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Divided Worlds?

Updated: Jan 10

Image credit: Encyclopaedia Britannica

The King Planet

Jupiter, at roughly 320 x the mass of the Earth and 2.5 x the mass of all other objects in the Solar System, is the largest object in our planetary neighbourhood with the exception only of the Sun. In dwarfing all other planets in the Solar System, Jupiter has long-held and continues to wield great power and influence to manipulate other planets and objects thereby determining the destiny of other worlds. Particular kinds of chemical data for meteorites have suggested that a major division separated reservoirs of Solar System materials close to and distant from the early-forming Sun - referred to as the Solar System dichotomy. This finding has led scientists to conclude that Jupiter and its immense gravitational clearing power were present early on, making this 'King planet' the earliest formed having grown its core in as little as 1 million years after the beginning of Solar System history. Hence, Jupiter alone was present as a witness to the ignition of the Sun's nuclear furnace during the first 5 million years of the evolution of our most immediate celestial setting. Though Jupiter may initially strike us as distant and disconnected from Earth - taking years for any spacecraft to travel to - it is so massif that it has the ability to sculpt the architecture of the Solar System and to determine the nature of chance events. Hence, the fate of our watery and relatively temperate world as a cradle for life that now supports the existence of human-kind, has been profoundly influenced by this so-called godfather of planets.

Image credit: NASA / Goddard

Everything In Its Proper Place?

The present-day architecture of our Solar System can be considered in two parts. The inner Solar System is closest to the Sun and comprises rocky bodies with metallic cores that include the planets of Mercury, Venus, Earth and Mars and their various moons, as well as the graveyard of shattered remnants of the early Solar system present in the asteroid belt. These materials of the asteroid belt are more mixed in including rocky, metallic, and carbon-rich bodies as identified by the characteristic signatures of their surfaces. The outer Solar System contrasts in being dominated by bodies dominated by gas and ice, and includes Jupiter and all that is found beyond it. Hence, Saturn, Uranus, and Neptune are among the outer Planets, the dwarf planet Pluto is included here, as are Kuiper Belt objects located at around 30 to 55 astronomical units (AU; where 1 AU = distance between Earth and the Sun, 93 million miles) from the Sun and the theorised Oort cloud hypothesised to be located somewhere very far out, likely between 2,000 and 200,000 AU. In being roughly donut shaped the Kuiper belt and Oort cloud are broadly analogous. These features are vastly different in terms of composition and mass relative to the asteroid belt. The Kuiper belt is one of the largest structures in the Solar System, and has been estimated to be between 20 and 200 x the mass of the asteroid belt. Relative to Earth's mass the Kuiper belt is not thought to exceed 10 percent, probably less. Knowledge of the Kuiper belt is currently limited because this is a region we are yet to explore with space craft. A similar status of restricted knowledge applies for the theorised Oort cloud that is considered to be where some icey comets originate from.

Given the division between rocky bodies in the inner Solar System and gaseous / icey worlds in the outer Solar System, traditional views held that planets orbited the Sun in a fixed manner as described by a clock-work orrery such as those seen in museums. With such mechanical models the Solar System is viewed as an orderly place in which the planets and most other features are considered to have formed in their present locations.

Image credit: Orrery, Kelvingrove Museum

However, the surfaces of the Moon, Mercury, Mars, asteroids, and the Earth are visibly scarred. The craters on Earth and Mars, and the heavily pock-marked 'death masks' of smaller bodies that cooled relatively quickly testify to these bodies being thwacked by many impactors - where one of the legacies of the Apollo missions is that age data for rocks of the Moon indicate that many such strikes occurred around 4.5 to 3.9 billion years ago (that is 4,500,000,000 to 3,900,000,000 years ago!).

Image: The cratered surfaces of the nearside (left) and farside of the Moon (right). Credit: NASA, GSFC, Arizona State University.

