Emergence, Part Two

2 08 2010

And so, Alpha A and Alpha B grew from T Tauri stars into Main Sequence stars. Their stellar winds weakened. The gas and dust of the protoplanetary disc dispersed as these gentler winds scattered the remnants into interstellar space.

As the dusty veil was brushed aside, a crowded solar system was revealed. Planetesimals without number looped through the system. There were asteroids, small and dark, and planetary embryos, each the size of a small moon. The total matter of more than a dozen Fram masses existed in this period of planetary formation, swimming in eccentric orbits.

Some bodies were drawn together by their mutual gravities, and accreted into larger objects. But many others were accelerated by the revolving, twin stars, or by the weak influence of Proxima, or by interaction with the orbital resonances of larger embryos. When these hastened planetesimals collided, they shattered spectacularly. More planetesimals, laboriously accreted over millions of years, were torn apart by the competing gravities of Alpha Centauri A and Alpha Centauri B.

For tens of millions of years these bodies glided through the system, were agitated by their parent stars, were drawn together or smashed apart. It was chaotic, violent, an interplanetary melee – a dance of mathematics and mass and resonance. Humans would later lend this meaning and call it the musica universalis.

Fram and Belgica grew from this churned belt of debris. They were probably the largest of the planetesimals of the Alpha Centauri system, although they were neither unrivalled nor alone. During the latter part of the first 100 million years of planetary formation, while its orbit was still highly eccentric, Belgica smashed into one of its largest neighbours. The impact was cataclysmic, perhaps the most violent of the system: its outer envelope of mantle and crust was blasted from the core, and the young planet lost much of its mass.

Yet it was this collision which probably bound Belgica to Alpha A. Instead of forming into a moon the way that Amundsen had formed around Fram, the material ejected by the collision crowded about the core of Belgica and collected in the wake of its orbit. This area of dense, ejected material pulled at Belgica with a weak but growing gravity. Over many more millions of years, Belgica’s eccentric orbit was slowed, and it fell into a more circular orbit of Alpha A. After billions of years, this material accreted into the misshapen lump that we now called Maud – Belgica’s smaller twin, trailing sixty degrees behind Belgia’s orbit in the Alpha A-Belgica L5 point.

It was not likely that Fram and Belgica formed around the same star. There was probably insufficient protoplanetary material around each to account for the mass of both planets; moreover, their competing gravities would have profoundly affected the development of both. More likely, Fram formed around Alpha B, and for the first 100 million years it was pounded by impacts as its greater gravity drew in the smaller planetesimals around it.

But not every planetesimal interacted with Fram in such a way as to be drawn toward it. Fram scattered many smaller bodies inward of its orbit, and exchanged angular momentum with these scattered bodies such that its orbit, little by little, was cumulatively drawn outward from Alpha B. Fram’s already eccentric orbit grew into a lengthening parabola that became more pronounced as it scattered more and more objects. After hundreds of millions of years, Fram’s aphelion drew farther and farther from Alpha B, until it began to be affected by the gravity of Alpha A; the aphelion of Fram’s orbit was always drawn toward Alpha A as it too rotated about the system barycentre.

For the next four and a half billion years, Fram’s wobbling orbit scattered other objects orbiting Alpha B. Its gravity distributed thousands of asteroids, comets and planetesimals outward into a sparse scattered disc. It trapped nearby asteroids in its Lagrange points. And Fram captured Sverdrup and Nansen, bound these small asteroids to its gravity well, and made them its moons.

The captured moons became gravitationally tied to Amundsen, and were forced into mean motion resonances. It is not likely that every object captured by Fram’s gravity fell into these resonances; Fram probably once had many more moons, in disordered, elliptical, highly eccentric orbits, which were lost to the gravities of Alpha A and B when they did not fall into resonance with Amundsen.

From this multiplicity of simple interactions, an elegant yet complex pattern emerged.

Through interaction with and ejection by Fram’s gravity well, perhaps ninety percent of the mass remaining after the earliest period of planetary development was pitched outward from the inner system and formed into the scattered disc. Fram’s eccentric orbit oscillated and was smoothed through these interactions, as the system sought to conserve angular momentum. In the absence of gas giants like Jupiter, Fram acted to clear much of the Alpha Centauri system of the remnants of the protoplanetary disc.

Yet Fram was never alone. There was no peak period of bombardment as in the Sol system – impacts were ongoing, a geologically regular occurrence. Fram was weathered by constant bombardment, craters overlaid with craters, mountain systems formed not through tectonic movement but the terrific force of impactors. Carbonaceous chondrites, lost in trans-solar orbits, and cold scattered disc and Oort objects, disrupted from their lonely exile by the passage of Proxima: the impact of these bodies over billions of years gave Fram its thick atmosphere.

Most recent of the large impact events was the collision that created Fram’s ring system. The impact had fractured Amundsen’s crust and pushed the moon beyond its Roche limit. The moon had been disintegrating for millions of years, and would continue to disintegrate for millions more. Tidal stresses continued to break apart the moon and spread its debris into a thickening ring.

The Alpha Centauri system had been shaped for five billion years. Volatile bodies had been distributed into a distant, scattered disc. Silicate asteroids had fallen into Fram’s Lagrange points, or into orbital resonances that kept them far from the planet. The system had fallen into equilibrium, and was as stable as it would ever be; now it was the turn of its recent inhabitants to shape this star system…