One of tens of thousands of small, rocky, planetlike objects in orbit about the Sun. Also called minor planets. (Chapter 7, Chapter 15)

asteroid belt

A region between the orbits of Mars and Jupiter that encompasses the orbits of many asteroids. (Chapter 7, Chapter 15)

average density

The mass of an object divided by its volume. 

(Chapter 7)

chemical composition

A description of which chemical substances make

up a given object. (Chapter 7)

comet

dynamo

The mechanism whereby electric currents within an

astronomical body generate a magnetic field. (Chapter 7)

*escape speed

ices

impact crater

Jovian planet

*kinetic energy

Kuiper belt

A region that extends from around the orbit of Pluto to about 500 AU from the Sun where many icy objects orbit the Sun.

(Chapter 7, Chapter 14, Chapter 15)

liquid metallic hydrogen

Hydrogen compressed to such a density that it

behaves like a liquid metal. (Chapter 7, Chapter 12)

magnetometer

meteoroid

A small rock in interplanetary space. (Chapter 7, 

Chapter 15)

Oort cloud

A presumed accumulation of comets and cometary

material surrounding the Sun at distances of roughly 50,000 AU.

(Chapter 7, Chapter 15)

spectroscopy

The study of spectra and spectral lines. (Chapter 5,

Chapter 6, Chapter 7)

terrestrial planet

High-density worlds with solid surfaces, including

Mercury, Venus, Earth, and Mars. (Chapter 7)

trans-Neptunian object

Any small body of rock and ice that orbits 

the Sun within the solar system, but beyond the orbit of Neptune.

(Chapter 7, Chapter 14)

All of the planets orbit the Sun in the

same direction and in almost the same plane. Most of the planets

have nearly circular orbits.

• The four inner planets are called terrestrial planets. They are

relatively small (with diameters of 5000 to 1 3,000 km), have high

average densities (4000 to 5500 kg/m3), and are composed pri-

marily of rocky materials.

• The four giant outer planets are called Jovian planets. They

have large diameters (50,000 to 143,000 km) and low average

densities (700 to 1700 kg/m3) and are composed primarily of light

elements such as hydrogen and helium.

Besides the planets, the solar system includes satellites of the planets, asteroids, comets, and trans-Neptunian objects.

• Seven large planetary satellites (one of which is the Moon) are comparable in size to the planet Mercury. The remaining satellites of the solar system are much smaller.

• Asteroids are small, rocky objects, while comets and trans- Neptunian objects are made of ice and rock. All are remnants left over from the formation of the planets.

• Most asteroids are found in the asteroid belt between the orbits of Mars and Jupiter, and most trans-Neptunian objects lie in the Kuiper belt outside the orbit of Neptune. Pluto is one of the largest members of the Kuiper belt.

Spectroscopy, the study of spectra, provides information about the chemical composition of objects in the solar system.

• The spectrum of a planet or satellite with an atmosphere reveals the atmosphere’s composition. If there is no atmosphere, the spectrum indicates the composition of the surface.

• The substances that make up the planets can be classified as gases, ices, or rock, depending on the temperatures at which they solidify.

When an asteroid, comet, or meteoroid collides

with the surface of a terrestrial planet or satellite, the result is an impact crater.

• Geologic activity renews the surface and erases craters, so a terrestrial world with extensive cratering has an old surface and little or no geologic activity.

• Because geologic activity is powered by internal heat, and smaller worlds lose heat more rapidly, as a general rule smaller terrestrial worlds are more extensively cratered.

Planetary magnetic fields are produced by the motion of electrically conducting liquids inside the planet. This mechanism is called a dynamo. If a planet has no magnetic field, that is evidence that there is little such liquid material in the planet’s interior or that the liquid is not in a state of motion.

• The magnetic fields of terrestrial planets are produced by metals such as iron in the liquid state. The stronger fields of the Jovian planets are generated by liquid metallic hydrogen or by water with ionized molecules dissolved in it.

astrometric method (for detecting extrasolar planets)

A technique for detecting extrasolar planets by

looking for stars that “wobble” periodically. (Chapter 8)

atomic number

center of mass

The point between a star and a planet, or between two stars, around which both objects orbit. (Chapter 8, Chapter 10,

Chapter 17)

chemical differentiation

The process by which the heavier elements in a

planet sink toward its center while lighter elements rise toward its surface. (Chapter 8)

chondrule

A glassy, roughly spherical blob found within meteorites.

