Module 10: How We Got Here: Formation of Our Solar System

The Nebular Theory of Solar System Formation

(slide 51)

Lunar Crater Chain

A crater chain is a line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena (plural catenae), as specified by the International Astronomical Union's rules on planetary nomenclature. Davy is a small lunar crater that is located on the eastern edge of the Mare Nubium. It overlies the lava-flooded remains of the satellite crater Davy Y to the east, a formation which contains a crater chain designated Catena Davy. There are 19 craters in the chain. To the southeast of Davy is the prominent crater Alphonsus (out-of-frame and to the upper right in this image).

Outer Planets and the Kuiper Belt

A plot of known Kuiper Belt objects shows that most of them orbit the Sun well beyond the orbit of Neptune (the outer blue orbit), which is 30 AU from the Sun. Also shown are planets inside Neptune's orbit—Uranus, Saturn, and Jupiter (innermost blue circle)—as well as Pluto's eccentric orbit (red ellipse). Since the belt was discovered in 1992, the number of known Kuiper belt objects (KBOs) has increased to over 1,000, and more than 100,000 KBOs over 100 km (62 mi) in diameter are believed to exist.

Giant impact hypothesis

According to the hypothesis, 4.533 billion years ago, shortly after the formation of the Earth, a Mars-sized planetesimal hit the Earth at an oblique angle, destroying the impactor and ejecting most of that body along with a significant portion of the Earth's felsic-rich (feldspar and silica) mantle into space. Current estimates based on computer simulations of such an event suggest that some 2% of the original mass of the impactor ended up as an orbiting ring of debris, and about half of this material coalesced into the Moon between one and one hundred years after the impact. An impact body, "Theia," could have formed at the Earth-Sun Lagrangian point L4 or L5, and then drifted into a chaotic orbit that would impact the Earth.

Accretion

Accretion is the process of growth by colliding and sticking. Large accretions of mass—planetesimals—are pulled into a spherical shape by gravity. Gravitational encounters between planetesimals could alter their orbits and result in collisions that smashed the smaller ones, leaving relatively few large planetesimals that grew into full-fledged terrestrial planets. Theoretical evidence for accretion comes from computer simulations.

Debris disk in a young star

An edge-on circumstellar dust disk—known as a debris disk—is seen extending outward to almost 60 AU from the young (12-million-year-old) star AU Microscopii. The star is an M-class variable (flare star) about 32.3 l-y (9.9 pc) distant is the constellation Microscopium.

Asteroids and Comets

Asteroids and comets are leftovers from the accretion process. Rocky asteroids formed inside the frost line. Icy comets formed outside the frost line. 433 Eros is an S-type near-Earth asteroid about 34.4 x 11.2 x 11.2 km (21.4 x 7.0 x7.0 mi) in size. Comets resemble asteroids, only they're made of ice, dust, and rock. When near the Sun, solar energy causes comet ices to vaporize and form a tail. Comet Tempel 1 is about 6.4 km (4 mi) across; Hartley 2 is 2.2 km(1.4 mi). Most comets are no bigger than 24 km (15 mi) in diameter.

Asteroids

Asteroids are rocky leftover planetesimals of the inner solar system. The asteroid belt exists most likely because no planet could form in this region owing to the gravitational influence of Jupiter. Recently, astronomers have found evidence of icy bodies within the asteroid belt. (A) Asteroid (253) Mathilde taken by the space probe NEAR Shoemaker on 27 June 1997 from a distance of 2400 km. The part of the asteroid visible has dimensions of 59 km x 47 km. (NASA) (B) Timelapse of the flyby of asteroid 2004 FH. 2004 FH is the center dot being followed by the sequence; the object that flashes by is an artificial satellite.

Comets

Comets are icy leftover planetesimals of the outer solar system. Oort cloud comets probably came from icy planetesimals ejected from the region of the gas giants. Kuiper belt comets were less affected by the gravity of the jovian planets, so they continue to orbit in the plane of planetary orbits. Comet with dust tail (top, in white) and ion tail (bottom, in blue).

