Chapter 8 Formation of the Solar System

8.1 & 8.2 The Origins of Our Solar System Our goals for learning: What properties of our solar system must a formation theory explain? What theory best explains the features of our solar system? Where did the solar system come from? What caused the orderly patterns of motion in our solar system?

What properties of our solar system must a formation theory explain? Patterns of motion of the large bodies Orbit in same direction and plane Existence of two types of planets Terrestrial and jovian Existence of smaller bodies Asteroids and comets Notable exceptions to usual patterns Rotation of Uranus, Earth’s moon, etc.

What theory best explains the features of our solar system? The nebular theory states that our solar system formed from the gravitational collapse of a giant interstellar gas cloud—the solar nebula (Nebula is the Latin word for cloud) Kant and Laplace proposed the nebular hypothesis over two centuries ago A large amount of evidence now supports this idea

Close Encounter Hypothesis A rival idea proposed that the planets formed from debris torn off the Sun by a close encounter with another star. That hypothesis could not explain observed motions and types of planets.

Where did the solar system come from? Nebular Cloud

Galactic Recycling Elements that formed planets were made in stars and then recycled through interstellar space when they died. Consequently, we are made of “star stuff”

Evidence from Other Gas Clouds Astronomical observations of nebula has shown that new stars are formed there. These interstellar gas clouds lend support to the nebular theory

What caused the orderly patterns of motion in our solar system?

What patterns can been seen comparing the inner four planets (Mercury, Venus, Earth, Mars) to the “Jovian” planets (Jupiter, Saturn, Uranus, Neptune)? The outer four are much more massive The inner four are much more dense The inner four are rocky, the outer four gaseous All of the above 1 and 3

What caused the orderly pattern of our Solar System? Heating: As the solar nebula collapsed due to gravity, the temperature increased. Spinning: As the nebula collapsed, the rotational rate increased due to conservation of angular momentum (Like a figure skater when they bring their arms in tight to their bodies). Flattening: Collisions between particles caused their motion to become less random and more orderly.

Heating – Conservation of Energy As the particles descended towards the center of the solar nebula due to gravity, they lost potential energy. While losing potential energy, they gained kinetic energy. The collapse of the nebula resulted in an increase in collisions between particles due to their proximity to one another. The collisions caused an increase in thermal energy, hence the temperature.

Conservation of Angular Momentum Rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted This phenomena is the same as that seen in figure skaters and divers.

Flattening Collisions between particles in the cloud caused it to flatten into a disk by reducing the up and down motions of the particles.

Circular Orbits Collisions between gas particles in cloud gradually reduce random motions and making them more orderly.

Flattening Spinning cloud flattens as it shrinks. Where is the evidence?

Disks around Other Stars Observations of disks around other stars support the nebular hypothesis

Why do we think that the solar system formed from a rotating, collapsing gas cloud that ended up as a disk orbiting the Sun? Most of the planets revolve and rotate in the same direction, in the same plane Conservation of angular momentum means a collapsing cloud will spin faster and faster We see clouds of gas and dust in space We see disks around young stars All of the above

What have we learned? Where did the solar system come from? Galactic recycling built the elements from which planets formed. We can observe stars forming in other gas clouds. What caused the orderly patterns of motion in our solar system? Solar nebula spun faster as it contracted because of conservation of angular momentum Collisions between gas particles then caused the nebula to flatten into a disk and caused the particles to move in nearly circular orbits. We have observed such disks around newly forming stars

8.3 The Formation of Planets Our goals for learning: Why are there two major types of planets? How did terrestrial planets form? How did jovian planets form? What ended the era of planet formation?

Why are there two major types of planet?

Why are there two major types of planets? Initial Composition of nebula: 98% Hydrogen and Helium 2% other stuff (heavier elements) If the whole nebula had a consistent composition throughout, then how did we end of with two different types of planets? Note: inner planets are comprised mostly of the 2% of heavier elements. outer planets are primarily composed mainly of the lighter elements.

As the Solar nebula collapsed, the inner parts of disk became hotter than outer parts. Rock and metals can be solid at much higher temperatures than ice.

Inside the frost line: Too hot for hydrogen compounds (CH4, NH3, H2O) to form ices. Outside the frost line: Cold enough for hydrogen compounds to exist in the solid phase.

How did terrestrial planets form? Small particles of rock and metal were present inside the frost line Planetesimals of rock and metal built up as these particles collided Gravity eventually assembled these planetesimals into terrestrial planets

Tiny solid particles stick together to form planetesimals. Gravity draws planetesimals together to form planets This process of assembly is called accretion

Accretion of Planetesimals Many smaller objects collected into just a few large ones

Why do we think the inner (terrestrial) planets became more dense than the outer planets? In the collapsing solar nebula, denser materials sank toward the center The sun’s gravity pulled denser materials toward the center The inner nebula was so hot that only metals and rocks were able to condense The rotating disk in which the planets formed spun lighter elements outward by centrifugal force

What do we think the composition of the solar nebula was? About half hydrogen and helium, half heavier elements (iron, carbon, silicon, etc.) About 98% hydrogen and helium, and 2% heavier elements A less hydrogen and helium, more heavier elements

What do we think condensed out of the solar nebula? Hydrogen and helium Hydrogen compounds (water, methane, ammonia) where the nebula was cold (<150 K) Rocky material where the nebula was 500– 1500 K (depending on the type of rock) Metal where the temperature was 1000–1500 K All except #1

How much of the solar nebula “condenses” in the inner regions where the temperature is greater than 150 K and in the outer regions where the temperatures are less than 150 K ? Nothing in the inner region and about 2% in the outer region. Nothing in the inner region and 100% in the outer region. About 0.6% in the inner region and 1.4% in the outer region. About 2% in both inner and outer regions.

