Monday, Nov. 17Syllabus, class notes, and homeworks are at:www.as.utexas.edu courses AST 301, LacyReading for this week: chapter 16We’ll go back to the old help session time and place thisweek: Wednesday at 5:00 in GRG 424Topics for this weekWhat evidence do we have that planets exist orbiting aroundother stars?Describe and compare briefly the compositions and orbits ofthe planets, asteroids, and comets.Describe the nebular theory of the formation of the solarsystem.How does the nebular theory explain the differences incomposition among the planets?Terrestrial PlanetsThe inner four planetsSizes similar to the Earth’sInteriors made of metals and rocksVery thin atmospheres (compared to the diameters of theplanets)Earth also has oceans (and Mars may once have). Thesealso make up a small fraction of the volume.Jovian planetsThe next four planetsInteriors made of ices and gasses (probably with smallrocky cores)Jupiter and Saturn are mostly gas.Uranus and Neptune are mostly (partially melted) ices.WarningRecently found planets around other stars often do notfollow the division into terrestrial and Jovian planets.In some cases giant planets are found very near their stars.In explaining the solar system we will assume that theplanets formed near their present positions.But this may not be correct.Nebular TheoryThe solar system formed from a part of a molecular cloud.Molecular Clouds:70% of mass is hydrogen28% of mass is helium1% of mass is solids (tiny rocks, soot, and metals)1% of mass is molecular (other than hydrogen)A region of the cloud was pulled together by gravity.It must have been rotating slowly.When it came together its rotation increased.The rotating gas cloud flattened.Planets formed in the disk left around the newly formedSun.The protoplanetary diskThe disk around the Sun was probably hot, and hottertoward its center.Ices and probably even rocks evaporated in the innerregion.As the disk cooled different materials condensed indifferent regions.Metals and compounds containing metals condensedclosest to the Sun.Between Venus and Mars rocks could also condense.Near Jupiter and Saturn ices could also freeze onto themetals and rocks.Seeing the gasBoth the silhouettes and the infrared emission picturesshow us where the dust is in protoplanetary disks.But dust is only 1% of the mass in disks.Can we see the gas?One way I’ve tried is by observing emission lines ofmolecular hydrogen (H2).Most of the gas is H2, but it doesn’t emit very strongly.We haven’t succeeded in taking pictures of the H2emission, but we can infer the distribution of the gasfrom the widths of the emission lines.Fast orbiting gas near a star emits broad lines because ofthe Doppler effect.Terrestrial planetsFrom Mars inward, only metals and rocks could condense.These materials first formed tiny grains.The grains stuck together to make bigger rocks.Once they got up about 1 km in size gravity could helpbring them together.When they were over about 100 km they could holdenough heat from radioactive decay to melt inside.The terrestrial planets formed from these planetesimals.The sizes of the terrestrial planets were limited by theamount of condensable materials in the inner solarsystem.Jovian planetsThe Jovian planets probably started to form like theterrestrial planets, by solids sticking together.But water, ammonia, and methane were also solids (ices)where they formed.So the planets grew faster and became more massive.Once they had more than a few times the mass of theEarth they had enough gravity to pull gasses in.Since there was much more hydrogen and helium thanother elements in the nebula, the Jovian planets couldbecome much more massive than the terrestrial planets.LeftoversThe asteroids between Mars and Jupiter may be leftoverrocky planetesimals (or they may be broken up smallplanets).The nuclei of comets are probably leftover icyplanetesimals.A puzzleWe have evidence for planets even more massive thanJupiter in orbit around other stars, orbiting closer to theirstars than Mercury is to the Sun.Ices could not have formed that close to a star, so theplanetesimals should not have been massive enough topull in hydrogen and helium.How can such massive planets be so close to
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