Earth's early atmosphere

Earth's primordial atmosphere is unknown, though it likely resembled our atmosphere prior to 4 billion years ago with large amounts of nitrogen, carbon dioxide, methane, ammonia, and water vapor.

Solar NebulaTemperature Gradient

Gravitational contraction generated heat, so that the nebula developed a temperature gradient similar to the one shown in the graph. Regions closer to the protostar were much hotter than regions in the outer part of the disk. The significance of the temperature gradient is reflected in the compositions and physical properties of the planets.

Jovian Planetary Cores Attract H and He Gases

Gravity of rock and ice in jovian planetary cores draws in hydrogen and helium gases that add to the overall mass. A 2008 simulation from U. C. Berkeley predicts a large core of iron, rock, and ice for Jupiter that is between 14 and 18 times the mass of the Earth. Clouds of hydrogen and helium gas are attracted to a large protoplanetary core. The gas

Ingredients of the Solar Nebula (abundances by % in parens)

H and He gas (98%) remained gaseous. H compounds (1.4%)—methane (CH4), ammonia (NH3), and water (H2O)—solidify into ices below 150 K. Rock (e.g., silicon-based minerals) (0.4%) condenses at 500-1,300 K. Metals (e.g., Fe, Ni, Al) (0.2%) condense at 1,000-1,600 K. Different types of matter condensed from the solar nebula at different temperatures. Gas discharge tubes of (A) hydrogen (H2) and (B) helium (He) in their plasma state.

The sinking solar Nebula

Heats up because gravitational P.E. is being converted into K.E., which heats the gas molecules (and follows the law of conservation of energy). Spins or rotates due to the conservation of angular momentum; a fast rotation insures that not all of the material in the nebula will collapse to the center. Flattens into a disk due to random collisions between particles in a spinning cloud. The shrinking solar nebula flattened into a disk as it rotated faster and faster.

Late-Heavy Bombardment

Leftover planetesimals bombarded other objects in the late stages of solar system formation. The Late Heavy Bombardment period lasted from 4.0-3.8 billion years ago.

Protoplanetary Disk

Like a raindrop forming in a cloud, a star forms in a diffuse gas cloud in deep space. As the star grows, its gravitational pull draws in dust and gas from the surrounding molecular cloud to form a swirling disk called a "protoplanetary disk." This disk eventually further consolidates to form planets, moons, asteroids, and comets.

Planets Rotate on their Axes

Like the Earth, most of the planets rotate on their axes counterclockwise. Only Venus and the dwarf planet Pluto rotate clockwise on their axes.

Mars Captures Phobos and Deimos

Long-standing theory has Mars capturing its two small moons. Shown here is a simulation of how Mars may have captured the two asteroid-like bodies that became its moons.

End of the Era ofPlanet Formation

Most of the H and He gas was swept into interstellar space by the solar wind. The young Sun's strong solar wind cleared the solar system soon after the planets formed, sealing the fate of the planets' composition. The young Sun's rapid rotation slowed considerably, but where did the angular momentum go?

Solar System's Location in Our Galaxy

Our solar system is located in the Orion Spur, an offshoot of the Sagittarius arm, which is a secondary arm of the Milky Way galaxy.

How did the terrestrial planets form (i.e., the process: two main steps)?

Rock and metals condensed inside the frost line and then collected into planetesimals. Planetesimals then accreted into planets.

Conservation of Angular Momentum

Rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted. The conservation of angular momentum is crucial in the formation of a star.

Galatic Recycling

Stars are great recycling factories. The elements that formed planets were made in stars and then recycled through interstellar space as stars age and then die. Following the Big Bang, the universe is composed of hydrogen (76%) and helium (24%) with a small amount of lithium (which is ultimately destroyed in the first stars). Subsequently, heavier elements come from thermonuclear processes in present in all stars and from the explosions of massive stars (supernovae). Gas that made up the solar nebula was the result of billions of years of galactic recycling that occurred before our solar system was born—giving the 98% hydrogen (H) + helium (He) / 2% heavier elements we see in the Sun today. Everything in our solar system—including us humans—is made from recycled "star stuff." (A) Hydrogen and (B) helium are the two most abundant elements in the universe and were created during the Big Bang.

