2020年9月19日托福阅读真题+题目+答案：Planetary Formation

2020年9月19日托福阅读真题+题目+答案：Planetary Formation

According to the condensation theory of planet formation, planets form out of a spinning disk of gas that surrounds a newborn star known as a protoplanetary disk. ⬛ The disk and star both originate in a rotating, collapsing cloud of material, and this process of collapse produces different abundances of materials in the disk at different distances from the star.⬛ In the higher temperature regions, comparable to the region around the planet Mercury in our solar system, the only kinds of material that can condense from the gas to the solid state (in this case, microscopic dust grains) are metals. ⬛Farther out, about where Venus, Earth, and Mars are now, the gas temperatures are lower. At these distances and temperatures, rocky materials such as silicates can also begin to form dust grains. ⬛Even farther out, the temperature gets low enough for water ice to form, and even farther from the star, ices of other compounds such as ammonia and methane can condense But how do young planetary systems go from making dust grains to making planets?

1.Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

O It is easiest for metals to condense from the gas to the solid state in higher temperature regions of the solar system.

O The only place in our solar system where metals can condense to form microscopic dust grains is in the region around the planet Mercury.

O Metals are the only materials that can condense to the solid state in the higher temperature regions of a solar system.

O The planet Mercury in our solar system formed from metals that condensed from the gas to the solid state.

2.According to the theory of planet formation discussed in paragraph 1, all of the following are true about the materials contained within the collapsing cloud EXCEPT:

O They form both a disk and a newborn star.

O As the cloud collapses, materials are more or less abundant at different locations.

O Which materials become solid and condense is determined by how far away the star is.

O Different materials present in the cloud result in differences in temperature in different parts of the cloud.

"The answer to that question, "explains astronomer David Jewitt, "is a process called binary accretion, where collisions between pairs of objects let larger and larger structures get put together. Collisions between grains, which are small and sticky, quickly lead to the construction of pebbles. Collisions between pebbles lead to rocks. Collisions between rocks lead to boulders. Collisions between boulders lead to planetesimals, rocky bodies the size of asteroids (bodies that orbit between Mars and Jupiter and range in size up to about 1,000 kilometers). If this accretion process happens far enough from the star, significant quantities of ices will be included in the planetesimals. This is the likely origin of comets. Eventually, the objects are large enough for gravity to begin compressing and heating the planetesimal interiors. As planetesimals grow even larger, gravity pulls them into a spherical shape, and the heaviest elements sink to the center of the body. Iron and nickel will, in this way, form the dense metallic cores of the young planets Eventually, full-sized terrestrial planets, the rocky cores of gas giants(such as Jupiter and Saturn), and moons are formed through the accretion process.

3.Paragraph 2 suggests that which of the following is necessary for the formation of a comet?

O A collision significant enough to remove the ice from a planetesimal

O The formation of a planetesimal at a great distance from a star

O Heavy elements sinking to the center of a planetesimal

O The presence of full-size terrestrial planets

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