The Formation of the Solar System Basic Facts to be explained : 1. Each planet is relatively isolated in space. 2. Orbits nearly circular. 3. All roughly orbit in the same plane. 4. Planets are all orbiting the Sun in the same sense in keeping with Sun s own rotation. 5. Most moons revolve about their planets in the same sense as the planets own rotation. 6. Planets are very different.
Evolutionary Models Models do not explain everything! (Not catastrophes :e.g.: major collisions.) Planets evolve after their initial formation e.g.:venus and Mars Greenhouse effects. Irregularities: eg: Retrograde motion. Nevertheless the Solar System has order (not random or chaotic) suggesting an origin ~4.6 billion years ago.
The Condensation Theory Nebular origins: Large gas cloud begins to collapse under its own gravity. Becomes denser and hotter. Sun forms at center. A star forming region in the Orion Nebula.
Nebular Collapse Conservation of angular momentum results in smaller disc rotating faster.
Swirling Solar Nebula Outer cooler regions of rotating gas cloud can coalesce into planets and moons. Rotation causes a shape change into flat disc.
In Summary: A large gas cloud begins to collapse under its own gravity. As it rotates it becomes flatter. It also becomes denser and hotter and a Sun forms at center. Planets form out of the material in the rotating disc.
Proto-Planetary Discs - Proplyds Evidence! Proplyds in the Orion Nebula.
Intersteller Dust Microscopic dust grains in space from old dead stars. Typically ~10μm. Help to cool warm matter, enhancing coalescence. Act as condensation nuclei like in rain drops. Hence essential for planet formation.
Dark Interstellar Dust Clouds in a Region of Star Formation.
Solar System Formation I (a) Flattened, rotating Solar Nebula. (b) Dust grains collide, stick, grow into planetesimals composition depends on formation region. Called Accretion
Solar System Formation II (c) Strong winds from proto-sun expel nebular gas. Some outer planetesimals (size of small moons) are both accreting and gravitationally growing.
T-Tauri Wind Larger planetary debris ended up in asteroid belt, Oort cloud or Kuiper belt. What happened to the interplanetary gases? Blown into interstellar space when Sun changed to the T- Tauri phase of its evolution. A Huge Shock wave!
T-Tauri Wind II Occurs just prior to nuclear burning starting, when only a few 100 million years old. Lots of outer proto-sun material ejected into space.
Solar System Formation III (d) Proto-planets continue to grow and gas giants are formed. (e) Over ~100 million years accretion and fragmentation eventually results in well separated planets.
Accretion Forms Inner Planets Initially many moon-sized planetesimals. ~100 million yrs for accretion circular orbits.
Formation of Gas Giants? Mechanisms for Jovian planets not as clear. Local instabilities in cooler outer regions enable quicker gravitational growth with less emphasis on accretion (except for moons). Possibility: formed further and have slowly migrated inwards! Chemical composition.
Heat, Condensation and Planetary Differentiation Hotter close to proto-sun. Materials condense out at various distances. Beyond 30AU methane condenses.
Terrestrial Planets Condensed when temperature was ~1000K. Heavier materials: Si, Fe, Mg, Al (oxides). Also present in outer regions it is just that inner regions are under represented in light material. Not as big as Jovian planets. Where does water and volatile gases come from?! Collisions with icy planetesimals and comets in eccentric orbits after formation!
Jovian Planets and Comets Beyond ~5AU cold enough for condensation into solid form: H 2, He, H 2 O, CH 4, NH 3. Speeds up formation process via Gravity. Hence large H 2 He rich planets. Planets gravity ejects interplanetary debris (over 100 million years) into Oort Cloud. Remants lie in the Kuiper Belt beyond Neptune s orbit.
The Oort Cloud and Kuiper Belt Net outflow of planetesimal material during solar evolution. Neptune moves outwards! Comets originate in these outer regions.
Comet Reservoirs Long and short period comets come from Oort and Kuiper regions.
Kuiper s Belt Object Discovery of 6 th Kuiper object in 1993. There are more being discovered all the time! (>1000)
Artist s Impressions! KBO s Photo of Eris and its moon Dysnomia previously known as Xena and Gabrielle Larger than Pluto
Long-period Comets Oort cloud may have trillions (~10 12 ) of comets Named after Dutch Jan Oort s postulate in 1950. Cloud may be 100,000AU diameter. Large, slow orbits: >100,000 s of years! Comets frozen ices do not come near the Sun. (Probably not even Pluto!) All orientations of orbit are possible.
Short-period Comets Periods of <~200 years. Hence do not venture far beyond Pluto s orbit. Kuiper belt 30-100AU. Heat up when travel near the Sun. Ices evaporate, dust ejected into a tail. Tail driven by the solar wind, which also creates ionised gases. Dramatic and Relatively Rare.
Gravitational Perturbations If an intermediate period comet passes by a Jovian Planet its path can be changed into a short period comet. Such interactions can also break up comets!
Cometary Tails Comet Hale-Bopp 1997 Two tails: (a) White dust is reflected sunlight, (b) Blue radiation from hot ions. (Period ~2400 yrs)
Halley s Comet Returns every 76 years. Visited in 1682, Halley (1705) predicted its period. A big success for Newton s Gravity. Giotto s view of core, 1986 Size ~ 15 x 10km
The Orbit of Halley s Comet Elliptical orbit extends to beyond Neptune.
Close up of Comet Tempel 1 Deep Impact Mission 2005
Impact with Tempel 1 July 4 2005 Found: Carbonates, Silicates Iron compounds Aromatic hydrocarbons
Other Comet Images McNaught s spectacular tail (2007) Comet Machholz (2004)
Even more Comet West (1975) Comet Swan (2006) Footprints on Wild 2!
Asteroids Rocks between Mars and Jupiter failed to aggregate into a planet. Most probably Jupiter s gravity is too strong. Perturbed trajectories inhibited coalescence.
Asteroid Belt Over ~100,000 rocky asteroids. Most are within a belt at 2.1-3.3AU. 3 largest asteroids are: Ceres ~940km, Pallas ~580km, Vesta ~540km, (in diameter). Ida ~60km with its moon Dactyl (~1.5km). Eros (probe landed on it!)
Close-ups of Eros Different: colourations, crater and rocks sizes, surface re-melting?
Ceres now a Dwarf Planet! Observed to have ~9 hour rotation! NASA s Dawn Mission to Vesta (2011) and Ceres (2015) launched in 2007.
A Role for Catastrophes? Condensation and Accretion theory explains the main features of the solar system. A List of outstanding issues: 1. Mercury s large iron core. 2. Venus slow rotation rate. 3. Earth-Moon system. 4. Mars curious basin and fissure.
Catastrophes? 5. Uranus large rotational tilt angle. 6. Triton s retrograde motion. Are catastrophes ( random one-off occurrences) needed to explain these events? Not all scientists agree