Largest known Kuiper Belt objects

Subsequent observations showed that Eris is not as large as first thought and is now estimated to be slightly smaller than Pluto. The four largest objects shown here (top row) are also dwarf planets. These presently known largest small bodies in the Kuiper Belt are likely not to be surpassed by any future discoveries according to Dr. Michael Brown, discoverer of Eris.

Planetary Migration

Supercomputer simulations show that there is a significant likelihood that the outer planets changed positions following their formation. In particular, Neptune and Uranus switched positions.

Earth-Moon formed later

The Earth and Moon were created from a giant collision between two planets the size of Mars and Venus. Until now it was thought to have happened when the solar system was 30 million years old or about 4.537 billion years ago. But new research shows that the Earth and Moon may have formed much later—perhaps up to 150 million years after the solar system's formation. The age of the Earth and Moon can be measured using radiometric dating of certain elements in the Earth's mantle. Radioactive hafnium-182 decays into the isotope tungsten-182. The tungsten isotope prefers to bond with metal, while hafnium prefers to bond to silicates (rock). It takes 50-60 million years for all hafnium to decay into tungsten. The new studies imply that the Moon-forming collision occurred after all of the hafnium had decayed completely into tungsten, which suggests that the event occurred up to 150 million years after the solar system's formation. Giant impact that formed the Earth-Moon system.

Oxygen levels on the early Earth

The Earth's atmosphere did not contain appreciable amounts of oxygen until about two billion years ago. Oxygen did not build up in the atmosphere until plants began to produce it as a by-product of photosynthesis.

The solar system is roughly divided into inner and outer regions. What "line" separates the two regions, and what number defines the line?

The frost line. 150 K

Composition of the solar system

The solar system is 98% H and He and only 2% everything else, but this 2% is enough to form the rocky terrestrial planets. (slide 27)

Orbits of Phobos and Deimos

(A) Orbits of Phobos and Deimos (to scale), seen from above Mars's north pole. Phobos orbits only 9,650 km (6,000 mi) above the planet's surface. Phobos is orbiting Mars 4 times faster than Deimos. (B) Phobos above Mars.

Jets from young stars

(A, B, C) Jets from young stars extend for hundreds to over a 1,000 AU. Compare these distances to the diameter of our solar system, which is about 100 AU.

What aspects of our solar system must a formation theory explain? (Hint: There are 4.)

1.Motions of large bodies. 2.Two types of planets. 3.The existence of small bodies: asteroids and comets. 4. Notable exceptions like Earth's moon.

A formation theory of our solar system must explain...

1.Patterns of motion of the large bodies Orbits are in same direction and plane 2.The existence of two types of planets Terrestrial (rocky) Jovian (gas-giant) 3.The existence of smaller bodies Rocky asteroids and icy comets 4.Notable exceptions to usual patterns Odd rotation of Venus and Uranus Earth's large moon

Conservation of Energy

As gravity causes the cloud to contract, it heats up. 1. A large gas cloud starts out having much more potential energy than kinetic energy (in the form of thermal energy or heat) 2. Under the force of gravity, the cloud begins to shrink. Gravitational potential energy is converted into thermal energy. 3. The process continues as more potential energy is converted to heat. The hottest spot is in the center. Collapsing gas clouds demonstrate the principle of conservation of energy.

Asaph Hall III (1829-1907)

Asaph Hall discovered the two moons of Mars—Phobos and Deimos—in 1877 at the U.S. Naval Observatory using a 26-in refractor. Hall discovered Phobos on August 17, 1877.

