Chapter 12

What theory describes the formation of our solar system from the collapse of a giant interstellar gas cloud?

The nebular theory.

Approximately how old is our solar system according to radiometric dating?

About 4.6 billion years.

What two elements made up almost all of the universe shortly after the Big Bang?

Hydrogen and helium.

By the time the solar system formed, what fraction of the original hydrogen and helium had been converted to heavier elements?

Roughly 2 %.

What kind of cosmic event may have triggered the collapse of the solar nebula?

A shockwave from a nearby supernova.

What physical law explains why the solar nebula spun faster as it contracted?

Conservation of angular momentum.

What process flattened the solar nebula into a disk?

Collisions among gas particles that reduced up-and-down motions.

Why do nearly all planets orbit in the same plane and direction?

They formed from a single, spinning, flattened disk of gas and dust.

What are the two main categories of planets in our solar system?

Terrestrial planets and jovian planets.

Why did two distinct planet types emerge in the nebula?

The inner disk was too hot for ices, so only rock/metal condensed there; beyond the frost line, ices could also condense.

What is the frost (ice) line?

The distance from the Sun beyond which temperatures were low enough for hydrogen compounds to freeze into ices.

How did the terrestrial planets grow?

By accretion of rocky/metal planetesimals inside the frost line.

What event ended most terrestrial planet growth?

The young Sun’s solar wind blew away remaining gas and dust.

What is differentiation inside planets?

The separation of materials by density as interior rock melts, forming metal cores and rocky mantles/crusts.

How did the jovian planets initially form?

Icy planetesimals (and possibly direct gas collapse) built large cores that then pulled in hydrogen and helium gas.

Where are most rocky asteroids found?

In the asteroid belt between Mars and Jupiter.

Why didn’t planetesimals in the asteroid belt merge into a planet?

Jupiter’s gravity stirred up their orbits, preventing accretion.

What are comets and where do they originate?

Icy leftover planetesimals that come from the Kuiper Belt and the distant Oort Cloud.

How can we distinguish Kuiper-Belt and Oort-Cloud comets by their orbits?

Kuiper-Belt comets orbit near the ecliptic in the planets’ direction; Oort-Cloud comets approach from random directions.

Roughly how massive is the Oort Cloud compared with Earth?

It may contain about 40 Earth masses of material.

What are Trojan asteroids?

Asteroids that share Jupiter’s orbit at its Lagrange points.

Define meteoroid, meteor, and meteorite.

Meteoroid: a small space rock; Meteor: the streak of light as it burns in the atmosphere; Meteorite: a fragment that reaches Earth’s surface.

Which meteorites preserve primitive solar-nebula material rich in carbon and water?

Carbonaceous chondrites.

What provides direct evidence of early condensation processes in the solar nebula?

Primitive meteorites.

How did Earth likely acquire most of its water?

Impacts of icy planetesimals and comets from the outer solar system.

What is a meteor shower and what causes one?

A cluster of meteors from a common radiant, produced when Earth passes through a comet’s debris stream.

What direct observational evidence supports the nebular theory around other stars?

Images of flattened disks of gas and dust surrounding young stars.

What is the astrometric ("wobble") method for finding exoplanets?

Detecting tiny, periodic shifts in a star’s position caused by an unseen planet’s gravity.

Why does the astrometric method favor detection of Jupiter-mass planets?

Only very massive planets induce a wobble large enough to measure easily.

What is the transit (photometry) method of planet detection?

Observing a slight dip in starlight when a planet passes in front of its star.

What planetary property can be estimated from the depth of a transit?

The planet’s size (radius).

What is the habitable zone?

The region around a star where surface liquid water could exist on a planet.

Why is the continuous habitable zone smaller than the instantaneous one for Sun-like stars?

Stars brighten over time, moving the habitable zone outward.

Which stars can have large, long-lasting habitable zones?

Low-mass stars that live 10–100 times longer than the Sun.

What was Kepler’s first planet found in its star’s habitable zone?

Kepler-22b.

What phenomenon cleared most leftover gas and dust after planet formation?

The intense solar wind of the young Sun.

How did magnetic braking affect the early Sun?

Interaction of the solar wind with magnetic fields slowed the Sun’s rotation.

How are oddities like Uranus’s tilt or Earth’s large Moon explained?

They likely resulted from giant impacts during the late stages of planet formation.

According to the giant-impact hypothesis, how did Earth’s Moon form?

Debris from a collision between Earth and a Mars-sized body coalesced into the Moon.

How does radiometric dating determine a rock’s age?

By comparing parent isotopes to their decay products, using known half-lives.

Roughly how long did it take for leftover debris to be largely cleared from the young solar system?

About 1 billion years.

How do jovian planets influence cometary orbits today?

Their gravity can fling comets into the Oort Cloud or send them toward the inner solar system.

What fraction of the solar nebula was hydrogen and helium?

About 98 % hydrogen and helium, 2 % heavier elements.

Who first proposed the nebular hypothesis of solar-system formation?

Immanuel Kant and Pierre-Simon Laplace in the 18th century.

What evidence supports that the Sun blew away remaining gas after planet formation?

Observations of young T Tauri stars with strong outflows that are clearing their disks.