Oct. 25th - Planet Formation

Question 1

Four criteria for the success of a solar system formation theory:

Answer 1

1. It must explain the patterns of planetary motion discussed in Chapter 7.
2. It must explain why planets fall into two major categories: small, rocky terrestrial planets near the Sun and large, hydrogen-rich jovian planets farther out.
3. It must explain the existence of huge numbers of asteroids and comets and why these objects reside primarily in the regions we call the asteroid belt, the Kuiper belt, and the Oort cloud.
4. It must explain the general patterns while at the same time making allowances for exceptions to the general rules, such as the odd axis tilt of Uranus and the existence of Earth’s large Moon.

Question 2

Timeline of the solar system formation hypotheses: Kant

Answer 2

1755: Kant proposed that our solar system formed from the gravitational collapse of an interstellar cloud of gas. Because an interstellar cloud is usually called a nebula (Latin for “cloud”), this idea became known as the nebular hypothesis
Theory popular until the 19th century (scientists had found a few aspects of our solar system that the hypothesis did not seem to explain well)

Question 3

Timeline of the solar system formation hypotheses: 20th Century

Answer 3

• 20th century: the nebular hypothesis faced competition from a hypothesis proposing that the planets represent debris from a near-collision between the Sun and another star.
• According to this close encounter hypothesis, the planets formed from blobs of gas that had been gravitationally pulled out of the Sun during the near-collision.
• It began to lose favor when calculations showed that it could not account for either the observed orbital motions of the planets or the neat division of the planets into two major categories (terrestrial and jovian).
• Moreover, the close encounter hypothesis required a highly improbable event: a near-collision between our Sun and another star. Given the vast separation between star systems in our region of the galaxy, the chance of such an encounter is so small that it would be difficult to imagine it happening

Question 4

Which hypothesis of solar system formation did scientists favour?

Answer 4

Using more sophisticated models of the processes that occur in a collapsing cloud of gas, scientists found that the nebular hypothesis offered natural explanations for all four general features of our solar system.

Indeed, so much evidence accumulated in favor of the nebular hypothesis that it achieved the status of a scientific theory —the nebular theory of our solar system’s birth.

Question 5

Putting Theories to the Test

In the case of a theory that claims to explain the origin of our solar system, one critical set of tests involves its ability to…

Answer 5

…predict and explain the characteristics of other solar systems

Question 6

The discovery of other planetary systems means the nebular theory passed its most important test:

Answer 6

Because it claims that planets are a natural outgrowth of the star formation process, it predicts that other planetary systems ought to be common, a prediction that has been borne out by observations. As a result, the nebular theory today stands on stronger ground than ever.

Question 7

Where did the solar system come from?

The nebular theory begins with the idea that…

Answer 7

…our solar system was born from the gravitational collapse of an interstellar cloud of gas (and dust), called the solar nebula, that collapsed under its own gravity.

Question 8

Where did the solar system come from?

Where did the gas that made up the solar nebula come from?

Answer 8

• According to modern science, it was the product of billions of years of galactic recycling that occurred before the Sun and planets were born.
• Recall that the universe as a whole is thought to have been born in the Big Bang, which essentially produced only two chemical elements: hydrogen and helium.
• Heavier elements were produced later, some through the nuclear fusion that makes stars shine, and most others through nuclear reactions accompanying the explosions that end stellar lives.
• The heavy elements then mixed with other interstellar gas that formed new generations of stars

Question 9

Where did the solar system come from?

Contents of the original Solar Nebula gas:

Answer 9

• The gas that made up the solar nebula contained (by mass) about 98% hydrogen and helium and 2% all other elements combined.
• The Sun and planets were born from this gas, and Earth and the other terrestrial worlds were made primarily from the heavier elements mixed within it.

Question 10

Explaining the major features of the solar system:

What caused the orderly patterns of motion?
How did the collapse begin?