Beta Pictoris

Beta Pictoris, 63.4 l-y (19.4 pc) distant, is the second brightest star in the constellation Pictor. It is 1.75 times as massive and 8.7 times as luminous as the Sun. The Beta Pictoris system is very young, only 8-20 million years old, although it is already in the main sequence stage of its evolution. β Pictoris is the title member of the Beta Pictoris moving group, an association of young stars which share the same motion through space and have the same age. This composite image represents the close environment of β Pictoris as seen in near infrared light. The exoplanet detected is more than 1,000 times fainter than β Pictoris, aligned with the disk, at a projected distance of 8 times the Earth-Sun distance. Because the planet is still very young, it is still very hot, with a temperature around 1475 K. (A) HD 141569 is a 5-million-year-old blue-white dwarf star approximately 320 l-y (98 pc) distant in the constellation Libra. The primary star has two red dwarf companions (orbiting each other). In 1999, a protoplanetary disk was discovered around the star. A gap in the disk indicates a possible extra-solar planet forming in the disk. The system is slightly tilted when viewed from Earth. (NASA, M. Clampin/ STScI, H. Ford/JHU, G. Illingworth/UCO-Lick, J. Krist/STScI, D. Ardila/JHU, D. Golimowski/JHU, the ACS Science Team and ESA) (B) HD 141569 and its two companions taken by HST in 2002.

Disk Shaped Solar Nebula

By the time the solar nebula shrank to a diameter of 200 AU, it had become a flattened, spinning disk. This process produced: the highest density and temperature at the center (i.e., the Sun) orderly motion nearly circular orbits (collisions between objects cause highly elliptical orbits to become more circular) Strong observational evidence supports the nebular theory of solar system formation. Additional support comes from computer simulations.

Summary for Building a Solar System via the Nebular Theory

Collapse of the nebula and formation of the protoplanetary disk and protosun. Condensation of planetessimals. Accretion of planetessimals to form planet seeds. Protoplanets continue to grow through accretion (terrestrials) and gas capture (jovians.). The strong solar wind of the young Sun clears away the remaining gas. Planets accreting out of a proto-planetary disk and clearing their orbits.

Planetary Disk Formation

Conservation of energy and angular momentum play an important role in star formation. Random collisions between particles help flatten the cloud into a disk. (A) The disk eventually thins as more material falls onto the star and the forming protoplanets. A hole in the disk near the star forms as material is completely incorporated into the star and planets. (B) Next, fully formed giant gaseous planets exist within the hole, even as new planets are still under construction in the outer parts of the disk. Rocky, terrestrial planets will build up from the many smaller objects now in orbit close to the star. (C) Ultimately, the remaining dust clears completely. The final stages of building Earth-like planets continue for another hundred million years or so in the form of catastrophic collisions between young bodies. After the dust settles, we have a fully formed solar system like our own.

Implications for formation of the Moon

Current theory has the Moon forming between 4.537 and 4.517 billion years ago (30-50 million years after the solar system's formation) after a Mars-sized object is believed to have collided with the Earth, creating the Moon by blasting pieces of the young planet into space. The early age of a "cool" Earth restricts theories for the formation of the Moon. Recent radiometric studies (see next slide) place the formation of the Earth-Moon system as late as 4.387 billion years ago (150 million years after the solar system's formation).

Giant impact origin of the Moon

During the late stages of planetary formation, a proto-planetary object the size of present-day Mars hit the proto-Earth. The collision vaporized the asteroid and threw out debris from the Earth. Some of the debris formed a ring around the Earth that coalesced into our Moon quite rapidly—in less than 100 years. The Moon likely formed as a result of a collision of Earth with a large impactor.

Evidence from other gas clouds

Evidence for the solar nebula theory comes from observations of other gas clouds, such as the Orion nebula, in which we can see stars that appear to be in the process of formation. M42, the Orion nebula, is located in the constellation Orion at a distance of about 1,350 l-y (0.4 kpc). It is a region of active star formation.

Giant gas interiors

Gas giants Jupiter and Saturn are composed largely of hydrogen and helium. Some planetary scientists like to call Uranus and Neptune "ice giants" as a significant portion of their interiors is composed of ices. Also, the two outermost planets contain methane, which colors their atmospheres blue.