(Chapter 8)

condensation temperature

The temperature at which a particular

substance in a low-pressure gas condenses into a solid. (Chapter 8)

conservation of angular

momentum

core accretion model

The hypothesis that each of the Jovian planets

formed by accretion of gas onto a rocky core. (Chapter 8)

disk instability model

The hypothesis that gases in the solar nebula

coalesced rapidly to form the Jovian planets. (Chapter 8)

extrasolar planet

* half-life

interstellar medium

Gas and dust in interstellar space. (Chapter 8,

Chapter 18)

jets

Kelvin-Helmholtz contraction

meteorite

nebular hypothesis

The idea that the Sun and the rest of the solar

system formed from a cloud of interstellar material. (Chapter 8)

planetesimal

A Moon-sized object formed by the coalescence of

planetesimals. (Chapter 8)

A disk of material encircling a protostar

or a newborn star. (Chapter 8, Chapter 18)

protosun

radial velocity method (for detecting extrasolar planets)

A technique used to detect extrasolar planets by

observing Doppler shifts in the spectrum of the planet’s star. 

(Chapter 8)

radioactive dating

radioactive decay

radioactive decay The process whereby certain atomic nuclei spontaneously transform into other nuclei. (Chapter 8)

solar nebula

solar wind

T Tauri wind

A flow of particles away from a T Tauri star. 

(Chapter 8)

transit

An event in which an astronomical body moves in front of another. (Chapter 8)

transit method (for detecting

extrasolar planets)

A method for detecting extrasolar planets that come between us and their parent star, dimming the star’s light. (Chapter 8)

Models of Solar System Formation

The most successful model of the origin of the solar system is called the nebular hypothesis. According to this hypothesis, the solar system formed from a cloud of interstellar material called the solar nebula. This occurred 4.56 billion years ago (as determined by radioactive dating).

The chemical composition of the solar nebula, by mass, was 98% hydrogen and helium (elements that formed shortly after the beginning of the universe) and 2% heavier elements (produced much later in the centers of stars, and cast into space when the stars died). The heavier elements were in the form of ice and dust particles.

• The nebula flattened into a disk in which all the material orbited the center in the same direction, just as do the present-day planets.

The terrestrial planets, the Jovian planets, and the Sun followed different pathways to 

formation.

• The four terrestrial planets formed through the accretion of dust particles into planetesimals, then into larger protoplanets.

• In the core accretion model, the four Jovian planets began as rocky protoplanetary cores, similar in character to the terrestrial planets. Gas then accreted onto these cores in a runaway fashion.

• In the alternative disk instability model, the Jovian planets formed directly from the gases of the solar nebula. In this model the cores formed from planetesimals falling into the planets.

• The Sun formed by gravitational contraction of the center of the nebula. After about 10⁸ years, temperatures at the protosun’s center became high enough to ignite nuclear reactions that convert hydrogen into helium, thus forming a true star.

Astronomers have discovered planets orbiting

other stars.

• Most of these planets are detected by the “wobble” of the stars around which they orbit.

• A small but growing number of extrasolar planets have been discovered by the transit method, by microlensing, and direct imaging.

• Most of the extrasolar planets discovered to date are quite massive and have orbits that are very different from planets in our solar system.

chromosphere

A layer in the atmosphere of the Sun between the

photosphere and the corona. (Chapter 16)

CNO cycle

conduction

convection

convective zone

corona

The Sun’s outer atmosphere, which has a high

temperature and a low density. (Chapter 16)

coronal hole

A region in the Sun’s corona that is deficient in hot gases.

(Chapter 16)

An event in which billions of tons of gas from

the Sun’s corona is suddenly blasted into space at high speed. 

(Chapter 16)

differential rotation

The rotation of a nonrigid object in which parts

adjacent to each other at a given time do not always stay close together. (Chapter 12, Chapter 16)

filament

granulation

The rice grain–like structure found in the solar

photosphere. (Chapter 16)

granule

helioseismology

The study of the vibrations of the Sun as a whole.