What do we think first caused tiny solid bits of material in the solar nebula to accrete and stick together? Gravity They were mostly made of sticky stuff Electrical forces (“static electricity”) Pressure

When the first solid bits in the solar nebula became large enough to be called planetesimals, what began to increase their growth rate? Gravity They were mostly made of sticky stuff Electrical forces (“static electricity”) Pressure Ice

How did jovian planets form? Ice could also form small particles outside the frost line. Larger planetesimals and planets were able to form. Icy planets were many times larger than Earth, even without H and He gases. Gravity of these larger planets was able to draw in surrounding H and He gases.

Gravity of rock and ice in jovian planets draws in H and He gases

Moons of jovian planets form in miniature disks

Why could the Jovian planets grow to be much larger than the terrestrial planets? They were further from the Sun and gravity was weaker They formed beyond the frost line where ices can condense so they included hydrogen compounds They were far enough from the Sun to escape the heavy bombardment that battered the early solar system

What appears to have happened to increase the mass of Jovian planets even further? They attracted more gas from interstellar space They added gas from the solar wind They added more mass from heavy bombardment Their gravity became strong enough to attract hydrogen and helium gas from the solar nebula

What ended the era of planet formation?

A combination of photons and the solar wind —outflowing matter from the Sun—blew away the leftover gases

Solar Rotation In nebular theory, young Sun was spinning much faster than now Friction between solar magnetic field and solar nebular probably slowed the rotation over time

What is the “solar wind?” Strong radiation that comes from the Sun Similar to winds on earth but faster and stronger Similar to winds on earth but less dense and weaker Atoms and parts of atoms ejected from the Sun at high speed

How do the “stellar winds” of young solar-type stars compare to the solar wind we see from the Sun today? Young stars don’t have winds Young stars have a less strong wind Young stars have much stronger winds

Could a solar system like ours have formed with the first generation of stars after the Big Bang? Possibly - there is no physical reason why not. No, there would not have been enough time to form planets. No, the expansion of the Universe would have torn the solar system apart. No, there would not have been enough metals and rock to form terrestrial planets. No, the stars would have died by now.

What have we learned? Why are there two types of planets? Only rock and metals condensed inside the frost line Rock, metals, and ices condensed outside the frost line How did the terrestrial planets form? Rock and metals collected into planetsimals Planetesimals then accreted into planets How did the jovian planets form? Additional ice particles outside frost line made planets there more massive Gravity of these massive planets drew in H, He gases

What have we learned? What ended the era of planet formation? Solar wind blew away remaining gases Magnetic fields in early solar wind helped reduce Sun’s rotation rate

8.4 The Aftermath of Planet Formation Our goals for learning: Where did asteroids and comets come from? How do we explain “exceptions to the rules”? How do we explain the existence of our Moon? Was our solar system destined to be?

Where do asteroids come from? A planet between Mars and Jupiter that broke up They are escaped small moons Leftover planetesimals from the inner solar system Leftover planetesimals from the outer solar system

Where do comets come from? A planet between Mars and Jupiter that broke up They are escaped small moons Leftover planetesimals from the inner solar system Leftover planetesimals from the outer solar system

Asteroids and Comets Leftovers from the accretion process Rocky asteroids inside frost line Icy comets outside frost line

How do we explain “exceptions to the rules”?

Heavy Bombardment Leftover planetesimals bombarded other objects in the late stages of solar system formation (first few hundred million years).

Origin of Earth’s Water Water may have come to Earth by way of icy planetesimals such as comets.

Captured Moons Phobos Deimos Unusual moons of some planets may be captured planetesimals

How do we explain the existence of our Moon?

Giant Impact Giant impact stripped matter from Earth’s crust. Stripped matter began to orbit… …then accreted into Moon.

Odd Rotation Giant impacts might also explain the different rotation axes of some planets such as Uranus and Venus.

Thought Question How would the solar system be different if the solar nebula had cooled, with a temperature half its actual value? a) Jovian planets would have formed closer to Sun b) There would be no asteroids c) There would be no comets d) Terrestrial planets would be larger

Was our solar system destined to be? Formation of planets in the solar nebula seems inevitable But details of individual planets could have been different

Thought Question Which of these facts is NOT explained by the nebular theory? There are two main types of planets: terrestrial and jovian. Planets orbit in same direction and plane. Existence of asteroids and comets. Number of planets of each type (4 terrestrial and 4 jovian).

What have we learned? Where did asteroids and comets come from? They are leftover planetesimals, according to the nebular theory How do we explain “exceptions to the rules”? Bombardment of newly formed planets by planetesimals may explain the exceptions How do we explain the existence of Earth’s moon? Material torn from Earth’s crust by a giant impact formed the Moon Was our solar system destined to be? Formation of planets seems invevitable. Detailed characteristics could have been different.