Forming the Solar Nebula

Gas that forms the solar nebula is initially spherical, spread out over a few light-years in diameter, low in density, and very cold (<30 K). The collapse can begin from a shock wave emanating from a nearby exploding star we call a supernova. As the cloud begins to shrink, gravity takes over, reducing the volume and diameter of the cloud and concentrating matter at the center. The collapsing solar nebula formed the glowing proto-Sun.

Uranus's Odd Axial Tilt

Giant impacts might also explain the different rotation axes of some planets such as Uranus. Uranus essentially rotates on its side compared to the other planets in the solar system.

Formation of the Jovian Planets

Ice could form small particles outside of the frost line. Because of low temperatures and abundance of gas far from the Sun, icy planetesimals could grow very large. These large planetesimals could attract and capture even more gas, growing into the giant, low-density planets we see today. Recent research indicates that gas giants form early, perhaps only 10 million years after the central star forms. The wide spacing of the outer planets is still to be fully explained. From largest to smallest, the jovian or gas giant planets (to relative scale) are Jupiter, Saturn, Uranus, and Neptune.

Immanuel Kant

Immanuel Kant (1724-1804) was probably best known as one of the preeminent philosophers of the late 18thcentury. Interestingly, he also thought about astronomy and put forth ideas that were well ahead of their time. In the General History of Nature and Theory of the Heavens (1755), Kant laid out the Nebular Hypothesis, in which he deduced that the solar system formed from a large cloud of gas (hence, the term nebula). He thus attempted to explain the order of the solar system, seen previously by Newton as being imposed from the beginning by God. Kant also correctly deduced that the Milky Way was a large disk of stars, which he theorized also formed from a much larger spinning cloud of gas. He further suggested the possibility that other nebulae might also be similarly large and distant disks of stars. These postulations opened new horizons for astronomy, and for the first time extended astronomy beyond the solar system to galactic and extragalactic realms.

Oldest Terrestrial Material

In 2001, a tiny grain of the mineral zirconium silicate, or zircon, from western Australia was dated by Simon Wilde (a geology professor at Curtin University, Perth, Australia), and scientists in Edinburgh, Scotland, to be 4.4 billion years old. This makes it the oldest terrestrial material found—only 100 million years older than the formation of the Earth itself. Other old zircons have been found in places such as northern Canada. The 4.4 billion-year-old zircon: Surface 2 of zircon W74/2-36. The U-Pb age of 4.4 billion years was determined by ion microprobe from the spot shown.

Flat planetary sytems

In 2012, a review of data from NASA's planet-hunting Kepler space telescope has found that more than 85 percent of known exoplanets have inclinations of less than 3°. This means these planets orbit around their respective stars in nearly the same plane as the other planets in their respective star systems. Seven of the 8 planets in our own solar system have inclinations under 3°. The thickness of a typical pancake corresponds to an inclination of about 6°, so most planetary systems are flatter than pancakes by a factor of two.

Solar Rotation

In nebular theory, the young Sun was spinning much faster than now. Friction between solar magnetic field and the solar nebula probably slowed the rotation over time. Gas and dust in the solar nebula orbits the young Sun relatively slowly as per Kepler's laws. The young Sun's strong magnetic field lines sweep through the solar nebula much faster due to the Sun's rapid rotation at this early stage in its evolution. The Sun's rotation rate slowed down as its magnetic field produced a drag as it swept through the charged particles in the solar nebula.

Close Encounter Hypothesis

In the early 20th century, a rival idea—the CloseEncounterHypothesis—proposed that the planets formed from debris torn off the Sun by a close encounter with another star. The close-encounter hypothesis was discarded because it... could not account for observed orbital motions of the planets. could not account for the two planet types (terrestrial/jovian). relied on an encounter that is statistically very unlikely. The Close Encounter Hypothesis is not only statistically unlikely, but observations of newly forming star systems show no evidence that this process is responsible for their formation.