Answer 10

• The solar nebula probably began as a large and roughly spherical cloud of very cold, low-density gas
• Initially, this material was probably so spread out—perhaps over a region a few light-years in diameter—that gravity alone may not have been strong enough to pull it together and start its collapse.
• Instead, the collapse may have been triggered by a cataclysmic event, such as the impact of a shock wave from the explosion of a nearby star (a supernova).

Question 11

Explaining the major features of the solar system:

What caused the orderly patterns of motion?
How did the collapse begin? - NEWTON’S LAWS

Answer 11

• Once the collapse started, gravity enabled it to continue. Recall that the strength of gravity follows an inverse square law with distance
• The mass of the cloud remained the same as it shrank, so the strength of gravity increased as the diameter of the cloud decreased.
• Because gravity pulls inward in all directions, you might at first guess that the solar nebula would have remained spherical as it shrank. Indeed, the idea that gravity pulls in all directions explains why the Sun and the planets are spherical. However, other physical laws also apply, and these explain how orderly motions arose in the solar nebula.

Question 12

Heating, spinning, and flattening:

Three key processes that altered the solar nebula’s density, temperature, and shape as it shrank:

Answer 12

1. Heating
2. Spinning
3. Flattening

Question 13

3 processes that altered the solar nebula as it shrank:

Heating

Answer 13

The temperature of the solar nebula increased as it collapsed. Such heating represents energy conservation in action.

As the cloud shrank, its gravitational potential energy was converted to the kinetic energy of individual gas particles falling inward.

These particles crashed into one another, converting the kinetic energy of their inward fall to the random motions of thermal energy. The Sun formed in the center, where temperatures and densities were highest.

Question 14

3 processes that altered the solar nebula as it shrank:

Spinning

Answer 14

Like an ice skater pulling in her arms as she spins, the solar nebula rotated faster and faster as it shrank in radius.

This increase in rotation rate represents conservation of angular momentum in action. The rotation of the cloud may have been imperceptibly slow before its collapse began, but the cloud’s shrinkage made fast rotation inevitable.

The rapid rotation helped ensure that not all the material in the solar nebula collapsed into the center: The greater the angular momentum of a rotating cloud, the more spread out it will be.

Question 15

3 processes that altered the solar nebula as it shrank:

Flattening

Answer 15

This flattening is a natural consequence of collisions between particles in a spinning cloud. A cloud may start with any size or shape, and different clumps of gas within the cloud may be moving in random directions at random speeds. These clumps collide and merge as the cloud collapses

Question 16

How did the formation of the spinning disk explain orderly motions?

Answer 16

The planets all orbit the Sun in nearly the same plane because they formed in the flat disk.

Though the small sizes of planets compared to the entire disk allowed some exceptions to arise. The fact that collisions in the disk tended to make orbits more circular explains why the planets in our solar system have nearly circular orbits.

Question 17

Testing the model

The heating that occurs in a collapsing cloud of gas means the gas should emit thermal radiation, primarily in the infrared

Answer 17

More direct evidence comes from flattened, spinning disks around other stars, some of which appear to be ejecting jets of material perpendicular to their disks.

These jets are thought to result from the flow of material from the disk onto the forming star, and they may influence the solar system formation processes

Question 18

Testing the model

Other support for the model comes from…

Answer 18

…computer simulations of the formation process
* A simulation begins with a set of conditions observed in interstellar clouds.
* Then, with the aid of a computer, we apply the laws of physics to predict the changes that should occur over time.

Question 19

Testing the model

Additional evidence that our ideas about the formation of flattened disks are correct comes from…

Answer 19

…many other structures in the universe.

We expect flattening to occur anywhere orbiting particles can collide, which explains why we find so many cases of flat disks, including the disks of spiral galaxies like the Milky Way, the disks of planetary rings, and the accretion disks that surround neutron stars and black holes in close binary star systems

Question 20

2 major types of planets:

The churning and mixing of the gas in the solar nebula should have ensured that the nebula had the same composition throughout.

