Discovering the Universe Eighth Edition Discovering the Universe Eighth Edition Neil F. Comins William J. Kaufmann III CHAPTER 5 Formation of the Solar System CHAPTER 5 Formation of the Solar System

WHAT DO YOU THINK? 1. How old is the Earth? How do we know??? 2. How old are the Sun and other planets? Were they created during the “Big Bang”? How do we know? 3. Have any Earthlike planets been discovered orbiting Sunlike stars? How can we tell?

In this chapter you will discover… how the solar system formed how the solar system formed why the early solar system was much more violent than it is today why the early solar system was much more violent than it is today how astronomers define various types of objects in the solar system how astronomers define various types of objects in the solar system how the planets are “grouped” how the planets are “grouped”

In this chapter you will discover… how moons formed throughout the solar system how moons formed throughout the solar system the “debris” in the solar system the “debris” in the solar system that disks of gas and dust, as well as planets, have been observed around a growing number of stars that disks of gas and dust, as well as planets, have been observed around a growing number of stars that newly forming stars & planetary systems are being observed that newly forming stars & planetary systems are being observed

The solar system exhibits clear patterns of composition and motion. These patterns are far more important and interesting than numbers, names, and other trivia.

Planets are very tiny compared to distances between them.

Over 99.9% of solar system’s mass Made mostly of H/He gas (plasma) Converts 4 million tons of mass into energy each second Sun

Made of metal and rock; large iron core Desolate, cratered; long, tall, steep cliffs Very hot and very cold: 425°C (day), –170°C (night) Mercury

Nearly identical in size to Earth; surface hidden by clouds Hellish conditions due to an extreme greenhouse effect Even hotter than Mercury: 470°C, day and night Venus

An oasis of life The only surface liquid water in the solar system A surprisingly large moon Earth and Moon to scale Earth

Looks almost Earth-like, but don’t go without a spacesuit! Giant volcanoes, a huge canyon, polar caps, and more Water flowed in the distant past; could there have been life? Mars

Much farther from Sun than inner planets Much farther from Sun than inner planets Mostly H/He; no solid surface Mostly H/He; no solid surface 300 times more massive than Earth 300 times more massive than Earth Many moons, rings Many moons, rings Jupiter

Jupiter’s moons can be as interesting as planets themselves, especially Jupiter’s four Galilean moons Io (shown here): Active volcanoes all over Europa: Possible subsurface ocean Ganymede: Largest moon in solar system Callisto: A large, cratered “ice ball”

Saturn Giant and gaseous like Jupiter Giant and gaseous like Jupiter Spectacular rings Spectacular rings Many moons, including cloudy Titan Many moons, including cloudy Titan Cassini spacecraft currently studying it Cassini spacecraft currently studying it

Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon. Artist’s conception The Rings of Saturn

Cassini probe arrived July (Launched in 1997)

Smaller than Jupiter/Saturn; much larger than Earth Smaller than Jupiter/Saturn; much larger than Earth Made of H/He gas and hydrogen compounds (H 2 O, NH 3, CH 4 ) Made of H/He gas and hydrogen compounds (H 2 O, NH 3, CH 4 ) Extreme axis tilt Extreme axis tilt Moons and rings Moons and rings Uranus

Similar to Uranus (except for axis tilt) Similar to Uranus (except for axis tilt) Many moons (including Triton) Many moons (including Triton) Neptune

Pluto and Eris Much smaller than other planets Much smaller than other planets Icy, comet-like composition Icy, comet-like composition Pluto’s moon Charon is similar in size to Pluto Pluto’s moon Charon is similar in size to Pluto

We know space is filled with gas and dust – the raw materials from which planetary systems form! Space isn’t empty…

…and its composition changes Spectra of exploding and old stars shows heavier elements being ejected, too

What features of our solar system provide clues to how it formed?

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

Two Major Planet Types Terrestrial planets are rocky, relatively small, and close to the Sun. Terrestrial planets are rocky, relatively small, and close to the Sun. Jovian planets are gaseous, larger, and farther from 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. Many rocky asteroids and icy comets populate the solar system.

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

What theory best explains the features of our solar system?

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

Detecting Planets around OTHER stars! KEPLER COROT WASP

BETA Pictoris – a Hint at Stars with Planetary Disks?

Planet Detection Indirect: Measurements of stellar properties revealing the effects of orbiting planets Indirect: Measurements of stellar properties revealing the effects of orbiting planets Direct: Pictures or spectra of the planets themselves Direct: Pictures or spectra of the planets themselves

Indirect: Gravitational Tugs The Sun and Jupiter orbit around a common center of mass. The Sun and Jupiter orbit around a common center of mass. The Sun wobbles around that center of mass with the same period as Jupiter. The Sun 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. Sun’s motion around solar system’s center of mass depends on tugs from all the planets. Astronomers who measured this motion could determine masses and orbits of all the planets. Astronomers who measured this motion 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. 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). However, these tiny motions are very difficult to measure (~0.001 arcsecond).

