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THE SOLAR SYSTEM: ORIGIN AND INFANCY

Written by Graham | Created: Tuesday 9th July 2019 @ 1818hrs | Revised: Thursday 1st October 2020 @ 1141hrs

SOMEWHERE IN A GALAXY...

In the beginning, there was a nebula, a gigantic molecular cloud some 20 parsecs or 65 light years in diameter. A long, long time ago - 4.6 billion years to be, if not exact, somewhere in the ballpark (if you'll excuse the management speak) - a part of this cloud, a fragment of about a parsec or so, underwent a process of gravitational collapse, eventually forming a series of cores some 2,000 to 20,000 astronomical units across. Among them was the pre-solar nebula, which eventually yielded a dense centre, which, in time, ignited to form our sun, while the remainder of the material began to circle around the protostar in the form of the protoplanetary disk. Another star which potentially emerged from the same medium at this time is HD 186302, which lies about 184 light years away in the constellation Pavo.

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PLANETARY FORMATION

Close to the sun, a series of planetesimals formed, composed of metals such as iron and aluminium, as well as silicates, growing to about 200 times smaller than the earth (0.05 M_⊕) by about 4.59 billion years ago.

Further out, beyond the "frost line," beyond which volatile icy compounds are able to maintain a solid state, other planetesimals were forming, reaching sizes of up to four times that of the earth within some 3,000,000 years. From these would develop the greatest planets of our solar system, the gas giants Jupiter and Saturn and, further out, the ice giants Uranus and Neptune.

The rapid increase in mass and size of the two gas giants had repercussions for the inner Solar System during this time: evidence from Ceres, a dwarf planet in the asteroid belt, suggests that, as Jupiter grew - and possible moved slightly inward towards the sun - it caused significant preturbations in the orbits of objects both inside and outside its orbit, leading to a Jovian Early Bombardment. Possibly added to this were the effects of Saturn's undergoing a similar growth, leading to the two planets, coming into orbital resonance, contributing to a Primordial Heavy Bombardment (about 4.568 billion years ago or shortly afterwards). Jupiter's growth also resulted in the dynamic excitation and clearing of the Asteroid Belt, leaving less than 1% of the original planetary embryos intact at around 3 AU. By this stage, the last remnants of nebular gas were dispersed.

AND THE EARTH WAS WITHOUT FORM AND VOID

The inner Solar System was a congested place just over 4.5 billion years ago, with between 50 and 100 protoplanets (large planetary embryos), ranging in size from about that of the moon to that of Mars. These were often given to violent interactions and collisions: one such catastrophe likely ripped the outer envelope of Mercury, leaving the chthonian planet-like world which exists today.

HOW TO SOLVE A PROBLEM LIKE THEIA

Another such coming together seems to have happened to the earth during its formative period: a Mars-sized object, given the name Theia seems to have careered into the earth, causing in turn the expulsion of masses of material which would eventually form the moon.

The earth itself is most widely held to have formed by about 4.54 billion years ago, by which stage it had more or less cleared its orbit of other planetary embryos (with the exception of the Earth trojan Theia mentioned above). The moon was formed by 4.533 billion years ago. In terms of geology sensu stricto (i.e. covering the earth only), the Hadean Eon is held to begin around 4.54 billion years ago, with the Pre-Nectarian period on the selenological timescale beginning at 4.533 billion years ago and the areological Pre-Noachian beginning with the formation of Mars in about 4.5 billion years ago.

LATE HEAVY BOMBARDMENT

One of the effects of Jupiter and Saturn's earlier resonances was preturbation in the orbits of Uranus and - especially - Neptune. The Nice model suggests that Neptune's original orbit was between those of Saturn and Uranus, and that its expulsion to the (relatively) outer reaches of the Solar System led to the period which brought the history of the Primordial Solar System to a close: the Late Heavy Bombardment. This period lasted from about 4.1 to 3.8 billion years ago, and led to the pockmarking of the moon in evidence today, as a result of Neptune's movements causing planetesimals in the outer Solar System to plummet towards the sun. Additionally, the LHB potentially caused the formation of the earth's oceans, by dint of the fact that many of these objects would have been icy and similar to the "dirty snowballs" which were believed to form the nuclei of comets, which contain water ice in addition to dust, rock, methane, ammonia and organic (carbon-based) compounds - thus very possibly providing the decisive contribution to the formation of life on this planet.

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YOU ARE HERE

While the possibility of life having once existed on Mars or even Venus, not to mention the possibility of it still clinging on on a remote moon of Jupiter or Saturn, this is strictly beyond the purview of this series. From now on, we will concentrate on developments taking place on our beautiful planet earth.