How, then, did the terrestrial planets end up so different in composition from the jovian planets?

Answer 20

The key clue comes from their locations:
* Terrestrial planets formed in the warm inner regions of the swirling disk
* Jovian planets formed in the colder outer regions.

Question 21

Condensation: sowing the seeds of planets

In the center of the collapsing solar nebula, gravity drew together enough material to form the Sun.
In the surrounding disk, however…

Answer 21

…the gaseous material was too spread out for gravity alone to clump it together.
* Instead, material had to begin clumping in some other way and then grow in size until gravity could start pulling it together into planets.
* In essence, planet formation required the presence of “seeds”—solid bits of matter from which gravity could ultimately build planets.

Question 22

Condensation: sowing the seeds of planets

The basic process of seed formation - what is it called?

Answer 22

• Much like the formation of snowflakes in clouds on Earth: When the temperature is low enough, some atoms or molecules in a gas may bond and solidify.
• The general process in which solid (or liquid) particles form in a gas is called condensation—we say that the particles condense out of the gas

Question 23

The ingredients of the solar nebula fell into four major categories:

Answer 23

• Hydrogen and helium gas (98% of the solar nebula). These gases never condense in interstellar space.
• Hydrogen compounds (1.4% of the solar nebula). Materials such as water (H2O), methane (CH4), and ammonia (NH3) can solidify into ices at low temperatures (below about 150 K under the low pressure of the solar nebula).
• Rock (0.4% of the solar nebula). Rocky material is gaseous at high temperatures, but condenses into solid form at temperatures below 500 K to 1300 K, depending on the type of rock.
• Metal (0.2% of the solar nebula). Metals such as iron, nickel, and aluminum are also gaseous at high temperatures, but condense into solid form at temperatures below 1000 K to 1600 K, depending on the metal.

Question 24

Because hydrogen and helium gas made up about 98% of the solar nebula’s mass and did not condense, the vast majority of the nebula…

Answer 24

…remained gaseous at all times.
* However, other materials could condense wherever the temperature allowed

Question 25

Further out and within the frost line…

Answer 25

A little farther out (near Mercury’s current orbit), it was cool enough for metals and some types of rock to condense into tiny solid particles, but other types of rock and all the hydrogen compounds remained gaseous.
* More types of rock could condense at the distances from the Sun where Venus, Earth, and Mars would form

Question 26

Frost Line

Answer 26

• In the region where the asteroid belt would eventually be located, temperatures were low enough to allow dark carbon-rich minerals to condense, along with minerals containing small amounts of water.
• It was cold enough for hydrogen compounds to condense into ices only beyond the frost line, which lay between the present-day orbits of Mars and Jupiter

Question 27

Inside the frost line versus beyond:

Answer 27

• Inside the frost line, only metal and rock could condense into solid “seeds.”
• Beyond the frost line, the solid seeds were built of ice along with metal and rock. Moreover, because hydrogen compounds were nearly three times as abundant in the nebula as metal and rock combined, the total amount of solid material was far greater beyond the frost line than within it

Question 28

Building the terrestrial planets

How did small seeds grow into planets?

Answer 28

The solid seeds of metal and rock in the inner solar system ultimately grew into the terrestrial planets we see today, but these planets ended up relatively small in size because rock and metal made up such a small amount of the material in the solar nebula.

Question 29

How did small seeds grow into planets? - ACCRETION

Answer 29

The process by which small “seeds” grew into planets is called accretion.
* Accretion began with the microscopic solid particles that condensed from the gas of the solar nebula.
* These particles orbited the forming Sun with the same orderly, circular paths as the gas from which they condensed
* Individual particles therefore moved at nearly the same speed as neighboring particles, so “collisions” were more like gentle touches

Question 30

Planetesimals

Answer 30

Although the particles were far too small to attract each other gravitationally at this point, they were able to stick together through electrostatic forces—the same “static electricity” that makes hair stick to a comb

Small particles thereby began to combine into larger ones. As the particles grew in mass, they began to attract each other through gravity, accelerating their growth into boulders large enough to count as planetesimals, which means “pieces of planets.