Heavy Bombardment Area

In the first few hundred million years of the solar system's existence, there were vast numbers of collisions between planetesimals. This period is known as the heavy bombardment. Impact craters are evidence of this period of bombardment. We know that objects like the Moon stopped changing billions of years ago, because we see extensive cratering from the heavy bombardment period. Niuafo'ou (Tin Can Island) issued a stamp showing the early heavy bombardment of Earth's crust by cosmic debris.

Inconsistencies with the Giant Impact Theory

Issues with the theory: The ratios of the Moon's volatile elements are not explained by the giant impact hypothesis. If the giant impact hypothesis is correct, they must be due to some other cause. The presence of volatiles such as water trapped in lunar basalts is more difficult to explain if the Moon was caused by an impact that would entail a catastrophic heating event. The iron oxide (FeO) content (13%) of the Moon, which is intermediate between Mars (18%) and the terrestrial mantle (8%), rules out most of the source of the proto-lunar material from the Earth's mantle. (slide 81)

Comets and Tidal Forces

Jupiter's enormous gravity can break apart fragile comets that pass by too closely. (A) Tidal forces from Jupiter broke apart comet Showmaker-Levy 9. (NASA) (B) A chain of craters named Enki Catena on Jupiter's moon Ganymede was imaged by the Galileo spacecraft. This chain of 13 craters probably formed by a comet which was pulled into pieces by Jupiter's gravity as it passed too close to the planet. Soon after this breakup, the 13 fragments crashed onto Ganymede in rapid succession.

Implications for formations of life

Low temperatures and water are preconditions for life. The earliest known biochemical evidence for life and for a hydrosphere is estimated at 3.85 billion years ago, and the oldest microfossils are 3.5 billion years old. The zircon discovery implies that water and thus life could have existed as long ago as 4.4 billion years. Perhaps life evolved and was extinguished several times by impacts before it finally took hold.

Accretion of Planetesimals

Many smaller objects collected into just a few large ones. 1. Central Star and Dust disk- orbiting dust grains accrete into planetesimals through colliding and sticking 2. Planetesimals grow to form "planetary embryos," the beginnings of protoplanets. They orbit the Sun in the same direction and in nearly the same place as the dust disk. 3. Near the Sun, rocky plotoplanets form. Farther out, gas-giant protoplanets accrete gas envelopes before the disk gas disappears 4. Gas-giant planets scatter or accrete most of the remaining planetesimals and embyros, leaving only relatively few large planets. Accretion built the inner rocky planets and the cores of the outer gas giants.

Moons of Mars Are Thought To Be Captured Asteroids

Mars has two known moons, (A) Phobos and (B) Deimos, which are thought to be captured asteroids. Both satellites were discovered in 1877 by Asaph Hall and are named after the characters Phobos (panic/fear) and Deimos (terror/dread) who, in Greek mythology, accompanied their father Ares, god of war, into battle. Phobos has an average diameter of 22.2 km (13.8 mi); Deimos is 12.6 km (7.8 mi).

Was Life on Mars Extinguished Prematurely by a Huge Impact?

Mars looks odd. The northern hemisphere is composed of barren plains and smooth sand dunes; the southern hemisphere is a chaotic, jagged terrain of mountains and valleys. It would appear this crustal dichotomy formed after a massive impact early in Mars's development. Evidence suggests that Mars lost its magnetism following a giant impact early in the planet's history that disrupted its molten core. Mars had a magnetic field that disappeared about 4 billion years ago around the same time that the crustal dichotomy appeared, suggesting a massive impact from a large asteroid. Earth's magnetic field deflects the solar wind and protects our atmosphere. Without such a protective shield, Mars likely lost its water and atmosphere and became the dry, barren place we see today. A giant impact explains why Mars's two hemispheres are so different.

The first billion years

Most evidence of the Hadean Eon (from Earth's creation to about 4.0 billion years ago—the beginning of the Late Heavy Bombardment) was destroyed by the bombardment and subsequent plate tectonics.

