Chapter 8Formation of Planetary SystemsOur Solar System and Beyond

Where did the solar system come from?

• According to the nebular theory, our solar system formed from a giant cloud of interstellar gas.

• (nebula = cloud)

Evidence from Other Gas Clouds• We can see

stars forming in other interstellar gas clouds, lending support to the nebular theory.

The Orion Nebula with Proplyds

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 must have increased as the cloud contracted.

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

Flattening

The spinning cloud flattens as it shrinks.

Formation of the Protoplanetary Disk

Disks Around Other Stars

• Observations of disks around other stars support the nebular hypothesis.

Why are there two major types of planets?

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

Conservation of Energy

Inner parts of the disk are hotter than outer parts.

Rock can be solid at much higher temperatures than ice.

Temperature Distribution of the Disk and 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

Fig 9.5

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.

Tiny solid particles stick to form planetesimals.

Summary of the Condensates in the Protoplanetary Disk

Gravity draws planetesimals together to form planets.

This process of assembly is called accretion.

Summary of the Condensates in the Protoplanetary Disk

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 from ices as well as metals

• The gravity of these larger planets was able to draw in surrounding H and He gases.

Moons of jovian planets form in miniature disks.

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

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

Heavy Bombardment

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

Giant Impact

Giant impact stripped matter from Earth’s crust

Stripped matter began to orbit

Then accreted into Moon

Origin of Earth’s Water

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

Odd Rotation

• Giant impacts might also explain the different rotation axes of some planets.

Captured Moons

• The unusual moons of some planets may be captured planetesimals.

Review of nebular theory

Thought Question

How would the solar system be different if the solar nebula had only been half as hot?

A. Jovian planets would have formed closer to the Sun.

B. There would be no asteroids.C. There would be no comets.D. Terrestrial planets would be larger.

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.

Radioactive Decay

• Some isotopes decay into other nuclei.

• A half-life is the time for half the nuclei in a substance to decay.

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 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form?

A. 1.25 billion years agoB. 2.5 billion years agoC. 3.75 billion years agoD. 5 billion years ago

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

Dating the Solar System

Dating the Solar System

• Radiometric dating tells us that the oldest moon rocks are 4.4 billion years old.

• The oldest meteorites are 4.55 billion years old.

• Planets probably formed 4.5 billion years ago.

How do we detect planets around other stars?

Planet Detection

• Direct: Pictures or spectra of the planets themselves

• Indirect: Measurements of stellar properties revealing the effects of orbiting planets

• For more check out this website: http://astro.unl.edu/naap/esp/detection.html

Direct Detection

• Special techniques for concentrating or eliminating bright starlight are enabling the direct detection of a very few extrasolar planets.

Indirect detection : Gravitational Tugs

• The Sun and Jupiter orbit around their common center of mass.

• The Sun therefore wobbles around that center of mass with the same period as Jupiter.

Gravitational Tugs• Sun’s motion around

solar system’s center of mass depends on tugs from all the planets.

• Astronomers who measured this motion around other stars could determine masses and orbits of all the planets.

Astrometric Technique• We can detect planets

by measuring the change in a star’s position in the sky.

• However, these tiny motions are very difficult to measure (~0.001 arcsecond).

Doppler Technique• Measuring a star’s

Doppler shift can tell us its motion toward and away from us.

• Current techniques can measure motions as small as 1 m/s (walking speed!).

First Extrasolar Planet Detected

• Doppler shifts of star 51 Pegasi indirectly reveal planet with 4-day orbital period

• Short period means small orbital distance

• First extrasolar planet to be discovered (1995)

First Extrasolar Planet Detected

• The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance.

Thought Question

A. It has a planet orbiting at less than 1 AU.B. It has a planet orbiting at greater than 1 AU.C. It has a planet orbiting at exactly 1 AU.D. It has a planet, but we do not have enough

information to know its orbital distance.

Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude?

Transits and Eclipses

• A transit is when a planet crosses in front of a star.• The resulting eclipse reduces the star’s apparent brightness and

tells us the planet’s radius.• When there is no orbital tilt, an accurate measurement of planet

mass can be obtained.

How do extrasolar planets compare with those in our solar system?

Measurable Properties

• Orbital period, distance, and shape• Planet mass, size, and density• Composition

Orbits of Extrasolar Planets• Most of the detected

planets have orbits smaller than Jupiter’s.

• Planets at greater distances are harder to detect with the Doppler technique.

Orbits of Extrasolar Planets• Most of the detected

planets have greater mass than Jupiter.

• Planets with smaller masses are harder to detect with the Doppler technique.

Planets: Common or Rare?

• Observations indicate ~2-5% of sun-like stars have Jupiter size planets

• The others may still have smaller (Earth-sized) planets that cannot be detected using current techniques.

• Current estimations range between 20 and 50% of sun-like stars having planets.

