Discovering the UniverseNinth Edition

Neil F. Comins • William J. Kaufmann III

CHAPTER 5Formation of the Solar Systemand Other Planetary Systems

A montage of the planets in our solar system presented in correct relative sizes. The orbits in the background are also drawn to scale.

WHAT DO YOU THINK?1. How many stars are there in the solar

system?2. Were the Sun and planets among the

first generation of objects created in the universe?

3. How long has Earth existed, and how do we know this?

4. What typical shape(s) do moons have, and why?

5. Have any Earthlike planets been discovered orbiting Sunlike stars?

In this chapter you will discover… how the solar system formed why the environment of the early solar system was much

more violent than it is today how astronomers define the various types of objects in

the solar system the relationships between planets, dwarf planets, small

solar system bodies, and other classifications of objects in the solar system

how the planets are grouped as they are how the moons formed throughout the solar system what the debris of the solar system is made of that disks of gas and dust, as well as planets, have been

observed around a growing number of stars that newly forming stars and planetary systems are

being discovered every year

How Stars Lose Mass

Antares is nearing the end of its existence. Strong winds from its surface are expelling large quantities of gas and dust, creating this nebula.

The “planetary nebula” Abell 39 exhibits a relatively gentle emission of matter; the central star shed its outer layers of gas and dust in an expanding spherical shell now about 6 ly across.

The Crab Nebula: A supernova is the most powerful known mechanism for a star to shed mass.

Dusty Regions of Star Formation

(a) These three bright young stars in the constellation Monoceros are still surrounded by much of the gas and dust from which they formed. This is a tiny part of a much larger cloud, known as the Cone Nebula. Astronomers hypothesize that the solar system formed from a similar fragment of an interstellar gas and dust cloud. (b) These are newly formed stars in the Orion Nebula. Although visible light from many of the stars is blocked by the nebula, their infrared emission travels through the gas and dust to us.

The Formation of the Solar System

Young Circumstellar Disks of Matter

Orion Nebula with Hubble Space Telescope. The four insets: false-color images of protoplanetary disks within the nebula. A newborn star is at the center of each disk.

This computer simulation shows the formation of the inner planets over time.

The Nice Model and the Outer Solar System

The outer planets formed much closer to the sun. Flinging material into the inner solar system caused them to spiral outward Later, they sent rocky and icy material out to the outer reaches of the solar system to stop their outward motion. This resulted in the formation of the Kuiper belt and Oort cloud.

Asteroid Gaspra, taken in 1991 by the Galileo spacecraft on its way to Jupiter. The asteroid measures 12 x 20 x 11 km. Millions of similar chunks of rock orbit the Sun between the orbits of Mars and Jupiter. Even smaller rocky bodies called meteoroids are scattered throughout the solar system.

Thousands of lunar craters were produced by impacts of leftover rocky debris from the formation of the solar system. Age-dating of lunar rocks brought back by the astronauts indicates that the Moon is about 4.5 billion years old. Most of the lunar craters were formed during the Moon’s first 700 million years of existence.

Here are classifications of solar system objects. A planet is an object that1) orbits the sun; 2) has enough mass so that its

own gravitational attraction causes it to be essentially spherical;

3) has enough gravitational attraction to clear its neighborhood of other orbiting debris.

A dwarf planets fulfills conditions (1) and (2), but not (3).A small solar system object only fulfills (1).

Different Classifications of Solar System Objects

Planetary orbits around the Sun, to scale. All orbits are counterclockwiseas viewed from above Earth’s North Pole. The orbits appear nearly circular, with Mercury having the most elliptical orbit.Edge view shows that all orbits are nearly in the ecliptic plane.

The Sun and the planets with size drawn to scale . The four planets that orbit nearest the Sun (Mercury, Venus, Earth, and Mars) are small and made of rock and metal. The next two planets (Jupiter and Saturn) are large and composed primarily of hydrogen and helium. Uranus and Neptune are intermediate in size and contain roughly equal amounts of ices, hydrogen and helium, and terrestrial material.

All of the objects in this image have the same mass (total number of particles). However, the chemicals from which they formhave different densities (number of particles per volume), so they each take up different amounts of space (volume).

A Circumstellar Disk of Matter

Hubble view of Beta Pictoris, with an edge-on disk of material 225 billion km (1500 AU) across that orbits the star 50 ly from Earth. Twenty million years old, this disk is believed to be composed primarily of icy bodies that orbit the star. The smaller disk is believed to have been formed by the gravitational pull of a roughly Jupiter-mass planet in that orbit.

The star Fomalhaut, blocked out, is surrounded by gas and dust in a ring whose center is separated from the star by 15 AU, nearly as far as Uranus is from the Sun. This offset is believed to be due to the gravitational effects of a giant planet orbiting Fomalhaut. This system is 25 ly from Earth. The dimmer debris in that system and between us and it scatters light that is considered “noise” in such images.

