Formation of Our Solar System Modified presentation originally created by the Lunar and Planetary Institute Image: Lunar and Planetary Laboratory:

Some data to explain: 1.Planets isolated 2.Orbits ~circular / in ~same plane 3.Planets (and moons) travel along orbits in same direction…. same direction as Sun rotates (counter- clockwise viewed from above) Lunar and Planetary Institute image at

Some more data to explain: 4.Most planets rotate in this same direction NASA images edited by LPI Mercury 0° Venus 177° Earth 23° Mars 25° Jupiter 3° Saturn 27° Uranus 98° Neptune 30°

And some more data to explain: 5.Solar System highly differentiated: Terrestrial Planets (rocky, dense with density ~4-5 g/cm3) Jovian Planets (light, gassy, H, He, density 0.7-2) Images: Lunar and Planetary Laboratory:

How Did We Get a Solar System? Huge cloud of cold, thinly dispersed interstellar gas and dust – threaded with magnetic fields that resist collapse Hubble image at Image: LPI

How Did We Get a Solar System? The cool nebular gas contained about five hydrogen molecules for every helium atom (H - 71%, He - 27% by mass). The temperature in the outer solar system was cold enough for H 2 molecules to form. Image: LPI

How Did We Get a Solar System? Other elements C, N, O bond with H to form CH 4, NH 3 and H 2 O. Astronomers call these “ices” because they can condense into solids in a cold nebula. There may have also been small amounts of CO and CO 2. About 0.5% of the nebula was metal and silicate rock. Image: LPI

Concentrations of dust and gas in the cloud; material starts to collect (gravity > magnetic forces) How Did We Get a Solar System? Hubble image at Image: LPI

How Did We Get a Solar System? Gravity concentrates most stuff near center Heat and pressure increase Collapses – central proto- sun rotates faster (probably got initial rotation from the cloud) Image: LPI

How Did We Get a Solar System? NASA artwork at Rotating, flattening, contracting disk - solar nebula! Equatorial Plane Orbit Direction

After ~10 million years, material in center of nebula hot enough to fuse H “...here comes the sun…” How Did We Get a Solar System? NASA/JPL-Caltech Image at

How Did We Get a Solar System? Hubble photo at Metallic elements (Mg, Si, Fe) condense into solids at high temps. Combined with O to make tiny grains Lower temp (H, He, CH 4, H 2 O, N 2, ice) - outer edges Planetary Compositions

How Did We Get a Solar System? Inner Planets: Hot – Silicate minerals, metals, no light elements, ice Begin to stick together with dust  clumps Image: LPI

How Did We Get a Solar System? Accretion - particles collide and stick together … or break apart … gravity not involved if small pieces Form planetesimals, up to a few km across Image: LPI

How Did We Get a Solar System? Gravitational accretion: planetesimals attract stuff Large protoplanets dominate, grow rapidly, clean up area ( takes ~10 to 25 My) Image: LPI

How Did We Get a Solar System? Outer Solar System Cold – ices, gases – 10x more particles than inner May have formed icy center, then captured lighter gases (Jupiter and Saturn first? Took H and He?) Image: LPI

How Did We Get a Solar System? Early burst of solar wind - sweeps debris out of system Gravitational accretion of gas for protoplanets in the coolest nebular parts Image: LPI

The Asteroid Belt Should have been a planet instead of a debris belt? Jupiter kept it from forming How Did We Get a Solar System? Eros image at

Beyond the Gas Giants - Pluto, Charon and the Kuiper Belt objects Chunks of ice and rock material Little time / debris available to make a planet – slower!! How Did We Get a Solar System?

Comets Dirty snowballs - small objects of ice, gas, dust, tiny traces of organic material Image from: How Did We Get a Solar System?

Early in the Life of Planets Planetesimals swept up debris Accretion + Impacts = HEAT Eventually begin to melt materials Iron, silica melt at different temperatures Iron sank – density layering Image from LPI:

Planetary Interiors Differentiation Separation of homogenous interior into layers of different compositions Early – hottest time – dense iron-rich material  core Releases additional heat Leaves mantle with molten ocean enriched in silica Crust eventually forms from lightest material Image from LPI:

Planetary Interiors Differentiation Continues! Radioactive decay = primary heat source Partial melting of mantle material  rising magma  volcanoes / lava flows Image from LPI:

When did Our Solar System Form … How do We Know? Image: Lunar and Planetary Laboratory:

When Did the Solar System Form? 4.56 billion years ago How do we know? (evidence for formation) Meteorite photo by Carl Allen at Lunar samples to 4.6 Ga Meteorites Ga Earth – 3.9 (or 4.4 Ga) Lunar meteorite at

How Do We Know How Our Solar System Formed?

Solar System Samples Meteorites Image: And

We Can Also Look Around …. Close-up of "Proplyds" in Orion Thanks Hubble! Hubble images at and

We Can Also Look Around …. This dusty disc around a newly formed star has a gap, thought to have been carved by newly forming gas giant planets clearing out their orbits as they circle the star. ALMA images at and artist’s impression

We Can Also Look Around …. This shows a dust trap in the disc that surrounds the system Oph-IRS 48. The dust trap allows tiny dust particles to clump together and grow to larger sizes. The green area is the dust trap, where the bigger particles accumulate. ALMA images at and artist’s impression