Summary of the lecture on 07-17-96

Formation of the Solar System

Galaxies not only consist of stars but also of clouds of interstellar matter (gas and dust), which When an interstellar cloud reaches a certain critical density, it collapses because the gravitational forces between the atoms, molecules, and dust grains overcome the internal thermal pressure of the cloud. If the cloud was spinning slowly before the collapse, it is going to speed up because the angular momentum is conserved . Material along the spin axis can reach the center more easily than material in the plane of the rotation. Therefore, a disk is formed. At the center of the disk, there is a concentration of mass which will eventually form the star - in the case of our solar system the sun. The material in the disk will form the planets. Because there is a temperature gradient in radial direction with the highest temperature close to the center of the disk and decreasing towards the edge, only silicates and other rocky materials will be in their solid state close to the center of the disk. Further out, water ice, amonic ice, etc. will also be solid. Dust grains will start sticking together forming larger and larger rocks. The process will be accelerated when the planets reach a size of about 100 km by "gravitational focusing". The outer planets are large enough and the temperature at the distance from the Sun is cool enough so that they could accrete gas from the cloud out of which the solar system was formed. Formation of the planets occured very quickly (~100 million years) which can be deduced from certain radioacticve isotopes found in meteorites.

Comets consist mainly of ices. The are thought to reside in a spherical cloud far beyond the orbit of Pluto called the Oort cloud. They are pulled into the inner solar system by gravitational resonances between the outer planets.

Last October, the first planet orbiting a solar type star was discovered around 51 Pegasus. Since then, half a dozen other planet detections have been made. These detections where all made by measuring the Doppler "wobble" of the star caused by the planet orbiting it. This method is the most sensitive to large planets with small orbital radii. The planets detected are all as massive as or more massive than Jupiter and are very close to their stars. This could, however, just be a selection effect.

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