1. The Origin of the Solar System

2. In the beginning, we started out looking like this, just a huge cloud of gas in space….

3.  A rotating cloud of gas contracts and flattens….  to form a thin disk of gas and dust around the forming sun at the center.  Planets grow from gas and dust in the disk and are left behind when the disk clears. Solar Nebula Theory

4. Dust Disks Around Stars  Very cold, low density disks observed (in the infrared) around stars. Debris left over from comets or collisions between small bodies (like asteroids). Evidence of planetary systems which have already formed.

5. Dust Disks Around Stars  Very cold, low density disks observed (infrared) around stars. Debris left over from comets or collisions between small bodies (like asteroids). Evidence of planetary systems which have already formed.  Dense disks of gas and dust observed (visible & radio) orbiting young stars. Stellar systems are too young for planets to have formed yet. Probable sites of ongoing planetary formation.

6. Examples of the Dust Disks around stars

7. Planet Building: the Condensation of Solids  Different materials condense from the gas cloud onto grains of elements (atoms of different gasses) at different temperatures.  The temperature due to the Sun varied with distance, so different materials condensed at different distances from the Sun.  Close to the Sun (1200-1500K): metal oxides and pure metals.  Farther out (~700-1200K): silicates and rocky material.  Outer regions (~50-200K): ices (water, methane & ammonia). First Important step in Planet formation

8. Planet Building: the Formation of Planetesimals  Planetesimals – small bodies on the order of kilometers in size.  Condensation – atoms of gas hit dust grains and stick, adding mass to the particle.  Accretion – solid particles collide and stick to one another.  Once particles were massive enough, the settled down into a disk rotating around the protosun (its not quite a star yet). Second Important step in Planet formation

9. Accretion Taking Place

10. Planet Building: the Growth of Protoplanets  As planetesimals grew, they became more massive, and therefore had stronger gravitational fields.  At a certain point, they were able to gravitationally hold an atmosphere.

11. Planet Building  Planetesimals contain both rock and metal.  A planet grows slowly from the uniform particles.  The resulting planet is of uniform composition.  Heat from radioactive decay causes differentiation.  The resulting planet has a metal core and low-density crust.  The first planetesimals contain mostly metals.  Later the planetesimals contain mostly rock.  A rock mantle forms around the iron core.  Heat from rapid formation can melt the planet.  The resulting planet has a metal core and low-density crust.

12. Planet-building processes  Dust grains stick together  planetesimals  Planetesimals stick together  protoplanets Terrestrial:  metallic / rocky  but small – not much material Jovian:  LOTS OF ICES, so quickly grew more massive  When ~15 x Earth’s mass, gravity strong enough to attract lots of H/He from solar nebula  got really really big – but not dense

13. The planets eventually formed and differentiated into: Terrestrial vs. Jovian Planets Planetary ringsNo ring system Farther away (from 5.2 to 30 AU)Close to the Sun (within 1.5 AU) Many moons (over 60)Few satellites (3) Faster rotators, differential rotationSlow rotators Lighter elements, H and HeHeavy gas atmospheres (N 2, O 2, CO 2 ) Low density, huge gaseous atmospheres Dense, rocky solid surfaces Large and massiveSmall size, low mass Jovian PlanetsTerrestrial Planets

14. Four stages of terrestrial planetary development  1. Differentiation early planet was molten heavy elements sunk, light elements rose On Earth:  Dense metal core  Less dense rocky mantle  Low-density rocky crust (outgassing made primitive atmosphere – more on that later)

15.  2. Cratering “heavy bombardment” period (first 0.5 billion years) many impacts with rogue planetesimals craters made (some huge) On Earth:  many craters later covered by ocean or erased by erosion) Four stages of terrestrial planetary development

16.  3. Flooding lava from below rain from atmosphere On Earth:  made oceans

17. Four stages of terrestrial planetary development  4. Slow surface evolution On Earth:  wind / water erosion  plate tectonics: moving sections of crust

18. Clearing of solar nebula  Sun pushed away remaining debris radiation pressure (light) solar wind (particles)  Planets swept up debris (craters) ejected debris

19. Clearing the Solar Nebula  Around 4.6 billion years ago, the cloud of gas (the solar nebula) vanished due to four effects: Radiation Pressure – light from the Sun exerted pressure on the particles, pushing them out of the solar system. The Solar Wind – a flow of atoms from the Sun’s upper atmosphere also helped push particles out of the solar system. As planets moved through their orbits, they swept up any material in their paths. Gravitational effects due to massive planets ejected particles out of the solar system.

20. Stellar Debris  Asteroids – rocky objects, mostly found between Mars and Jupiter (in the Astreroid Belt ~ 2.8 AU). Range in size up to 100 km in diameter. Irregularly shaped, and cratered. Remnants of planet formation.  Comets – small icy bodies (dirty snowballs). Large elliptical orbits can bring comets in close to the Sun. Recent studies suggest they are at least 50% rock and dust.  Meteoroids – specks of dust and rock which encounter Earth’s atmosphere and either burn up or fall to the ground. (Most only about 1g in mass). Meteors – Flash across the sky as the meteoroid burns up. Meteorite – remnant of a meteoroid that reaches the ground.

21. Up close and personal with an asteroid A Comet

22. Stellar Motions Due to Planets  Technically, planets don’t orbit around a star, but around the common center of mass.  If planets are massive enough, the center of mass is not located at the center of the star, and the star orbits around this point as well.  This motion can be detected through Doppler shifts in the star’s spectrum.

23.  Approximately the same age: Earth rocks Moon rocks Martian meteorites asteroidal meteorites  ~ 4.6 billion years  Determined by radioactive dating: compare original amount of radioactive element with an amount present now “half-life”: time it takes for ½ of radioact. elem. to decay into non-radioact. elem. Using Radioactive Dating, We’ve Discovered:

24. Explaining the Solar System  Terrestrial: small, dense, low mass  Jovian: large, low density, high mass Condensation sequence and accretion  Terrestrial: heavy gas atmospheres  Jovian: lighter elements Jovian planets can gravitationally hold onto lighter gas  Terrestrial: few satellites, no ring system  Jovian: many satellites, planetary rings Jovian planets gravitationally stronger  Existence of comets and asteroids Leftover material from the formation of the solar system.

25. Evidence of Extrasolar Planets  Two methods which suggest the existence of extrasolar planets: Detection of dust which accompanies planets around stars. Detection of stellar motions due to the presence of orbiting planets.

26. Known Extrasolar Planets  Most known extrasolar planets are high-mass and low- period planets. (Selection effect) High-mass: the greater the mass, the greater the wobble produced in the star’s motion. Low-period: the lower the period, the shorter the period over which the wobble occurs.  How can high-mass, low-period planets form? In dense disks, friction may slow the planet’s down, causing them to spiral inward.