Solar System

Formation

Main Features

• The Sun, 9 planets, 60–70 satellites,asteroids, comets

• Held together by Sun’s gravity

• Disk-shaped: most planets and satellitesorbit the Sun in a plane

• All orbits are counter-clockwise (viewed from

above Earth’s North Pole)

• Most rotations are also counter-clockwise(Notable exceptions: Venus and Uranus)

Main FeaturesContinued

• Four innermost planets are rocky (theTerrestrial planets): iron and silicates aretheir dominant constituents

• Next four planets are made mostly of gases(the Jovian planets): hydrogen, helium andgaseous compounds (methane, ammonia)

• Most Solar System objects are 4.5 billionyears old• From radio-isotope dating of oldest rocks

Formation

• Formed from a large interstellar gas cloud

• Cloud contained hydrogen, helium in roughlyprimordial proportions (71%; 27%)

• Also contained vapors of heavier elements(carbon, oxygen, nitrogen, silicon, iron, …)and their compounds

• Primordial gases, enriched by previousgenerations of stars that lived and died duringthe preceding ~10 billion years.

FormationContinued

• Cloud begins to collapse towards its densestpart, pulling in other parts by gravity• Collapse usually initiated by birth (or death!) of

some neighboring star

• Slow rotation of the cloud, accentuated by thecollapse

• Cloud flattens into disk shape

• Such disks have been observed in regionswhere star formation is now taking place

FormationContinued

• Condensation: sticking together of gasmolecules to form small droplets or solid flakes• Iron: 1300 K

• Silicates: 1100 K

• Water: 300 K

• Iron and silicates accreted everywhere, but water andother volatile compounds mostly far from the proto-Sun

• Accretion: larger aggregates grew by the stickingtogether of smaller ones. Collisions broke apart aswell as “assembled” large chunks

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FormationContinued

• The larger chunks grew by attracting more materialthrough gravitation to form planetesimals

• Planets formed by mergers of planetesimals• The larger planetesimals became centers of growth

because of their stronger gravity

• As they grew, their gravity got stronger, thus attractingmore matter, and so on …

• The ones forming in the cold, outer regions could growvery big by attracting and holding the abundant gaseousmaterials

FormationContinued

• Planets heated and differentiated• Heat released by in-falling material

• Heat released by radioactivity in rocks

• Rings of material assembled around new planets toultimately form most “normal-sized” satellites

• Cratering caused by falling debris

• Atmospheres around terrestrial planets formed byvolcanic outgassing

• Cleaning up

FormationCleaning up

• Gas and dust driven away by vigorouswind from the newly-formed Sun

• Other debris:• Impacted planets and satellites, merging into

them• Deflected by planets into orbits that took it far

away from the Sun• Jupiter was especially effective in doing this

because of its great mass