Surprising Characteristics

• Some extrasolar planets have highly elliptical orbits.

• Some massive planets orbit very close to their stars: “Hot Jupiters.”

Hot Jupiters

Do we need to modify our theory of solar system formation?

Revisiting the Nebular Theory

• Nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU).

• The discovery of “hot Jupiters” has forced a reexamination of nebular theory.

• “Planetary migration” or gravitational encounters may explain “hot Jupiters.”

Planetary Migration• A young planet’s

motion can create waves in a planet-forming disk.

• Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.

Gravitational Encounters

• Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit.

• Multiple close encounters with smaller planetesimals can also cause inward migration.

Modifying the Nebular Theory

• Observations of extrasolar planets have shown that the nebular theory was incomplete.

• Effects like planet migration and gravitational encounters might be more important than previously thought.

Chapter 12Asteroids, Comets, and Dwarf Planets

Their Nature, Orbits, and Impacts

Asteroid Facts

• Asteroids are rocky leftovers of planet formation.• The largest is Ceres, diameter ~1,000 km. • There are 150,000 in catalogs, and probably over a

million with diameter >1 km.• Small asteroids are more common than large asteroids.• All the asteroids in the solar system wouldn’t add up to

even a small terrestrial planet.

Asteroid Orbits• Most asteroids orbit

in a belt between Mars and Jupiter.

• Trojan asteroids follow Jupiter’s orbit.

• Orbits of near-Earth asteroids cross Earth’s orbit.

Origin of Asteroid Belt• Rocky planetesimals

between Mars and Jupiter did not accrete into a planet.

• Jupiter’s gravity, stirred up asteroid orbits and prevented their accretion into a planet.

Origin of Meteorites

• Most meteorites are pieces of asteroids.

Meteor Terminology

• Meteoroid: A small piece of rock or dust in space. Not as big as an asteroid.

• Meteor: A meteoroid that is falling through the Earth’s atmosphere. (shooting stars)

• Meteorite: A rock from space that falls through Earth’s atmosphere and reaches Earth..

Comet Facts

• Formed beyond the frost line, comets are icy counterparts to asteroids.

• The nucleus of a comet is like a “dirty snowball.”• Most comets do not have tails.• Most comets remain perpetually frozen in the

outer solar system.• Only comets that enter the inner solar system grow

tails.

Anatomy of a Comet• Coma is atmosphere

that comes from heated nucleus.

• Plasma tail is gas escaping from coma, pushed by solar wind.

• Dust tail is pushed by photons.

Growth of Tail

Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit.

Meteors in a shower appear to emanate from the same area of sky because of Earth’s motion through space.

Kuiper belt:On orderly orbits from 30–100 AU in disk of solar system

Oort cloud:On random orbits extending to about 50,000 AU

Only a tiny number of comets enter the inner solar system; most stay far from the Sun.

How did they get there?

• Kuiper belt objects are believed to have formed in the their current location along with the formation of the rest of the solar system. They orbit in a flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets.

• Oort cloud objects were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction.

Is Pluto a Planet?

• Much smaller than the eight major planets• Not a gas giant like the outer planets• Has an icy composition like a comet• Has a very elliptical, inclined orbit• Pluto has more in common with comets than with the

eight major planets.

Discovering Large Iceballs

• In summer 2005, astronomers discovered Eris, an iceball even larger than Pluto.

• Eris even has a moon: Dysnomia.

Dwarf Planets• There are many icy

objects like Pluto on elliptical, inclined orbits beyond Neptune.

• The largest ones are comparable in size to Earth’s Moon.

• If the object is massive enough for its own gravity to make it spherical(ish) then it is called a dwarf planet.

Kuiper Belt Objects• These large, icy

objects have orbits similar to the smaller objects in the Kuiper Belt that become short period comets.

• So are they very large comets or very small planets?

HST’s view of Pluto and moons

9.4 Cosmic Collisions: Small Bodies Versus the Planets

Our goals for learning:• Have we ever witnessed a major impact?• Did an impact kill the dinosaurs?• Is the impact threat a real danger or just

media hype?• How do other planets affect impact rates

and life on Earth?

Mass Extinctions

• Fossil record shows occasional large dips in the diversity of species: mass extinctions.

• The most recent was 65 million years ago, ending the reign of the dinosaurs.

Iridium: Evidence of an Impact

• Iridium is very rare in Earth surface rocks but is often found in meteorites.

• Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact.

• Dinosaur fossils all lie below this layer.

Iridium Layer

Dinosaur fossils in lower rock layers

No dinosaur fossils in upper rock layers

Thin layer containing the rare element iridium

Likely Impact Site• Geologists found a

large subsurface crater about 65 million years old in Mexico.

Frequency of Impacts• Small impacts

happen almost daily.

• Impacts large enough to cause mass extinctions are many millions of years apart.