Question 31

Planetesimals - Destructions

Answer 31

• With different orbits crossing each other, collisions between planetesimals tended to occur at higher speeds and hence became more destructive. Such collisions tended to shatter planetesimals rather than help them grow. It therefore took some combination of large size and luck (in not suffering major collisions) for a planetesimal to be able to continue its growth.
• The fact that our solar system contains only four terrestrial planets today (Mercury, Venus, Earth, and Mars) tells us that only four of the inner solar system’s large planetesimals survived to become planets.

Question 32

Making the Jovian planets

Accretion should have occurred similarly in the outer solar system, but condensation of ices meant both that there was more solid material and that this material contained ice in addition to metal and rock.

What is the leading Jovian planet formation theory?

Answer 32

• The leading model for jovian planet formation holds that the largest ice-rich planetesimals became sufficiently massive for their gravity to capture some of the hydrogen and helium gas that made up the vast majority of the surrounding solar nebula.
• This added gas made their gravity even stronger, allowing them to capture even more gas. Ultimately, the jovian planets accreted so much gas that they bore little resemblance to the icy seeds from which they grew.
• Each jovian planet came to be surrounded by its own disk of gas, spinning in the same direction as the planet rotated
• Moons that accreted from ice-rich planetesimals within these disks ended up with nearly circular orbits going in the same direction as their planet’s rotation and lying close to their planet’s equatorial plane. Recent observations seem to confirm that moons can form in this way.

Question 33

Clearing the nebula:

The vast majority of the hydrogen and helium gas in the solar nebula never became part of any planet. So what happened to it? WHAT DID THIS MEAN FOR PLANET FORMATION?

Answer 33

Apparently, it was cleared away by a combination of high-energy radiation (ultraviolet and x-rays) from the young Sun and the solar wind—a stream of charged particles (such as protons and electrons) continually blown outward in all directions from the Sun.

The clearing of the gas sealed the compositional fate of the planets. If the gas had remained longer, it might have continued to cool until hydrogen compounds could have condensed into ices even in the inner solar system.

In that case, the terrestrial planets might have accreted abundant ice, and perhaps hydrogen and helium gas as well, changing their basic nature.

At the other extreme, if the gas had been blown out earlier, the raw materials of the planets might have been swept away before the planets could fully form.

Question 34

The clearing of the nebula also helps explain what was once considered a surprising aspect of the Sun’s rotation: If the young Sun really did rotate fast, as the nebular theory seems to demand, how did its rotation slow down?

Answer 34

• Angular momentum cannot simply disappear, but it can be transferred from one object to another—and then the other object can be pushed away
• The young Sun’s rapid rotation would have generated a magnetic field far stronger than that of the Sun today, which in turn would have led to the strong solar wind and to strong surface activity (such as large sunspots and frequent solar flares) that would explain the young Sun’s intense emission of ultraviolet and x-ray radiation.
• This high-energy radiation ionized gas in the solar nebula, creating many charged particles, while the magnetic field swept through the nebula with the Sun’s rapid rotation
• Because charged particles and magnetic fields tend to move together, the magnetic field dragged the charged particles along faster than the rest of the nebula, effectively slowing the Sun’s rotation and transferring some of the Sun’s angular momentum to the nebula.
• When the nebula was cleared into interstellar space, the gas carried the angular momentum away, leaving the Sun with the greatly diminished angular momentum and slow rotation that we see today

Question 35

Where did asteroids and comets come from?