Inferring planets that aren’t seen!

Doppler Technique Measuring a star’s Doppler shift can tell us its motion toward and away from us. 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!). 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 Doppler shifts of star 51 Pegasi indirectly reveal planet with 4-day orbital period Short period means small orbital distance Short period means small orbital distance

1st Extrasolar Planet Detected The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance. 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?

Thought Question 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. 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. 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. When there is no orbital tilt, an accurate measurement of planet mass can be obtained.

Direct Detection Special techniques for concentrating or eliminating bright starlight are enabling the direct detection of planets.

Direct Detection

The Process of Science Observation: TMR-1 “detected” in 1998 Observation: TMR-1 “detected” in 1998

The Process of Science Observation: TMR-1 “detected” in 1998 Observation: TMR-1 “detected” in 1998 Hypothesis: It is a planet, connected by a disk to a double (binary star) Hypothesis: It is a planet, connected by a disk to a double (binary star)

The Process of Science Observation: TMR-1 “detected” in 1998 Observation: TMR-1 “detected” in 1998 Hypothesis: It is a planet ? Hypothesis: It is a planet ? Critical Tests: Spectra Critical Tests: Spectra

The Process of Science Observation: TMR-1 “detected” in 1998 Observation: TMR-1 “detected” in 1998 Hypothesis: It is a planet ? Hypothesis: It is a planet ? Critical Tests: Spectra Critical Tests: Spectra Result: A background star! Result: A background star!

The Process of Science

How do extrasolar planets compare with our solar system?

Measurable Properties Orbital period, distance, and shape Orbital period, distance, and shape Planet mass, size, and density Planet mass, size, and density Composition (by spectra) Composition (by spectra)

Orbits of Extrasolar Planets Most of the detected planets have orbits smaller than Jupiter’s. Most of the detected planets have orbits smaller than Jupiter’s. Planets at greater distances are harder to detect with the Doppler technique. 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. Most of the detected planets have greater mass than Jupiter. Planets with smaller masses are harder to detect with the Doppler technique. Planets with smaller masses are harder to detect with the Doppler technique.

Planets: Common or Rare? One in ten stars examined so far have turned out to have planets. One in ten stars examined so far have turned out to have planets. The others may still have smaller (Earth- sized) planets that cannot be detected using current techniques. The others may still have smaller (Earth- sized) planets that cannot be detected using current techniques.

Surprising Characteristics Some extrasolar planets have highly elliptical orbits. Some extrasolar planets have highly elliptical orbits. Some massive planets orbit very close to their stars: “Hot Jupiters.” Some massive planets orbit very close to their stars: “Hot Jupiters.” See “SuperWASP” See “SuperWASP”SuperWASP

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). 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. The discovery of “hot Jupiters” has forced a reexamination of nebular theory. “Planetary migration” or gravitational encounters may explain “hot Jupiters.” “Planetary migration” or gravitational encounters may explain “hot Jupiters.”

Planetary Migration A young planet’s motion can create waves in a planet- forming disk. 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. 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. 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. 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. 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. Effects like planet migration and gravitational encounters might be more important than previously thought.

What have we learned? How do we detect planets around other stars? How do we detect planets around other stars? — A star’s periodic motion (detected through Doppler shifts) tells us about its planets. — Transiting planets periodically reduce a star’s brightness. — Direct detection is possible if we can block the star’s bright light.

What have we learned? How do extrasolar planets compare with those in our solar system? How do extrasolar planets compare with those in our solar system? — Detected planets are all much more massive than Earth. — Most have orbital distances smaller than Jupiter’s, with highly elliptical orbits. — “Hot Jupiters” have been found. Do we need to modify our theory of solar system formation? Do we need to modify our theory of solar system formation? — Migration and encounters may play a larger role than previously thought.

Summary of Key Ideas

Formation of the Solar System Hydrogen, helium, and traces of lithium, the three lightest elements, were formed shortly after the creation of the universe. The heavier elements were produced much later by stars and are cast into space when stars die. By mass, 98% of the observed matter in the universe is hydrogen and helium. Hydrogen, helium, and traces of lithium, the three lightest elements, were formed shortly after the creation of the universe. The heavier elements were produced much later by stars and are cast into space when stars die. By mass, 98% of the observed matter in the universe is hydrogen and helium. The solar system formed 4.6 billion years ago from a swirling, disk-shaped cloud of gas, ice, and dust, called the solar nebula. The solar system formed 4.6 billion years ago from a swirling, disk-shaped cloud of gas, ice, and dust, called the solar nebula. The four inner planets formed through the accretion of dust particles into planetesimals and then into larger protoplanets. The four outer planets probably formed through the runaway accretion of gas and ice onto rocky protoplanetary cores over millions of years, but possibly by gravitational collapse in under 100,000 years. The four inner planets formed through the accretion of dust particles into planetesimals and then into larger protoplanets. The four outer planets probably formed through the runaway accretion of gas and ice onto rocky protoplanetary cores over millions of years, but possibly by gravitational collapse in under 100,000 years.

