Astrochemistry Episode 3 (Solar System Formation)

Anakin: *exasperated* And here you are, destroying another one of my good days...

Obi-Wan: God, Anakin you really need to get your priorities straight. The last time we talked you actually seemed capable of using that little brain of yours. A fleeting glimpse of that would be nice!

Anakin: *sarcasm evident* Well, to be fair, I was actually interested in what you had to say for a change…

Obi-Wan: Perfect! Today I’ll be trying to drill in new information about a few theories explaining the formation of an ancient star system, the Solar System, which can be extrapolated to other similar systems.

Anakin: Shoot

Obi-Wan: So to begin, one of the more recent theories regarding the same has been The Capture Theory which was proposed by Michael Mark Woolfson in 1964. (338 bby)

Anakin: And what exactly does this "Capture Theory" say anyway?

Obi-Wan: It theorizes that the Solar System was formed from interactions between the Sun and a low-density protostar. The Sun's gravity would have drawn material from the diffuse atmosphere of the protostar, which would then have collapsed to form the planets. The capture theory predicts a different age for the Sun than for the planets. As captured planets would have initially eccentric orbits, this gave rise to the possibility of a collision.

Anakin: How exactly does this theory explain the formation of planets, if I may ask?

Obi-Wan: Well, for starters, they theorized that a filament was thrown out by a passing proto-star which was captured by the Sun, resulting in the formation of planets. According to their theory, there were six planets each corresponding to a point mass in the filament. The two nearest planets- Enyo(about double the mass of Neptune) and Bellona(about one-third the mass of Uranus) were believed to have collided with each other. Enyo was ejected out of the Solar system while Bellona split to form Earth and Venus. Later, it was found that both Enyo and Bellona were gas giants more massive than Jupiter. Their collision caused deuterium-deuterium chain reactions that shattered both of these. Materials left over from Enyo formed Venus while that from Bellona formed Earth. This theory also states that Mars and the Moon were previously moons of Enyo and Mercury is either a fragment of Bellona or another moon of Enyo.

Anakin: That seems pretty impressive. And also, how about the other celestial objects? What does the theory have to say about them?

Obi-Wan: The Enyo-Bellona collision also formed the asteroid belt, Kuiper belt, Oort cloud, and comets. Pluto, either a fragment or moon of one of the planets, passed close to Neptune's satellite Triton, causing it to assume its retrograde orbit. All of this elaborate information doesn't mean that this theory is without faults though.

Anakin: Faults? This seemed like a pretty tight argument to me. What were the so-called faults then?

Obi-Wan: Surely you must know- nothing, and I mean nothing, is devoid of faults, Anakin. No attempt was made to consider the effect of external gravitational fields on the evolution of the captured planetary filament. On top of this, the ejected material would have needed to condense considerably soon after it was ejected from the star if it were to survive the large gravitational field gradient of the Sun at perihelion. The theory failed after a hypothetical experiment was set up where the planet was effectively reduced to a two-dimensional object after a short time due to the 'pinch type force' exerted on it by the sun and the other star.

Anakin: That is a lot of shortcomings for a standard theory. Is there any other alternative to this theory?

Obi-Wan: Well yes, although I cannot account for its legitimacy. The Modern Laplacian Theory, an alternative to Capture Theory, states that a concentric system of orbiting gaseous rings condensed to form each of the planets and regular satellites present in the solar system. These rings were believed to have been shed by an envelope of gases a long time back, which later gravitationally contracted to form the central body.

Anakin: How exactly does this theory account for planet formation?

Obi-Wan: So, the first stage in the formation of each planet involved the condensate grains settling into the mean orbit of the gaseous rings. Then, this material was gathered by gravitation along the mean orbit to form a dense core. During the formation of these rock and ice cores of the major planets, most of the mass of the gas ring was lost through thermal evaporation. Once the cores were formed, these cores acted as gravitational sinks for the gases remaining in the parent gas rings.

Anakin: Do tell me more about the gas rings and the satellites, will you?

Obi-Wan: If we study the chemical composition of the regular satellites, they can tell us a lot about the thermodynamic state of the gas rings from which they are assumed to have condensed. Thus, we might be able to see a gradient in the materials making up these satellites. That is, the satellites would become denser and rockier as they move closer to the planets.

Anakin: I assume this theory has drawbacks of its own too, seeing as "nothing, I mean, nothing is devoid of faults"?

Obi-Wan: Of course it does. The Modern Laplacian Theory is based on the idea that when a rotating protostar (gaseous cloud that underwent gravitational collapse to form the sun) contracts, it starts showing convection currents (in the sun, itself). Stellar convection currents are involved in the case of stars. Most stars have convective outer envelopes(proven by observation) but the Laplacian theory suggests that the radiation is only towards the inside because the outer region is quiescent or inactive. Convection currents in the sun are actually found in the outermost layer of the sun- contradictory to the theory.

Anakin: All of this was based on the physics of supersonic convection currents? I assume there's more?

Obi-Wan: There's definitely a lot more to it than that, young Padawan. Rayleigh-Taylor Instability is one such not so little issue. It is basically an instability of an interface that occurs when a lighter fluid pushes a heavier fluid. Take for example the instability in plasma fusion reactors and behaviour of water suspended above oil in the gravity of Earth. According to the theory, convection currents should support a thin, dense layer. This is incompatible with RT. Moreover, to test the results, the focus was shifted by researchers to the outer rings, which evolved from the thin, dense layer. Analysis of a thin gas ring showed that it would be unstable. Hence, the full three-dimensional motion of the ring was simulated. The results, however, showed that the ring keeps on changing its angular velocity distribution and spreads. So a ring, if at all formed, will actually be a disc (in contradiction to the MLT).

Anakin: That is actually quite disheartening, to be honest. So despite all these efforts, the complete understanding of the topic still remains elusive?

Obi-Wan: Of course, it is only natural that we fail to understand it completely. After all, perfection is only an asymptote that we can strive to achieve. So a perfect theory regarding the same is still distant but there are alternatives to choose to believe in.

Anakin: This whole discussion, although a bit confusing, was rather informative, I'd say. I'd definitely like it if you told me more.

Obi-Wan: Alas! I'm afraid this is the end, Anakin. From now on, you must seek knowledge on your own devices. There’s a whole new world out there, a world bustling with new hopes and promises. Dive deep into it.

Reference: Wikipedia