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Chapter 5

What are the main characteristics of our solar system?

four small planets close to the Sun and four large planets far from the Sun

The formation of terrestrial-type planets around a star is most likely to have occurred by what process?

accretion, or slow accumulation of smaller particles by mutual gravitational attraction

The average density of the large, outer planets is

close to the density of water.

What fraction of the mass of Earth is made up of the elements hydrogen and helium?

much less than 1%

At what point in time do we say that the protosun became the Sun?

when thermonuclear fusion reactions began at its center

Most of the mass of the universe—98%—is in the form of

hydrogen and helium

The region at the farthest limits of the solar system in which a large number of objects composed of rock and ice circle the Sun in a roughly spherical region is called the

Oort comet cloud

The average density of a planet is

its total mass divided by its total volume

Which planets do not have any natural satellites?

Mercury and Venus

The planet with the greatest mean density is


How do we measure the mass of an extrasolar planet?

We use Newton's law of gravity, using the measured distance of the planet from its star and the planet's gravitational pull on the star

What is the basic difference between comets and asteroids?

Comets are mostly composed of ices, whereas asteroids are mainly composed of rocks

In our solar system, which of the following planets is not a member of the terrestrial group?


Which of the following characteristics is not typical of our planetary system?

The physical size of most of the planets is about the same

Pluto is a 2:3 resonance orbit with Neptune. What does this mean?

During the time Neptune orbits the Sun three times, Pluto orbits it twice

Fusion is the process by which

elements are transformed into heavier elements by nuclear reactions.

The largest of the terrestrial planets is


The asteroid belt exists between the orbits of which planets?

Mars and Jupiter

The smallest planet in our solar system is


A curious fact about the structure of the planet Jupiter, compared with the structure of Earth, is that it has

much greater mass but much lower average density

At the present time, the discoveries of how many extrasolar planets have been confirmed?


Viewed from a position above the north pole of the Sun, the direction of the motion of the planets in their orbits is


What is surprising about the extrasolar planets that have so far been discovered?

Many of the extrasolar planets are giant planets like Jupiter, orbiting at distances characteristic of terrestrial planets like Earth, where giant planets cannot form

What is the Kuiper belt?

relatively flat distribution of objects in the plane of the ecliptic, extending from around the orbit of Pluto out to about 50 AU from the Sun

Accretion is the process by which

dust grains and ice crystals coalesce to form planetesimals

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