(Chapter 16)

hydrogen fusion

The thermonuclear conversion of hydrogen into

helium. (Chapter 16)

hydrostatic equilibrium

A balance between the weight of a layer in a

star and the pressure that supports it. (Chapter 16)

limb darkening

luminosity (of the Sun)

magnetic-dynamo model

A theory that explains the solar cycle as a

result of the Sun’s differential rotation acting on the Sun’s magnetic field. (Chapter 16)

magnetogram

An image of the Sun that shows regions of different

magnetic polarity. 

(Chapter 16)

magnetic reconnection

An event where two oppositely directed magnetic fields approach and cancel, thus releasing energy. 

negative hydrogen ion

A hydrogen atom that has acquired a second

electron. (Chapter 16)

neutrino

photosphere

plage

plasma

* positron

prominence

proton-proton chain

A sequence of thermonuclear reactions by which

hydrogen nuclei are built up into helium nuclei. (Chapter 16)

radiative diffusion

The random migration of photons from a star’s

center toward its surface. (Chapter 16)

radiative zone

solar flare

solar neutrino

A neutrino emitted from the core of the Sun. 

(Chapter 16 )

solar neutrino problem

The discrepancy between the predicted and observed numbers of solar neutrinos. (Chapter 16)

spicule

A narrow jet of rising gas in the solar chromosphere. 

(Chapter 16)

sunspot

sunspot cycle

The semiregular 11-year period with which the number of sunspots fluctuates. (Chapter 16)

sunspot maximum/minimum

That time during the sunspot cycle when

the number of sunspots is highest/lowest. (Chapter 16)

supergranule

thermal equilibrium

A balance between the input and outflow of heat

in a system. (Chapter 16, Chapter 27)

thermonuclear fusion

The combining of nuclei under conditions of high

temperature in a process that releases substantial energy. (Chapter 16)

A splitting or broadening of spectral lines due to a

magnetic field. (Chapter 16)

The Sun’s energy is produced by hydrogen fusion, a sequence of thermonuclear reactions in which four hydrogen nuclei combine to produce a single helium nucleus.

• The energy released in a nuclear reaction corresponds to a slight reduction of mass according to Einstein’s equation E =mc².

• Thermonuclear fusion occurs only at very high temperatures; for example, hydrogen fusion occurs only at temperatures in excess of about 107 K. In the Sun, fusion occurs only in the dense, hot core.

A theoretical description of a star’s interior can be calculated using the laws of physics.

• The standard model of the Sun suggests that hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius.

• The core is surrounded by a radiative zone extending to about 0.71 solar radius. In this zone, energy travels outward through radiative diffusion.

• The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure. In this zone, energy travels outward primarily through convection.

Conditions in the solar interior can be inferred from measurements of solar neutrinos and of solar vibrations.

• Neutrinos emitted in thermonuclear reactions in the Sun’s core have been detected, but in smaller numbers than expected. Recent neutrino experiments explain why this is so.

• Helioseismology is the study of how the Sun vibrates. These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun.

The Sun’s atmosphere has three main layers: the photosphere, the chromosphere, and the corona. Everything below the solar atmosphere is called the solar interior.

• The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphere. Its spectrum is similar to that of a blackbody at a temperature of 5800 K. Convection in the photosphere produces granules.

• Above the photosphere is a layer of less dense but higher-temperature gases called the chromosphere. Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules.

• The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density. Activity in the corona includes coronal mass ejections and coronal holes. The solar corona blends into the solar wind at great distances from the Sun.

The Sun’s surface features vary in an 11-year cy-

cle. This is related to a 22-year cycle in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity.

• Sunspots are relatively cool regions produced by local concentrations of the Sun’s magnetic field. The average number of sunspots increases and decreases in a regular cycle of approximately 1 1  years, with reversed magnetic polarities from one 11-year cycle to the next. Two such cycles make up the 22-year solar cycle.

• The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field. These changes are caused by convection and the Sun’s differential rotation.

• A solar flare is a brief eruption of hot, ionized gases from a sunspot group. A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona.