7.2 Origin of the Solar System Textbook pp 223-230

Observations to account for: 1. The Solar System is flat, with all the planets orbiting in the same direction.

2. There are two types of planets, with the rocky ones near the Sun and the gaseous or liquid ones farther out.

3. The composition of the outer planets is similar to the Sun’s, while that of the inner planets is like the Sun’s minus the gases that condense only at low temperatures.

4. All the bodies in the Solar System whose ages have so far been determined are less than about 4.6 billion years old.

The Solar Nebular Theory The Solar Nebular Theory was first proposed in the 18th century by German philosopher Immanuel Kant and French mathematician Pierre Simon Laplace. Their original proposal was that the Solar System originated from a rotating, flattened disk of gas and dust.

This theory offers a natural explanation for the flattened shape of the system and the common direction of the planets around the Sun.

The modern form of the theory proposes that the Solar System was born 4.6 billion years ago from an interstellar cloud, an enormous rotating aggregate of gas and dust.

Interstellar Clouds Interstellar clouds are the raw material of the Solar System and all stars and celestial bodies. Typical clouds are made of mostly hydrogen (71%) and helium (27%) with tiny traces of other chemical elements. Along with these gases float tiny dust particles called interstellar grains.

Interstellar Grains Range in size from large molecules to micrometers or larger. Believed to be a mixture of silicates, iron compounds, carbon compounds & ice water bits. A pair of SiC stardust grains from the Murchison meteorite.

Formation of the Solar Nebula Our own interstellar cloud began its transformation when gravitational attraction between the particles in the densest parts of the rotating cloud caused it to collapse inward.

This drawing shows how forming planets (planetesimals) draw interstellar gas and grains to themselves. 

A possible solar system forming around another star as seen by the Hubble Space Telescope. 

Why this is important - The position of the planets in the solar nebula greatly affected their (1) size and (2) composition. This is caused by the varying temperatures within the nebula.

Condensation in the Solar Nebula Condensation occurs when a gas cools and its molecules stick together to form liquid or solid particles. For condensation to happen, the gas must cool below a critical temperature (depending upon the substance and its surrounding pressure).

For example… If we cool vaporized iron (at 2000 K) to 1300 K, tiny flakes of iron will condense from it. If there is a mixture of vaporized iron, silicate and water, the material with the highest vaporization temperature will condense FIRST. In this case it will be the iron…

However… The condensation will STOP if the temperature never drops sufficiently low. This happened in the Solar Nebula. Because the Sun heated the inner part of the disk, the temperature from the Sun to almost the orbit of Jupiter never dropped low enough to condense water and similar substances. OTOH, iron and silicate could – and DID condense, forming the inner planets.

This drawing depicts the planets inside the solar nebula when the solar system formed.  The nebula was much warmer close to the proto-sun. The blue line shows the point at which the temperature became cold enough for gases to become ice.

At this point and further out - beginning with the forming Jupiter - the materials that forming planets (proto-planets) began to extract from the cloud were ice, as well as rocky material and gas molecules.

Retention of ice resulted in these proto-planets becoming giant, massive planets. Planets which formed closer to the proto-sun were smaller, and more rocky.

Formation of Moons Moon formation was very much like a scaled down-version of planet formation. The moons of the outer planets probably were formed from planetesimals orbiting the growing planets. Once a body grew massive enough so that its gravitational force could draw in additional material, it became ringed with debris, which eventually formed moons.

Many of these moons would be large enough to be full-fledged rocky planets if they orbited the Sun. A few even have thin atmospheres.

Final Stages of Planet Formation The last stage of planet formation was a rain of planetesimals that blasted out the huge craters such as those we see on the Moon and all other bodies in the Solar System with solid surfaces.

Occasionally, an impacting body was so large it altered the planet it struck so that it blasted its way its crust (Mercury), reverse its orbit (Venus) or tilt its rotational axis (Uranus).

Although most planetesimals were consumed by planet building, some survived to form small moons, asteroids, or comets. Planetesimals between Mars and Jupiter, stirred by Jupiter’s gravitational force, were unable to assemble into a planet. We see them today as the asteroid belt.

Formation of Atmospheres Atmospheres formed last. The inner and out planets formed their atmospheres differently – the outer planets probably captured their hydrogen-rich atmospheres from the solar nebula.

Inner planets, on the other hand, were not massive enough and were too hot to capture gas, and are therefore deficient in hydrogen and helium. They most probably created their original atmospheres from volcanic eruptions and by retaining gases from infalling comets and icy planetesimals that vaporized on impact.

Very small bodies such as Mercury and our Moon keep very little if any atmosphere at all because atmospheric gases tend to easily escape from them.

Cleaning Up the Solar System It didn’t take long for the planets to form from the solar Nebula – only a few hundred years at most. The rain of falling planetisimals may have taken longer.

The last process to occur before the Solar System became what we see today was to have the residual gas and dust removed. This was probably accomplished by the Sun, with its intense heat driving a flow of gas outward from its atmosphere. We could think of it as a “cosmic broom.”