Page 321 - ABCTE Study Guide_Neat

major group of planets. Being similar to Jupiter, they are called Jovian planets. They are basically big balls
of gas, so they are often referred to as gas giants. Pluto does not fit into this classification
scheme. Pluto has properties more like the major moons of the Jovian planets, and in 2006 was
reclassified as a dwarf planet.

Astronomers think that the solar system formed when a large cloud of gas and dust began to collapse.
Such a cloud is called a nebula, so this theory is called the nebular theory. Once the collapse starts,
gravity pulls the cloud particles closer together and toward the center. As it collapses, it begins to spin
faster and flatten. Most of the matter was used to form the Sun, and a small amount of the material in the
flattened disk spun into eddies and formed the planets. In the inner part of the solar system, the Sun's
heat evaporated most of the hydrogen and helium and other gases. Hence these inner planets are mostly
metal and rock, while the outer planets are, mostly gases such as hydrogen, helium, ammonia, and
methane. Because planets are natural byproducts of the formation of stars, most stars should have
planets orbiting them.

As large as it may seem to us, our solar system is on a cosmic scale quite small. Our Sun is one of
hundreds of billion stars in our Milky Way galaxy. Our galaxy is only one of an unknown number of billions
of galaxies in the universe.

Question

Which best describes the relative size and composition of terrestrial planets compared to Jovian planets?

A Larger, composed of rock and metal

B Smaller, composed of rock and metal

C Larger, composed of gas

D Smaller, composed of gas

Answer

Answer B is correct. Remember that the four Jovian planets are larger and primarily made up of gas. The
Jovian planets are: Jupiter, Saturn, Uranus, and Neptune. The terrestrial planets are: Mercury, Venus,
Earth, and Mars.

The Earth Makes Its Closest Approach to the Sun in mid-January, So Why Is That the Coldest
Time of the Year?

Although it's counter-intuitive, the changing distance between the Sun and Earth does not cause the
seasons. The earth does follow an elliptical orbit around the Sun, but it is close enough to being circular
that the difference in distance cannot cause the seasons.

The tilt of the Earth's axis, and the revolution of the earth around the sun, causes the seasons. As the
Earth orbits the Sun, the tilt of Earth's rotational axis causes an apparent north-south movement of the
Sun in the sky. On the summer solstice, June 21, Earth's northern hemisphere is tilted 23.5° towards the