Major and more recent strikes, such as that on Earth linked to the demise of dinosaurs around 66 million years ago, prove that such catastrophic events have played an important role in snuffing out some of the competition and paving the way for human-kind. Further challenges to earlier theories of an orderly Solar System include the expectantly small size of Mars (roughly one tenth the mass of Earth), and the compositional mix and relatively meagre mass of the asteroid belt (around 4 percent of the mass of the Moon). Significantly, recent astronomical studies of solar systems across the wider universe drawing on a number of specialist telescopes have shown populations of so-called exoplanets that are dramatically different to our own. Some of these other planetary systems are home to rocky bodies many times greater in size and mass than the Earth - so-called 'super-Earths'; these can even occur as several bodies packed in closer to one another than we could ever have expected. Some exoplanetary systems even have gas giant planets in orbits sizzlingly close to their stars - these are termed 'Hot Jupiters'. Why are they so near to their stars?

Image credit: NASA, Ames Research Center, Natalie Batalha, Wendy Stenzel

What has become clear from these studies of exoplanets is that the make-up, orbital characteristics, and order of planets known from our own Solar System is not typical of those that we now know from elsewhere - both in terms of stars broadly like our Sun and for stars of different types. For a video of some of the relative orbits among exoplanetary systems as determined by the now retired Kepler space telescope see -

Importantly, the way in which planets are distributed relative to their star and each other can vary enormously, and in a number of cases the only explanation is that those planets did not form in their present locations but have moved around. Is planetary pinball relevant to our own Solar System?

A Wild Ride

To account for the present distribution and size of planets and asteroids, scientists have proposed that early in the history of the Solar System, and while there remains gas in the disk of material from which the Sun and other objects formed, Jupiter brings about a dance of the planets during the first 5 million years of Solar System history. The gas outward from Jupiter pushes it spiralling inward toward the Sun. As Jupiter rampages inward its enormous gravity clears much in its wake, and chaotically shifts the materials of the inner Solar System that shall grow into what we now know as Mercury, Venus, Earth and Mars. In snowploughing some of the of the inner rocky materials into the Sun during a voyage of destruction, Mars - the runt among our neighbouring rocky planets - is starved of the building blocks that may otherwise have allowed it to grow to the size of Earth or bigger.

Before Jupiter reaches the Sun and obliterates every last rocky world, Saturn - another gas giant of enormous mass and significant gravitational influence -is forming and also migrates toward the Sun while gas remains present in the disk from which planets are growing. At a critical point Jupiter and Saturn become locked in what is termed an 'orbital resonance', a particular gravitational dance in which the tug of one on the other is enhanced. Consequently, Jupiter is captured from the verge of total destruction and cast back out away from the Sun where Saturn retreats also. The inward then outward travel of these two gas giant planets, termed 'The Grand Tack', throws shockwaves through the Solar System. This reverse of the marauding ride taken by these bruising giant planets scatters rocky materials outward and throws icy materials inward from beyond the so-called celestial 'snow line' (a reference to the distance at which water freezes). Hence, this tacking of the gas giants predicts a mix of materials that started their lives as both dry (inward of Jupiter's initial location) and wet (outward) being present in the inner Solar System during the subsequent growth of rocky bodies from embryonic 'planetesimals' headed toward planets of the sizes we know today.

The chaos continues. At around 600 million years or so into Solar System history the presence of Neptune and Uranus becomes more widely influential - the so-called 'Nice model' applies. These icy planets are thought to have formed much further inward than where they are located today, with a mass of smaller icy planetesimals outward of them. With Neptune and Uranus located close to where the Grand Tack has left Saturn, Jupiter and Saturn hit another resonance that brings upheaval to the Solar System. Neptune and Uranus are flung outward toward their present-day locations and the act of doing so flings icey planetsimals / cometary materials all over the Solar System thereby providing an explanation for the cratering history of the Moon and its companions, as well as potentially adding to the total amount of water on our now blue-green living world. A ninth planet may even have been present during this early part of Solar System history. Physical models predict a 'Planet 9', another icy world, which was sacrificed via ejection from the Solar System as Neptune and Uranus were thrown outward; a mysterious additional planet for which scientists may recently have captured the faintest of distant signals of. Together, the Grand Tack and Nice model testify to the past crimes of Jupiter and shatter earlier ideas of an orderly and relatively quiet history for our Solar System.

