Chapter 2: Space

The illustration shows the Milky Way galaxy as it might appear if observed from a distance. How does the Milky Way compare to other galaxies of the Universe?

• The Milky Way is the largest galaxy.

• The Milky Way is the only galaxy with a black hole at its center.

• The Milky Way has a unique spiral shape.

• The Milky Way includes Earth, but otherwise it is similar to other galaxies.

Answer explanation

The Milky Way includes Earth and shares characteristics with other galaxies, such as containing stars, gas, and dust. While it has a spiral shape, many other galaxies also exhibit similar structures.

The universe contains a huge number of galaxies, and scientists continue to research a precise number. Which best describes a galaxy?

• a star and the planets in orbit around it

• a pattern of stars that appears in the night sky

• a huge system of stars and other objects that are held together by gravity

• a collection of gases that gravity may collapse into a star over time

Answer explanation

A galaxy is best described as a huge system of stars and other objects that are held together by gravity, distinguishing it from individual stars or patterns seen in the night sky.

The four inner planets are often called terrestrial planets, meaning they are like Earth. How are the four inner planets similar to one another?

• They all have oceans composed of liquid water.

• They all have surfaces made of rock.

• They all have similar average temperatures.

• They all are surrounded by an oxygen-rich atmosphere.

Answer explanation

The four inner planets—Mercury, Venus, Earth, and Mars—are all terrestrial, meaning they have rocky surfaces. Unlike the other options, which are incorrect, this characteristic is a defining feature of these planets.

The Sun provides light and heat to the planets of the solar system. How else does the Sun affect the planets?

• Solar storms provide the energy for volcanic eruptions.

• Gases from the Sun supply the atmospheres of the planets.

• Magnetic forces from the Sun hold the planets in their orbits.

• Gravity from the Sun holds the planets in their orbits.

Answer explanation

The correct answer is that gravity from the Sun holds the planets in their orbits. This gravitational force is essential for maintaining the stability of the solar system, keeping the planets in their respective paths around the Sun.

The artistic rendition shows a NASA rover on the surface of Mars. Why would rovers be a poor choice for studying the outer planets, such as Saturn and Jupiter?

• The outer planets are made of gas, so a rover could not roll across them.

• The outer planets have many volcanoes, so a rover could be buried in melted rock.

• The outer planets have no atmosphere, so a rover could not function properly.

• The outer planets are far from the Sun, so a rover might freeze in the cold temperatures.

Answer explanation

Rovers are designed to traverse solid surfaces, but the outer planets like Saturn and Jupiter are primarily composed of gas. Therefore, a rover would have no solid ground to roll across, making it ineffective for exploration.

How are asteroids different from the planets of the solar system?

• Asteroids are made of rock, water, ice, and gases.

• Asteroids may approach the Sun, but they do not orbit around the Sun.

• Asteroids move in orbits around a planet, not around the Sun.

• Asteroids are smaller than planets and typically have irregular shapes.

Answer explanation

Asteroids differ from planets primarily in size and shape. They are generally much smaller than planets and often have irregular shapes, unlike the round shapes of planets.

Earth is one of eight planets of the solar system. Which description best explains the position of Earth among the planets?

• A. Earth is the planet closest to the Sun.

• B. Earth is the largest planet within the asteroid belt.

• C. Earth is the third of the inner planets in order of closeness to the Sun.

• D. Earth is the first of the outer planets in order of closeness to the Sun.

Answer explanation

Earth is the third planet from the Sun, following Mercury and Venus, making option C the correct choice. It is not the closest (A), not the largest within the asteroid belt (B), and not an outer planet (D).

Until 2006, scientists described Pluto as an unusual planet. Then they classified Pluto as a dwarf planet, a new category of solar system objects. Which was the most likely reason that scientists changed their classification of Pluto?

• A. Pluto does not orbit around the Sun.

• B. Pluto is more like other objects that are not planets.

• C. Pluto is too distant to receive much light or heat from the Sun.

• D. Pluto is made entirely of gas, not rock.

Answer explanation

Scientists reclassified Pluto as a dwarf planet because it shares more characteristics with other small celestial objects, rather than fitting the traditional definition of a planet, which includes clearing its orbit.