Solar System Formation Unit 5.6

Solar System Formation During the five decades, interplanetary probes have vastly increased our knowledge of the solar system.

Solar System Formation However, we can understand the overall organization of our planetary system without dwelling on these details. Indeed, some key elements of the modern theory of planetary formation predate the Space Age by many years.

Solar System Formation Presented here is the “standard” view of how the solar system came into being. This picture will underlie much of our upcoming discussion of the planets, their moons, and the contents of the vast spaces between them.

Nebular Contraction One of the earliest heliocentric models of solar system formation may be traced back to the 17th century French philosopher Rene Descartes.

Nebular Contraction Imagine a large cloud of interstellar dust and gas called a nebula, a light- year or so across.

Nebular Contraction Now suppose that, due to some external influence, such as a collision with another interstellar cloud or the explosion of a nearby star, the nebula starts to contract under the influence of its own gravity.

Nebular Contraction As it contracts, it becomes denser and hotter, eventually forming a star – the Sun – at its center.

Nebular Contraction Descartes suggested that, while the Sun was forming in the cloud’s hot core, the planets and their moons formed in the cloud’s cooler outer regions.

Nebular Contraction In other words, planets are the by-products of the process of star formation.

Nebular Contraction In 1796, the French mathematician- astronomer Pierre Simon de Laplace developed Descarte’s ideas in a more quantitative way.

Nebular Contraction He proved that due to the conservation of angular momentum, the nebula spins faster as it contracts. This increase in the rotational speed would decrease the size of the rotating mass, resulting in the nebula’s shape to change as it collapses.

Nebular Contraction Centrifugal forces tend to oppose the contraction in directions perpendicular to the rotation axis. The end result is that the nebula collapses most rapidly along that axis.

Nebular Contraction The mass eventually flattens into a pancake-shaped primitive solar system, known as the solar nebula.

Nebular Contraction If we now simply suppose that the planets formed out of this spinning material, then we can already understand the origin of much of the large-scale architecture observed in our planetary system today.

Nebular Contraction The first thing to note is that the planets’ orbits are circular and they move in the same sense in nearly the same plane.

Nebular Contraction The planets inherited all these properties from the rotating disk in which they were born. The idea that the planets formed from such a disk is called the nebular theory.

Nebular Contraction Astronomers are fairly confident that the solar nebula formed a disk, because similar disks have been observed around other stars. Scientific theories must continually be tested and refined as new data becomes available.

Solar System Formation While Laplace’s description of the collapse and flattening of the solar nebula was basically correct, we now know that a disk of warm gas would not form clumps of matter that would subsequently evolve into planets. These clumps would disperse, not contract

Solar System Formation The model currently favored is the condensation theory, which rests squarely on the old nebular theory, combining its basic physical reasoning with new information about interstellar chemistry to avoid most of the original theory’s problems.

The Condensation Theory The key new ingredient is the presence of interstellar dust in the solar nebula. Astronomers now recognize that the space between the stars is strewn with microscopic dusty grains, an accumulation of the ejected matter of many long-dead stars.

The Condensation Theory These dust particles probably formed in the cool atmospheres of old stars and then grew by accumulating more matter from the interstellar gas within the Milky Way galaxy. The result is that our entire galaxy is littered with mini chunks of ice and rocks having typical sizes in millimeters.

The Condensation Theory Dust grains play two important roles in the evolution of a gas cloud. First, dust helps to cool warm matter by efficiently radiating its heat away in the form of infrared radiation.

The Condensation Theory As the cloud cools, its molecules move around more slowly, reducing the internal pressure and allowing the nebula to collapse more easily under the influence of gravity.

The Condensation Theory Second, acting as condensation nuclei – microscopic platforms to which other atoms can attach, forming larger and larger balls of matter – the dust grains greatly speed up the process for collecting enough atoms to form a planet.

The Condensation Theory This mechanism is similar to the way raindrops form in Earth’s atmosphere: Dust and soot in the air act as condensation nuclei around which water molecules cluster.

The Condensation Theory Thus, according to the modern condensation theory, once the solar nebula had formed and begun to cool, dust grains formed condensation nuclei around which matter begin to accumulate. This vital step greatly hastened the critical process of forming the first small clumps of matter.

The Condensation Theory Once these formed, they grew rapidly by sticking to other clumps. As the clumps grew larger, their surface areas increased and consequently, the rate at which they swept up new material accelerated.

The Condensation Theory Gradually, the accreted matter grew into objects of pebble size, baseball size, basketball size, and larger. Eventually, this process of accretion – the gradual growth of small object by collision and sticking – created objects a few hundred kilometers across.

The Condensation Theory By that time, their gravity was strong enough to sweep up material that would otherwise not have collided with them, and their rate of growth accelerated, allowing them to form still larger objects.

The Condensation Theory Because larger bodies have stronger gravity, eventually almost all the original material was swept up into a few large protoplanets – the accumulations of matter that would in time evolve into the planets we know today.

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