Formation of our solar system The nebular hypothesis

Formation of our solar system: The nebular hypothesis (Kant, 1755) Hydrogen (H), He (He) and “stardust” (heavier elements that were formed in previous stars)

Our Solar System - Origins • Observations – Ordered motions of objects – Two types of planets – Terrestrial (Solid) vs. Jovian (Gas) • For example, Mars vs. Jupiter – Asteroids and comets – Exceptions to ordered motions • Tilt of Earth’s axis, Earth’s Moon, Pluto • Is there a theory that can explain what we see? Yes!!

Nebular Theory • Solar System formed through the collapse of a large cloud of gas under its own gravity Eagle Nebula Lagoon Nebula

Formation of our solar system: The nebular hypothesis (Kant, 1755)

Gravitational Collapse n Lets look at an animation!

Angular Momentum Conservation also explains why objects rotate faster as they shrink in radius:

Formation of our solar system: The nebular hypothesis (Kant, 1755)

From Cloud to Solar System • Energy Conservation – As the nebula contracts, the energy is concentrated in a smaller area. This, in turn, heats the cloud • Ang. Momentum Cons. – Causes the cloud to spin faster • Cloud Flattens – The lighter material floats to the outer edges. – The more dense material stays towards the center. – In our solar system, this was the formation of our inner solid and outer gaseous planets.

Does this theory make sense? • We observe many other stars forming from interstellar clouds with circumstellar disks • What has yet to be explained?

How do planets form? • Through accretion (collecting of material) – Small particles are able to build larger complexes of particles through electromagnetic forces – Eventually the complex becomes large enough to “attract” pieces through gravitation – planetesimals – Only the largest planetesimals survive to become planets – The smaller pieces collide and merge with the larger ones or are broken up by gravitational forces • Question: Why are all of the planets very nearly spherical in structure?

Why are there two types of planets? • It all boils down to temperature • Gas in the solar nebula was mostly hydrogen and helium, but there were trace amounts of hydrogen compounds, rock, and metals • Condensation (gas solidifying into solid) occurs at different temperatures • Warmer temperatures closer to the center of the disk (Sun) • Temperature decreases as we move towards the edge of the disk

Planet Formation • Terrestrial planets (Mercury, Venus, Earth, and Mars) all formed beyond a distance of 0. 3 AU – Rock could not condense within this distance • Jovian planets (Jupiter, Saturn, Uranus, and Neptune) all formed beyond the frost line (3. 5 AU) – Hydrogen compounds formed the cores of these planets • Continues to support nebular theory • What about planet orbits? – But more questions remain

The solar system: Sun, terrestrial, and Juvian planets Terrestrial (inner) planets Juvian (outer) planets • Small • contains the heavier elements (Fe, Si, O) • rocky shell over a metallic core • Accretion began 4. 567 billion years ago • It took about 0. 1 billion years (100 million years) for planets to form • mostly volatile gases (H, He) • same composition as the sun, but internal pressure is too low for nucleo-synthesis to take place 99. 8 % of the total mass of the solar system resides in the sun

Earth formation Nebula forms out of H, He clouds and fusion products from now dead stars. Gravity pulls gas and dust inward, and rotation makes a accretionary disk. The proto-sun is at the center and fusion begins when it is hot enough. Gravity reshapes the proto-Earth into a sphere Heavier elements concentrate in the inner rings. Collisions and gravitational attraction leads to the formation of planetesimals. Soon after Earth formed, a Mars-size planet collided with Earth, blasting debris into space that will form our Moon. Volcanic gases form the atmosphere, and as Earth cools, moisture condenses and rains into oceans. Some gases may have come from passing comets.

Asteroids and Comets Ida • Asteroids – Rocky leftover planetesimals • Comets - Icy leftover planetesimals Hale-Bopp

Other Solar Systems? • Detected over 100 planets • Can not detect directly – Even for the largest planets, light from the star overwhelms any light from the planet • Use indirect evidence to search for planets – Gravitational tugs