Astronomy 311: Lecture 2 - Solar System Formation• Solar Nebula Theory– Cold dense molecular gas cloud starts to contract - it satsified theJeans mass criterion. Spins, heats up and flatt ens out. Proto-Sun atthe center.– Angular momentum transferred from the protoSun to outside throughviscous drag.– Initially temperature rises in the SN, but eventually it starts to cool.Material starts to condense from gas to solid in dust/grains followingwhether its a refractory or volatile material.– Less than 5AU, ices never condense.– Greater than 5AU ices do condense.– These dust grains condense to central plane of the nebula.– Grow by ”gentle collisions”: sticking together by magnetism/electrostaticcharges etc. This is also called coagulation.– Normal grains in the interstellar medium are maybe between 100 ×10−10m to 0.2 × 10−6m and consist of carbons and silicates mainly.– In the SN, coagulation results in these grains becoming 0.1 − 10km insize in a few hundred thousand years.– This ” grain growth” is a very hot to pic today. Chondrules are carbonrich materials embedded in silicate rich surroundings seen in meteoritesand represent the possible types of grains available for coagulation.– A planetismal is something about 10km in diameter.– Only 10km lar ge ”rocks” are generally large enough to attract othergrains/rocks to it through gravitational attraction.– That is after a rock reaches 10km in size it gr ows further throughencouraging collisions through gravitational attraction.– This leads to one large body or planetary embryo forming by swal-lowing up all the other smaller bodies: runaway growth is where oneplanetisimal grew more than o thers. Larger bodies grew more rapidlythan smaller ones while the total mass in smaller bodies declined.– At 1AU, such planetary embryos are about 1/10 mass of the currentEarth and takes about 104− 105years to come about.– In the outer part of the SS, planetary embryos could be, perhaps anEarth size mass and take about 105years to f orm.– The terrestrial planets formed by collisions b etween planetary embryosin about 108years.1– This took longer in the outer SS, maybe 109− 1010years.– In the outer SS, planetary embryos could attract large volumes ofgaseous H/He - atmosphere.– Jupiter formed at the same time as the terrestrial planets becauseotherwise there would have been a planet between Mars and Jupiter.Jupiter’s large gravitational influence prevented the rocks in t he as-teroid belt from f orming a planet.– The Solar Wind/T Tauru emissions swept of excess gas into spaceoutside the SS. So Saturn/Neptune/Jupiter did not capture as muchgaseous H/He as Jupiter. This wind also removed primitive atmo-spheres from the terrestrial planets including the Earth.– Oort clouds probably formed by the gravitational influence of Jupiter.– Major satellites of Jupiter/Saturn/Uranus/Neptune probably formedvia ”mini-solar nebulas”.– The Jovian planets are larger because further away from the Sun, t he”larger volume” makes it easier for planetary embryos to gr ow bigger.– However, some astronomers think that the large gas giants formedcloser to the Sun and ”migrated outward”: planetary migration be-cause of the discovery of the ”hot Jupiters”: large planets at only0.5AU from their star.– Perhaps the KBO’s originated like this.– Planet formation took few tens of millions o f years. Age is roughly 4.6billion years. Well supported by evidence.– Observational evidence?∗ The spectrum of an object is the variation of light intensity fromthat object a s a function of wavelength.∗ Frequency, wavelength of light.∗ Frequency x wavelength = speed of light.∗ Atoms/molecules have an energy level structure and jump froma lower/higher energy level to a higher/lower energy level by ab-sorbing/emitting a photon of the appropriate energy leading to anabsorption/emission spectrum.∗ Absorption/emission spectrum is dark/bright lines on a bright/dakrbackground.∗ Pattern o f absorption/emission lines unique to atom/compound.∗ Absorption spectrum occurs when a hot source shines through acold cloud: absorption lines reveal information about cold cloud.∗ Emission spectrum when a hot souce emits light: emission linesreveal information about hot source.2∗ In both cases information can be density, temperature etc.∗ In studying protoplanetary disks, light from cent ral star or pro-tostar is processed by the disk and an analysis of the resultinginformation can reveal information about things like the composi-tion/density/temperature and gra in size of the disk