The process of planet formation also explains the origin of the many asteroids and comets in our solar system (including those large enough to qualify as dwarf planets):

Answer 35

• They are “leftovers” from the era of planet formation
• The asteroids and comets that exist today probably represent only a small fraction of the leftover planetesimals that roamed the young solar system. The rest are now gone.
• Some of these “lost” planetesimals may have been flung into deep space by gravitational encounters, but many others must have collided with the planets.

Question 36

When impacts occur on solid worlds, they leave behind…

Answer 36

…impact craters as scars.
* Impacts have thereby transformed planetary landscapes and, in the case of Earth, altered the course of evolution.

Question 37

Heavy Bombardment Period

Answer 37

Although impacts occasionally still occur, the vast majority of these collisions occurred in the first few hundred million years of our solar system’s history

Question 38

How, then, did Earth come to have the water that makes up our oceans and the gases that first formed our atmosphere?

Answer 38

The likely answer is that water, along with other hydrogen compounds, was brought to Earth and other terrestrial planets by the impacts of water-bearing planetesimals that formed farther from the Sun.

As these planetesimals became part of the forming Earth, their gaseous content became trapped on or within our planet.

Recent evidence (based on ratios of deuterium to ordinary hydrogen) suggests that most of the planetesimals that brought water came from the outer portion of the asteroid belt: these planetesimals formed beyond the frost line and therefore contained small amounts of ice

Question 39

How do we explain “exceptions to rules”

Captured moons: how do we explain moons with less orderly orbits, such as those that go in the “wrong” direction (opposite their planet’s rotation) or that have large inclinations to their planet’s equator?

Answer 39

These moons are probably leftover planetesimals that originally orbited the Sun but were then captured into planetary orbit.

It’s not easy for a planet to capture a moon. An object cannot switch from an unbound orbit (for example, an asteroid whizzing by Jupiter) to a bound orbit (for example, a moon orbiting Jupiter) unless it somehow loses orbital energy

Question 40

How can planetesimals get caught up in an orbit?

Answer 40

For the jovian planets, captures probably occurred when passing planetesimals lost energy to friction in the extended and relatively dense gas that surrounded these planets as they formed.

The planetesimals would have been slowed by friction with the gas, just as artificial satellites in low orbits are slowed by drag from Earth’s atmosphere.

If friction reduced a passing planetesimal’s orbital energy enough, the planetesimal could have become an orbiting moon.

Because of the random nature of the capture process, captured moons would not necessarily orbit in the same direction as their planet or in its equatorial plane.

Question 41

Giant Impacts:

How can we NOT explain the presence of Earth’s moon?

Answer 41

• Capture processes cannot explain our own Moon, because it is much too large to have been captured by a small planet like Earth.
• We can rule out the possibility that our Moon formed simultaneously with Earth, because if both had formed together, they would have accreted from planetesimals of the same type and would therefore have approximately the same composition and density.
• But this is not the case: The Moon’s density is considerably lower than Earth’s, indicating that it has a very different average composition.

Question 42

Today, the leading hypothesis suggests that the moon formed as the result of…

Answer 42

…a giant impact between Earth and a huge planetesimal.
* The giant impact hypothesis holds that a Mars-size object hit Earth at a speed and angle that blasted Earth’s outer layers into space.
* A recently proposed variation on the giant impact model suggests that the giant impact might have melted and vaporized our entire planet, forming a spinning, donut-shaped cloud of vaporized rock. In this model, the Moon forms rapidly (within hundreds of years) in the outer part of this cloud, while the Earth more gradually re-forms in the center of the cloud.

Question 43

Strong support for the giant impact hypothesis comes from two features of the Moon’s composition.

Answer 43

• First, the Moon’s overall composition is quite similar to that of Earth’s outer layers—just as we would expect if it were made from material blasted away from those layers.
• Second, the Moon has a much smaller proportion of easily vaporized ingredients (such as water) than Earth. This fact supports the hypothesis because the heat of the impact would have vaporized these ingredients. As gases, they would not have participated in the subsequent accretion of the Moon.