Image credit: ScienceNews

The destiny of all planets in our Solar System and the ability of Earth to support life has been transformed by Jupiter's riotous journey. Jupiter's dominant presence and its ability to both give and to take away in determining the course of life on Earth continues to be felt to this day. The gravitational force of the King planet shields Earth from being pummelled by objects from further out, torments and squeezes its innermost moon Io resulting in the most volcanically active body in the Solar System, and retains a vice-like grip on the asteroid belt. Yet in being the conductor of the asteroid belt, Jupiter is occasionally responsible for nudging asteroids into one another and / or sending impactors toward the Earth. The pounding from the larger of such incoming objects can be linked to the formation of metal deposits critical to modern-society or can strike with such force that conditions reminiscent of a nuclear winter are brought about stifling whatever unfortunate creatures may be around; profoundly influencing the course of life and lifestyles. Thus, Jupiter endures as both a perilous threat and protector of humanity and other worlds.

Meteor Crater, Arizona, USA. A tourist attraction. Image credit: The Barringer Crater Company.

What Are The Fresh Challenges?! Could There Be Life Elsewhere?

The puzzle of Solar System history is one that we are yet to put together in completeness. Many important questions remain challenging to resolve, yet we diligently press on with the latest study approaches and amassed sets of bright minds in our endeavours to address these. Exciting new areas of investigation to unlock the story of our planetary neighbourhood and Earth's formation include the ability to explore with ever increasing levels of scrutiny the chemical fingerprints of past Solar System events, as well as capabilities to model and measure physical constraints at greater levels of complexity and accuracy than we have known before. For example, included among the latter are advances in understanding conditions favourable to the retention of impactor materials among forming worlds during collisional processes, and the exciting fast-emerging area of study of the history of ancient magnetic signals unpicked from among extremely small minerals (collectively termed 'nanophases') of meteorites. As a planetary geochemist myself I am always especially excited about the fortuitous falls and finds of new meteorite samples to carefully examine, and the original ways that we develop to interrogate and satisfactorily explain their chemistries through data modelling. Bringing findings together from these areas, as well as with data from space missions, is often very useful to science and enjoyable to those of us trained in the questioning and solving of problems.

The specialist chemical tracers that have provided strong evidence of division among the types of materials present in the Solar System during the growth of worlds after the inception of what is inferred to be Jupiter's formation have recently be revisited. These observations are part of the chemical fingerprints extracted from meteorites that are key to grounding the models developed above. Yet many of the specialist chemical tracers used for this purpose are challenging and time consuming to measure, and so have often been limited to a relatively modest number of powders created from some carefully selected samples among the greater number of all available meteorites. A tantalising set of findings for specific components of key meteorites (see #8 below) has provided new evidence blurring the previously clear groupings indicative of division in the early Solar System, thereby threatening to unravel our theories of Solar System history and its workings. This recent result raises exciting new questions that must be carefully considered, further tested, and addressed to understand if these findings testify to a short period of time before Jupiter grew large enough to exert a gravitational force sufficiently significant to act as a barrier firmly dividing material types in the Solar System. Other authors have suggested that a pressure maximum in the gas and dust of the disk from which the Solar System formed could potentially account for the early presence of a divide; this hypothesis provides an alternative to the relatively popular Jupiter-(Saturn) migration model as a means of explaining the now questioned chemical groupings among meteorite types. Crucially, the scientific community must now be engrossed in clarifying if the new findings for specific components of meteorites call for a degree of revision to current theories of our Solar System's history in part founded on the chemical dichotomy that has been called into question. Is this finding reproducible and will we find that it applies to other diagnostic chemical markers among meteorites? If so, why? In particular we must interrogate the possibility of a leaky barrier between distinct types of materials (simplistic viewed as 'wet' and 'dry') lasting for longer than previously known and anticipated to overlap the time at which the rocky worlds of the inner Solar System rapidly grew. Is this a mix suggestive of a different type of chaos during the birth of habitable Earth? If so, how best shall we bring our various scientific capabilities together to go about decoding it?

