The Birth of the Solar System

Phil Plait, Bad Astronomy Blog

Discover Magazine

 

Much like all stars, the Sun was formed by the collapse of a dust and gas cloud called a nebula. Some perturbation causes the nebula to start rotating, and in order for the nebula to keep from dissipating, the cloud must start to collapse and shrink in size. This is a law of physics called conservation of angular momentum.

A good example of the conservation of angular momentum is to think of a figure skater. When the skaters spin on their skates with their arms outstretched, the skaters spin slowly. As they bring their arms in closer to their bodies, the rate of spin increases. To keep the same angular momentum, the angular speed must increase as the outmost portion of the rotation decreases.

Angular Momentum

The University of Reading

L = R*ω
where

• L is the angular mometum, which is the same for the left and right pictures above
• R is the distance from the center of spin to the outermost portion of the body spinning (this is shown as I above with is actually the moment of Inertia around the center of spin)
• ω is the angular speed, i.e. how fast the angle is changing over time.

When a nebula does this, not only does the cloud shrink in radius, but the center becomes more dense. As the density increases, so does the temperature. When the temperature reaches about 15 million Kelvin, it is hot enough that the protons in the center can overcome their natural repulsion due to electromagentic forces to fuse together. This is the beginning of the proton-proton chain which was discussed way in the beginning. As the temperature begins to rise and fusion takes place, the radiation pressure from the core of the protostar stops the collapse of the cloud, putting the star in hydrostatic equilibrium. The radiation pressure is enough to push back against the gravitational force trying to squash the star.

Astronomy Online

The rest of the cloud is now free to consolidate into planetessimals, freeing up the area around their orbits. The inner part of the solar nebula contained mostly refractory elements, leading up to more dense planetessimals and the outer part contained a lot more volatile elements, leading to planetessimals that were both rocky and icy. As the planetessimals starting colliding with each other, they stuck together, melting, and allowing the heaviest elements to sink to the center of the bodies. As more and more planetessimals collided, the bodies grew bigger and bigger, getting to a point where the bodies were able to form spherical shapes, and leading to the planets we see today.

Oort Cloud

Graphical Representation of Oort Cloud (obviously, not to scale)

The Oort Cloud is a spherical region of the Solar System that can effectively thought of as the boundary between our Solar System and interstellar space. Unlike the Kuiper Belt, the Oort Cloud does not lie mainly in the plane of the Solar System, but it does, however, contain objects similar to the Kuiper Belt. The Oort Cloud objects are mainly water ice and ammonia ice bodies.

The Kuiper Belt is home to short-period comets, which generally are comets with elliptical orbits and periods of less than 200 years. Oort Cloud comets have much longer periods (if they are periodic) of more than 200 years, and may have elliptical, parabolic, or hyperbolic orbits. The difference between elliptical orbits and parabolic and hyperbolic orbits, is that elliptical orbits are closed (comets with these orbits return periodically), while parabolic and hyperbolic orbits are open (comets with these orbits come towards the Sun once, never to return). When the eccentricity of an orbit equals 1, that is what defines a parabolic orbit, while eccentricities greater than one have hyperbolic orbits.

Different Orbits

There is a theory that the objects in the Oort Cloud formed closer to the Sun, but were bumped into the region of the Oort Cloud by gravitational interactions with the outer planets. It is about 100,000 AU in diameter with two main regions: the spherical outer Oort Cloud and a disk-like inner Oort Cloud (also called Hills Cloud). The cloud is named after Dutch astronomer Jan Oort and is sometimes referred to the Opik-Oort Cloud in conjection with Estonian astronomer Ernst Opik who postulated in 1932 that the long-period comets came from a cloud on the outer edges of the Solar System. Jan Oort revived this theory to help resolve a paradox on the stability of comets. Comets could not have formed in their orbits because the Sun's radiation would cause them to boil away.