Phobos Formed from Re-Accretion

New research indicates that Phobos formed relatively near its current location via re-accretion of material blasted into Mars' orbit by some catastrophic event, such as a huge impact. This could be an event similar to how Earth's moon formed. Recent thermal infrared observations from the Mars Express Planetary Fourier Spectrometer (PFS), show poor agreement with any class of chondritic meteorite such as would be found in bodies from the asteroid belt. They instead argue in favor of the in-situ scenarios. Other observations of Phobos' surface appear to match up with the types of minerals identified on the surface of Mars. Spatial locations of PFS and observations of Phobos used for the compositional analysis.

Solar Wind Clears Leftover Gases in Young Solar System

Outflowing matter from the Sun—the solar wind—blew away the leftover gases. The solar wind from the early Sun was much stronger than it is today.

Stages of a Comet's Formation

Periodic comets originate in the Kuiper Belt and have very elliptical orbits. Comet Halley (1P/Halley) is the most famous periodic comet and the first to be understood as such. A periodic comet with a highly elliptical orbit will typically form its dust and ion tails when it is about 1 AU from the Sun.

Pierre Simon de Laplace

Pierre Simon de Laplace (1749-1827) was a French mathematician and astronomer whose work was pivotal to the development of mathematical astronomy and statistics. He summarized and extended the work of his predecessors in his five-volume Celestial Mechanics (1799-1825). This work translated the geometric study of classical mechanics to one based on calculus. In statistics, the so-called Bayesian interpretation of probability was developed mainly by Laplace. Laplace developed the nebular hypothesis of the formation of the solar system, first suggested by Emanuel Swedenborg and expanded by Immanuel Kant. According to Laplace's description of the hypothesis, the solar system had evolved from a globular mass of incandescent gas rotating around an axis through its center of mass. As it cooled, this mass contracted, and successive rings broke off from its outer edge. These rings, in turn, cooled and finally condensed into the planets, while the Sun represented the central core which was still left. In addition to outlining the idea of the nebular hypothesis, Kant had also suggested "meteoric aggregations" and tidal friction as causes affecting the formation of the solar system. Laplace was probably aware of this, but, like many writers of his time, he generally did not reference the work of others.

What began our Solar System

Radiometric dating has also indicated that the source of the "trigger" to begin our solar system formation came from the explosion of a nearby star (a supernova), probably about 4.6 billion years ago.

Flattening

Random collisions between particles in the contracting cloud cause it to flatten into a disk. (A) All stars form from clouds of gas and dust. Eventually, gravity causes the cloud to collapse; since the cloud is spinning, material falls in along the "poles" faster than it does near the "equator." This flattening results in a disk-like object. (B) Material slowly moves to the center of this disk, forming a new star. While the star continues to grow, lumps form in the disk that will ultimately become planets. Conservation of energy and angular momentum play an important role in star formation. Random collisions between particles help flatten the cloud into a disk. 1. Because of conservation of energy, the cloud heats up as it collapses (P.E becomes K.E) Because of conservation of angular momentum, the cloud spins faster as it contracts (m x v x r) 2. Random collisions between particles in the contracting cloud flatten the cloud into a disk. 3. The result is a spinning, flattened disk, with mass concentrated near the center and the temperature highest near the center (where the protostar forms)

Condensation in the Early Solar System

Refractory materials (rock, metals) do not melt at high temperature. The outer disk has volatile materials like ices. Volatile materials can melt or evaporate at moderate temperatures. Refractory materials condensed closer to the proto-Sun while more volatile materials condensed farther away in regions that are colder.

To the edge of our Solar System

Regions of our solar system are plotted on a logarithmic scale. The units are AUs. The planets range on a scale of 1-10, the solar system on 100, the Oort Cloud on 1,000-10,000. The closest star is between 105 and 106 AUs, which is the reason for switching to light-years.

Saturn's Rings May Be Remains of Ripped-Apart Moon

Results of a computer simulation showing how Saturn's gravity likely stripped icy material from a Titan-size moon, creating the planet's rings. (Left) Material stripped after 8 hours. (Right) Material stripped after 25 hours. In the simulation, the rocky core of the moon eventually collides with Saturn. This model explains the odd iciness of Saturn's rings and inner satellites, and also describes events that are a natural part of a giant planet's formation and youth.