We are only just beginning to learn about extraordinary worlds across the Milky Way galaxy and are in the infant stages of peering more clearly beyond to planets across the wider universe. We already know that some planets outside of our own solar system can be positioned in ways that do not resemble our closest neighbours. These distant exoplanets can even have extremely fast orbits, dizzying rotation speeds, and some rotate counter to the direction of spin of their star - the latter considered an indicator of impacts by massif objects during their formation, or possibly in some cases a consequence of an otherwise visiting object being captured. We have not yet been contacted by extraterrestrial life of intelligence equal to or far surpassing our own, and many distant planets are not thought to be fit for inhabiting. Yet, life is likely elsewhere and the possibility of worlds being far more hospital to life than even Earth's environments offer is a tantalising matter. More can be read on that subject here -

A Little Suggested Reading

[1] Dauphas, N., Marty, B. and Reisberg, L., 2002. Molybdenum nucleosynthetic dichotomy revealed in primitive meteorites. The Astrophysical Journal Letters, 569(2), p.L139.

[2] Scott, Edward RD, Alexander N. Krot, and Ian S. Sanders. "Isotopic dichotomy among meteorites and its bearing on the protoplanetary disk." The Astrophysical Journal 854, no. 2 (2018): 164.

[3] Kruijer, T.S., Burkhardt, C., Budde, G. and Kleine, T., 2017. Age of Jupiter inferred from the distinct genetics and formation times of meteorites. Proceedings of the National Academy of Sciences, 114(26), pp.6712-6716.

[4] Walsh, K.J., Morbidelli, A., Raymond, S.N., O'Brien, D.P. and Mandell, A.M., 2011. A low mass for Mars from Jupiter’s early gas-driven migration. Nature, 475(7355), pp.206-209. (open access preprint) and

[5] Tsiganis, K., Gomes, R., Morbidelli, A. and Levison, H.F., 2005. Origin of the orbital architecture of the giant planets of the Solar System. Nature, 435(7041), pp.459-461.

[6] Gomes, R., Levison, H.F., Tsiganis, K. and Morbidelli, A., 2005. Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets. Nature, 435(7041), pp.466-469.

[7] Levison, H.F., Morbidelli, A., Tsiganis, K., Nesvorný, D. and Gomes, R., 2011. Late orbital instabilities in the outer planets induced by interaction with a self-gravitating planetesimal disk. The Astronomical Journal, 142(5), p.152.

[8] Williams, C.D., Sanborn, M.E., Defouilloy, C., Yin, Q.Z., Kita, N.T., Ebel, D.S., Yamakawa, A. and Yamashita, K., 2020. Chondrules reveal large-scale outward transport of inner Solar System materials in the protoplanetary disk. Proceedings of the National Academy of Sciences, 117(38), pp.23426-23435.

[9] Brasser, R. and Mojzsis, S.J., 2020. The partitioning of the inner and outer Solar System by a structured protoplanetary disk. Nature Astronomy, 4(5), pp.492-499.

[10] Zhu, M.H., Artemieva, N., Morbidelli, A., Yin, Q.Z., Becker, H. and Wünnemann, K., 2019. Reconstructing the late-accretion history of the Moon. Nature, 571(7764), pp.226-229. (Open access available only via direct request from the first author.)

[11] Bryson, J.F., Weiss, B.P., Biersteker, J.B., King, A.J. and Russell, S.S., 2020. Constraints on the Distances and Timescales of Solid Migration in the Early Solar System from Meteorite Magnetism. The Astrophysical Journal, 896(2), p.103.

[12] Friendly articles debating possible detection of planet 9.

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