Off-Center Disk

This infrared image of an almost-extrasolar planet was taken at the European Southern Observatory. Neither object is quite large nor massive enough to be a star, and evidence suggests that 2M1207b did not form from a disk of gas and dust surrounding the larger body; hence, it is not a planet. This system is about 170 ly from the solar system .

Three Traditional Methods of Detecting Exoplanets

(a) A planet and its star both orbit around their common center of mass, always staying on opposite sides of that point. The star’s motion around the center of mass provides astronomers with the information that a planet is present. (b) As a planet moves toward or away from us, its star moves in the opposite direction. Using spectroscopy, we can measure the Doppler shift of the star’s spectrum, which reveals the effects of the unseen planet or planets. (c) If a star and its planet are moving across the sky, the motion of the planet causes the star to orbit its center of mass. This motion appears as a wobbling of the star across the celestial sphere. (d) If a planet happens to move in a plane that takes it across its star (that is, the planet transits the star), as seen from Earth, then the planet will hide some of the starlight, causing the star to dim. This change in brightness will occur periodically and can reveal the presence of a planet.

This figure shows the separation between extrasolar planets and their stars. Note that many systems have giant planets that orbit much closer than 1 AU from their stars. (MJ is shorthand for the mass of Jupiter.) For comparison, the solar system is shown at top.

Microlensing Reveals an Extrasolar Planet

Gravitational fields cause light to change direction. The deflection itself is too small to be detectable, but it does increase detectably the apparent brightness of the distant star. A planet makes it non-syymetric.

A Star with Three Planets

The star Upsilon Andromedae has at least three planets, discovered by measuring the complex Doppler shift of the star. This star system is located 44 ly from Earth, and the planets all have masses similar to Jupiter’s. (b) The orbital paths of the planets, labeled B, C, and D, along with the orbits of Venus, Earth, and Mars, are drawn for comparison.

Summary of Key Ideas

Formation of the Solar System Hydrogen, helium, and traces of lithium, the three

lightest elements, were formed shortly after the formation 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 planets and other debris in the solar system today formed from gas, ice, and dust in the solar nebula orbiting the protosun.

The outer solar system, beyond the snow line, had both dust and ice (including hydrogen and helium), while inside the snow line, such ices were vaporized by the protosun.

Formation of the Solar System Jupiter and Saturn were initially worlds of rock and metal

that pulled onto themselves large amounts of hydrogen and helium, along with some water.

Uranus and Neptune were also initially worlds of rock and metal, but they attracted more water and less hydrogen and helium than the other giant planets.

The Nice model of solar system formation proposes that in the outer solar system, Jupiter formed first, followed by Saturn, and then by Neptune and Uranus, which were flung out to their present orbits by gravitational forces from Jupiter and Saturn.

The four inner planets formed through the collisions of Moon-sized bodies, probably after the outer four planets were formed.

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.

For 800 million years after the Sun formed, impacts of asteroidike objects on the young planets dominated the history of the solar system.

Categories of Solar System Objects Astronomical objects smaller than the eight planets are

classified as dwarf planets or small solar system bodies (SSSBs).

A variety of other names, including asteroids, comets, meteoroids, trans-Neptunian objects, plutinos, plutoids, Kuiper belt objects (KBOs), and Oort cloud objects, overlap with the designations “dwarf planet” and “SSSB”.

KBOs and Oort cloud objects are trans-Neptunian objects—they orbit farther from the Sun than the outermost planet.

To date, five objects—Pluto, Ceres, Eris, Haumea, and Makemake—have been classified as dwarf planets.

Other objects orbit the Sun beyond Neptune. At least 1500 KBOs have been observed. A few potential Oort cloud objects have also been identified.

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, 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.

Comparative Planetology Pluto, once considered the smallest planet, has a size,

density, and composition consistent with other large Kuiper belt objects (KBOs).

Asteroids are rocky and metallic debris in the solar system, are larger than about 10 m in diameter, and are 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.

At least 506 exoplanets have been discovered orbiting other stars.

Most of the exoplanets that have been discovered have masses roughly equal to the mass of Jupiter.

Exoplanets are discovered indirectly as a result of their effects on the stars they orbit.

Key Termsaccretionalbedoasteroidasteroid beltaverage densitycometcraterdense coredwarf planetJeans instabilityKuiper beltKuiper belt object (KBO) meteoroidmetals

microlensingmoon (natural satellite)Nice modelOort cloudorbital inclinationplanetplanetesimalprotoplanetary disks (proplyds)protosunsmall solar system body (SSSB) solar nebulasolar systemterrestrial planettrans-Neptunian object (TNO)

WHAT DID YOU THINK? How many stars are there in the solar system? One, the Sun.

WHAT DID YOU THINK? 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.

WHAT DID YOU THINK? 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.

WHAT DID YOU THINK? 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, which pulls 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? Not really. Most exoplanets are Jupiterlike 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.