ASTR100 (Spring 2008) Introduction to Astronomy The Formation of Planets Prof. D.C. Richardson Sections

What features of our solar system provide clues to its origin?

Motion of Large Bodies All large bodies in the solar system orbit in the same direction and in nearly the same plane. Most also rotate in that direction.

Two Major Planet Types Terrestrial planets are rocky, relatively small, and close to the Sun. Jovian planets are gaseous, larger, and farther from the Sun.

Swarms of Smaller Bodies Many rocky asteroids and icy comets populate the solar system.

Notable Exceptions Several exceptions to normal patterns need to be explained.

What theory best explains the features of our solar system?

The Nebular Theory According to the nebular theory, our solar system formed from a giant cloud of interstellar gas. (nebula = cloud) Orion Nebula

The Nebular Theory We can see stars forming in other interstellar clouds, lending support to the nebular theory. Proplyds in the Orion Nebula

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

Conservation of Angular Momentum The rotation speed of the cloud from which our solar system formed increased as the cloud contracted.

Conservation of Angular Momentum Rotation of contracting cloud speeds up for same reason a skater speeds up as she pulls in her arms…

Collisions Collisions between particles in the cloud caused it to flatten into a disk.

Collisions Collisions between gas particles in a cloud gradually reduce random motions.

Collisions Collisions between gas particles also reduce up and down motions.

Collisions The spinning cloud flattens as it shrinks.

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

Why are there two major types of planets?

Conservation of Energy As gravity causes the cloud to contract, it heats up.

Disk Temperature Inner parts of disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice.

The Frost Line Inside the frost line: too hot for hydrogen compounds to form ices. Outside the frost line: cold enough for ices to form.

Formation of Terrestrial Planets 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.

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

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

Formation of Jovian Planets Ice could also form small particles outside the frost line. Larger planetesimals and planets were able to form. Gravity of larger planets was able to draw in surrounding H and He gases.

Formation of Jovian Planets The gravity of rock and ice in jovian planets draws in H and He gases.

Formation of Jovian Moons Moons of jovian planets form in miniature disks.

The Solar Wind Radiation and outflowing matter from the Sun—the solar wind—blew away the leftover gases.

Where did asteroids and comets come from?

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

Heavy Bombardment Leftover planetesimals bombarded other objects in the late stages of solar system formation.

Origin of Earth’s Water Water may have come to Earth by way of icy planetesimals from the outer solar system.

How do we explain the existence of our Moon and other exceptions to the “rules”?

Giant Impacts Earth’s Moon was probably created when a giant planetsimal slammed into the newly forming Earth. Other large impacts may explain other exceptions, like Venus’ rotation and Uranus’ tilt. Movie 1Movie 2

Captured Moons The unusual “irregular” satellites of some planets may be captured planetesimals.

Review of nebular theory Fig 6.24

When did the planets form? We cannot find the age of a planet, but we can find the ages of the rocks that make it up. We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it.

The decay of radioactive elements into other elements is a key tool in finding the ages of rocks.

Age dating of meteorites that are unchanged since they condensed and accreted tell us that the solar system is about 4.6 billion years old.

Thought Question Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 3 atoms of argon-40 for every 1 atom of potassium-40. How old is the rock? billion years billion years. 3.5 billion years. 4.It is impossible to determine.

Thought Question Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 3 atoms of argon-40 for every 1 atom of potassium-40. How old is the rock? billion years billion years. 3.5 billion years. 4.It is impossible to determine.