It is now generally accepted that, from a geochemical perspective, our Solar System is lumpy. In fact it comes in two very distinct lumps. This was pointed out by Paul Warren in his 2011 paper. He showed that when various isotopes (e.g. ⁵⁴Cr, ⁶²Ni, ⁵⁰Ti), measured in a wide range of extraterrestrial materials, are plotted against each other, two discrete clusters are defined. And between the clusters is a big gap. Some cosmochemists have even started calling it the “Warren gap”. Paul Warren suggested that one of the groups, dominated by carbonaceous chondrites, was from the outer Solar System and the other, including rocks from Earth, Mars, the Moon and a large range of melted and not so melted asteroids, were from the inner Solar System.

OK, that might not sound like such an amazing suggestion. We know that the inner and outer bits of our Solar System are quite different. The inner bit contains the rocky planets, like Earth and Mars, and the outer bit the gas and ice giants. They look pretty different. But from a geochemical point of view, the puzzling thing is that gap.

The early Solar System was a dynamic place. Stuff should have been mixing rapidly as gas and dust spiraled towards the growing Sun and at the same time the energetic young Sun ejected material outwards during high energy outbursts. All this has been seen around other nearby young stars. Why should a compositional gap have been preserved? It was a mystery.

Then in a paper published in 2017,  Thomas Kruijer and co-authors fingered Jupiter. Its large size is consistent with it being one of the earliest planets to form in the protoplanetary disc. kruijer et al 2017 argued that once it started to form it acted as a barrier preventing mixing between the inner and outer Solar System. It is an idea that has taken hold and is now widely accepted and cited by planetary scientists.

But now, in a new paper in Nature Astronomy, Ramon Brasser and Stephen Mojzsis have called into question the role of Jupiter. They modelled the growth of the Solar System’s largest planet and found it was too slow to account for the compositional gap. So, if not Jupiter, what was it that so efficiently split the Solar System into two chemical lumps?

In their paper, Ramon Brasser and Stephen Mojzsis suggest that the protoplanetary disc was divided into segments by pressure maxima. One such zone may have been located near the formation region of Jupiter. They go on to speculate that the early Solar System may have had multiple rings, such as those observed by ALMA (Atacama Large Millimeter/submillimeter Array) in the disc around the young star TW Hydrae. It is an intriguing possibility. You can be sure geochemists will start taking a closer look at their data to see if multiple clusters are present.  However, I think it is going to be a tough job to displace Jupiter from its current starring role. We shall see!

ALMA image of protoplanetary disc around the young star TW Hydrae. The gaps in the disc are generally considered to be due to planets that are actively being formed about the central star.  (image credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO))

Featured image: This image of Jupiter was created by Kevin M. Gill using the Juno Cam imager. Image credit: NASA/JPL-CALTECH/SWRI/MSSS/KEVIN M. GILL

Upper image: Artist’s impression of Solar System: Image credit: NASA/Jet Propulsion Laboratory-Caltech