Formation of the Solar System The Sun formed at the center of the solar nebula. After about 100 million years, the temperature at the protosun’s center was high enough to ignite thermonuclear fusion reactions. The Sun formed at the center of the solar nebula. After about 100 million years, the temperature at the protosun’s center was high enough to ignite thermonuclear fusion reactions. For 800 million years after the Sun formed, impacts of asteroid-like objects on the young planets dominated the history of the solar system. For 800 million years after the Sun formed, impacts of asteroid-like objects on the young planets dominated the history of the solar system.

Comparative Planetology The four inner planets of the solar system share many characteristics and are distinctly different from the four giant outer planets. The four inner planets of the solar system share many characteristics and are distinctly different from the four giant outer planets. The four inner, terrestrial planets are relatively small, have high average densities, and are composed primarily of rock and metal. The four inner, terrestrial planets are relatively small, have high average densities, and are composed primarily of rock and metal. Jupiter and Saturn have large diameters and low densities and are composed primarily of hydrogen and helium. Uranus and Neptune have large quantities of water as well as much hydrogen and helium. Jupiter and Saturn have large diameters and low densities and are composed primarily of hydrogen and helium. Uranus and Neptune have large quantities of water as well as much hydrogen and helium.

Comparative Planetology Pluto, once considered the smallest planet, has a size, density, and composition consistent with the known Kuiper Belt Objects (KBOs). Asteroids are rocky and metallic debris in the solar system, larger than about a kilometer in diameter, and found primarily between the orbits of Mars and Jupiter. Meteoroids are smaller pieces of such debris. Comets are debris that contain both ice and rock.

Planets Outside Our Solar System Astronomers have observed disks of gas and dust orbiting young stars. Astronomers have observed disks of gas and dust orbiting young stars. At least 250 extrasolar planets have been discovered orbiting other stars. At least 250 extrasolar planets have been discovered orbiting other stars. Most of the extrasolar planets that have been discovered have masses roughly the mass of Jupiter. Most of the extrasolar planets that have been discovered have masses roughly the mass of Jupiter. Extrasolar planets are discovered indirectly as a result of their effects on the stars they orbit. Extrasolar planets are discovered indirectly as a result of their effects on the stars they orbit.

Key Terms accretion albedo asteroid asteroid belt average density comet core-accretion model crater gravitational instability model meteoroid microlensing moon (natural satellites) orbital inclination planet planetesimal protoplanet protoplanetary disks (proplyds) protosun solar nebula solar system terrestrial planet

WHAT DID YOU THINK? Were the Sun and planets among the first generation of objects created in the universe? Were the Sun and planets among the first generation of objects created in the universe? No. All matter and energy were created by the Big Bang. However, much of the material that exists in our solar system was processed inside stars that evolved before the solar system existed. The solar system formed billions of years after the Big Bang occurred. No. All matter and energy were created by the Big Bang. However, much of the material that exists in our solar system was processed inside stars that evolved before the solar system existed. The solar system formed billions of years after the Big Bang occurred.

WHAT DID YOU THINK? How long has Earth existed, and how do we know this? How long has Earth existed, and how do we know this? Earth formed along with the rest of the solar system, about 4.6 billion years ago. The age is determined from the amount of radioactive decay that has occurred in it. Earth formed along with the rest of the solar system, about 4.6 billion years ago. The age is determined from the amount of radioactive decay that has occurred in it.

WHAT DID YOU THINK? What typical shape(s) do moons have, and why? What typical shape(s) do moons have, and why? Although some moons are spherical, most look roughly like potatoes. Those that are spherical are held together by the force of gravity, pulling down high regions. Those that are potato-shaped are held together by the electromagnetic interaction between atoms, just like rocks. These latter moons are too small to be reshaped by gravity. Although some moons are spherical, most look roughly like potatoes. Those that are spherical are held together by the force of gravity, pulling down high regions. Those that are potato-shaped are held together by the electromagnetic interaction between atoms, just like rocks. These latter moons are too small to be reshaped by gravity.

WHAT DID YOU THINK? Have any Earthlike planets been discovered orbiting Sunlike stars? Have any Earthlike planets been discovered orbiting Sunlike stars? Not really. Most extrasolar planets are Jupiter-like gas giants. The planets similar in mass and size to Earth are either orbiting remnants of stars that exploded or, in the case of Gliese 581, a star much less massive and much cooler than the Sun. Not really. Most extrasolar planets are Jupiter-like gas giants. The planets similar in mass and size to Earth are either orbiting remnants of stars that exploded or, in the case of Gliese 581, a star much less massive and much cooler than the Sun.