Lost planetary worlds: Evidence of the unquiet early Solar System

This post was contributed by Birkbeck student, Anja Lanin. Anja attended Professor Hilary Downes’ lecture, ‘Lost Planetary Worlds’ during Birkbeck Science Week 2016.

Solar System

Professor Hilary Downes has been a research scientist in the Department of Earth and Planetary Sciences at Birkbeck for 30 years. Insights from her own and other workers’ research have left her with a strong interest in the evolution of the Solar System. As it turns out, the orderly Solar System we observe today in fact started out as everything but quiet and orderly. In its early days it was a place of violent collisions between planetary bodies. Many of these have been almost completely lost. Almost! We have evidence of their existence, ranging from the macroscopic to the elemental, and this was the subject of Professor Downes’ enthusiastic talk ‘Lost Worlds of the Solar System.’

Theory: Computer models

Starting her talk by showing a real image of a planet-forming region around stars, as observed by telescope, as well as computer models which together may suggest the organised formation of planets within an accretionary disk, Professor Downes moved on to theoretical considerations of a very different-looking chaotic early Solar System.

Computer simulations of Jupiter’s growth, for example, indicate that many planetary embryos were sent onto wildly eccentric orbits. Other models show planets such as Jupiter and Saturn moving repeatedly closer and then away from the sun causing gravitational chaos in the inner Solar System before the system became more settled.

Evidence from our Solar System planets – shaken and stirred!

The audience were then presented with some very odd and interesting facts about our planets. For one thing, they do not orbit the sun in the original plane (the location of the previous accretionary disk) of the Solar System. Some seem to defy the laws of physics by floating above and some below. Furthermore, some of the planets’ axial tilts have gone ‘wonky.’

While Jupiter and Mercury spin textbook-style perpendicular to the plane, all other planets have been knocked around to the extreme that Uranus has been completely knocked over and is now spinning parallel to the plane. Venus is even more special – it is rotating in the opposite direction to all other planets! These characteristics, according to Professor Downes, strongly suggest violent collisions of the planets with other planetary material.

‘Tangible’ evidence: meteorites within meteorites

So what happened to the impactors? We can actually study collisional space debris which comes to us in the form of meteorites. For many of these meteorites the parent body, for example a planet or an asteroid, is known, but, as Professor Downes emphasises, there are many parentless ungrouped meteorites. Perhaps the most interesting of these are brecciated meteorites which contain fragments of other meteorites. What do we learn from these fragments?


Real science reveals real mysteries….

As indicated in the talk, planetary science students at Birkbeck are actively accessing technology (e.g. electron microprobe) that allows them to study the mineralogy and basic chemical make-up of meteorites. This is one way that allows them to determine whether or not meteoritic material comes from a classified or unclassified parent body.

Something that cannot be analysed at Birkbeck yet!, but also yields very important clues, are oxygen isotope ratios. Each known planetary body has a unique oxygen fingerprint, so that previously unregistered ratios hint at lost parent bodies. Professor Downes, relating to her own group’s research, points out a particularly interesting brecciated meteorite fragment, which, surprisingly, turned out to be granitic, i.e. it is mineralogically and texturally similar to granites found on Earth (some of us recognise the rock from kitchen counter tops!).

However! – its oxygen chemistry indicates that it comes neither from Earth nor is it related to the other meteoritic material in which it was included as a fragment. It is therefore not related to the asteroid from which the rest of the meteorite is derived. In addition, a strange associated glass is high in sulfur (S) and chlorine (Cl), and no planet in the Solar System except Mars contains sulfur and chlorine. But the oxygen chemistry again suggests it is not from Mars. Thus, this glass may represent another lost planetary body or planet possibly disintegrated during the early collisional chaos!

There are many examples of odd, unexplained finds in meteorites. Even opal, which we recognise as a semi-precious stone, has been found by Profesor Downes and her colleagues, although its extraterrestrial origin is still unclear. Perhaps a water or ice-rich meteorite crashed into an asteroid and all that is left of this ice or water world is this little piece of opal?

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