saturn and Uranus. If so, then meteorites should fall into two primary groups – not, of course, based on whether they included chondrules (since both explosions would have produced melt droplets) but on isotopic differences. Nucleosynthesis took place in all the planets and, given that the two bodies would not have had the same mass, the extent to which that happened would have depended on how massive they were.

The predicted dichotomy is seen in the fundamental distinction between carbonaceous and non-carbonaceous meteorites, cutting across the division of meteorites into chondrites, achondrites and irons. Analyses of low-abundance elements such as Ti, Cr, Ni and Mo show that carbonaceous meteorites tend to have higher proportions of the neutron-rich isotopes (Warren 2011, Dauphas & Schauble 2016). For example, a graph of 95Mo plotted against 94Mo, will show one ratio for carbonaceous chondrites, achondrites and irons and another, lower ratio for non-carbonaceous chondrites, achondrites and irons. (Although most iron meteorites are carbon-poor, some are classified as carbonaceous because of their molybdenum isotope ratios.) Intermediate compositions do not occur, nor is there any change in the ratio over time. Since chondrules, matrix and the corresponding irons all show the same ratios, the components must have had the same ancestry.

So what accounts for the dichotomy? That the isotopic ratios reflect nucleosynthetic processes is generally agreed. However, in orthodox cosmology nucleosynthesis takes place only in stars. One seems forced to conclude, consequently, that two supernovae contributed to the nebula and that the isotopes retained their distinctive ratios in discrete ‘reservoirs’, though the bulk composition of the reservoirs was identical. This is implausible to say the least. The same problem arises when trying to account for compositionally distinct chondrule groups: if chondrules are condensates from the nebula, how can different regions of the nebula have remained compositionally distinct for two million years? The alternative explanation is that the discrete reservoirs were in fact planets, and the degree to which neutron-heavy isotopes were synthesised depended on planetary size.

The carbonaceous chondrites and achondrites are sso called because, compared to ordinary chondrites (which make up the majority) and the bulk composition of Earth, they are enriched in carbon. Much of the excess carbon inheres in the minerals calcite and dolomite. The minerals are secondary, because they precipitated from a solution of water and CO2 gas (carbonic acid) and of Ca and Mg ions exsolved by the acid from the rock minerals. The carbon and oxygen originated from the Sun, at a time when it was generating and blasting out significant amounts of C, N and O. The water, on the other hand, must have come from the primeval Oort Cloud, diffusing in the opposite direction towards the Sun but impeded by the solar wind. At the relevant time the front where they met and beyond which C, N and O were able to interact with the water lay somewhere beyond Jupiter. We may deduce therefore that carbonaceous meteorites represent the debris from an exploded planet in that region and there acquired their carbon secondarily. Non-carbonaceous meteorites represent the debris from the nearer planet. Interestingly, the isotopic dichotomy is also seen in nitrogen itself. Relative to the terrestrial standard, carbonaceous irons and comets are enriched in 15N, which has one more neutron than the main isotope, 14N, whereas non-carbonaceous irons are depleted (Füri & Marty 2015). The source of 15N was the Sun. Earth was protected from the solar wind to some extent by its magnetic shield.

The idea that carbonaceous chondrites formed beyond Jupiter and the other chondrites this side of Jupiter was first proposed by Paul Warren (2011). The giant gas and ice planets formed in the outer nebula, since it was colder, so chondrites forming there were rich in volatiles. Later the gravitational influence of Jupiter drew some of the chondrites into the main asteroid belt. Although the main asteroid belt is not sharply zoned because of mixing and drifting, carbonaceous asteroids are more frequent in the middle to outer belt and non-carbonaceous asteroids in the inner belt. A particular example of inward migration is the comet Wild 2. As we have seen, frozen volatiles co-exist with rock-derived material that was subjected to melting and rapid cooling.

CAIs are typically much more abundant in carbonaceous chondrites (0.5–3% by volume) than in enstatite and ordinary chondrites (less than 0.1%) (Desch et al. 2018). As CAIs require the highest melting temperatures, the bigger of the two planets that exploded must have been the one between Saturn and Uranus, consistent with the higher proportions of neutron-rich isotopes in carbonaceous chondrites.