Ring Formation is Ongoing

Ring particles are constantly being ground down in size from impacts by the countless small particles that orbit the Sun. Thus, ring particles must be continuously supplied to the rings to replace those destroyed. The likely source is the small moon lets that orbit a Jovian planet in its equatorial plane. These new ring particles are released by impacts on these small moons within the rings. (A) The tiny moons Pandora (left) and Prometheus (right) orbit on either side of Saturn's F ring. Only Prometheus acts as a ring shepherd. (NASA/JPL) (B) An artist's concept of a close-up view of Saturn's ring particles. The particles (blue) are composed mostly of water ice, but are not uniform. They clump together to form elongated, curved aggregates, continually forming and dispersing. The space between the clumps is mostly empty. The largest individual particles shown are a few meters (yards) across. (NASA/JPL/U Colorado) (C) A scan across Saturn's incredible halo of ice rings yields a study in precision and order.

Details of the Zircon Discovery

Scientists analyzed oxygen isotope ratios, measured rare earth elements, and determined element composition in a grain of zircon that measured little more than the diameter of two human hairs. The oxygen isotopes and rare earth analysis show us a high oxygen isotope ratio that is not common in other such minerals from the first half of the Earth's history. In other words, the chemistry of the mineral and the rock in which it developed could only have formed from material in a low-temperature environment at Earth's surface. The zircon discovery implies that 4.4 billion years ago, temperatures had cooled to the 100-degree Celsius range, permitting the formation of a solid surface and possibly water. These results may indicate that the Earth cooled faster than anyone thought and that the surface was not an ocean of hot magma. Previously, the oldest evidence for liquid water on Earth, a precondition and catalyst for life, was from a rock estimated to be 3.8 billion years old.

Self-Gravity

Self-gravity holds stars and planets together and is the gravitational attraction between the parts of a planet or star that pulls all the parts toward its center. This inward force is opposed by either structural strength (e.g., rock) or the outward force resulting from gas pressure within a star. The balance between gravity pulling inward and pressure pushing outward is called hydrostatic equilibrium.

Solar System Formation Sequence

Self-gravity of particles in the molecular cloud causes the cloud to contract Conservation of angular momentum in the contacting cloud causes the cloud to rotate. Collisions of particles cause the cloud to collapse into a disk. As gravity concentrates mass in the center, the proto-Sun forms Gravity and centrifugual force balance each other; objects begin to accrete, forming lanes in the dusty disk that eventually become planetary orbits. Shown are the general steps in the formation of a solar system based on the nebular theory.

Formation of theTerrestrial Planets

Small particles of rock and metal that had condensed from the initial solar nebula were present inside the frost line. As these small particles of rock and metal collided, planetesimals built up. Gravity eventually assembled these planetesimals into the terrestrial planets through accretion. (B) Vesta, one of the largest asteroids in the solar system, has a mean diameter of 525 km (326 mi).It is the second-most-massive asteroid after the dwarf planet Ceres. Vesta is the last remaining rocky protoplanet —with a differentiated interior—of the kind that formed the terrestrial planets.

What caused the orderly patterns of motion in our solar system? (Hint: The solar nebula did what two things to make this occur?)

Solar nebula spun faster as it contracted because of conservation of angular momentum. Collisions between gas particles then caused the nebula to flatten into a disk.

Radioactive Decay

Some isotopes decay into other nuclei. A half-life is the time for half the nuclei in a substance to decay. Knowing the proportions of the two isotopes can tell us the age of the rock.

Fission TheoryIn

The Fission Theory of the Moon's formation is depicted in a 1922 article in the London Illustrated News. According to the theory, a rapidly rotating molten fluid Earth flattens into an oblate spheroid (the "egg-shape"). As the speed of rotation increases, the Earth then becomes more pear-shaped (a shape seen today). The resulting equatorial bulge lengthens and ultimately breaks off, creating the Moon. The Moon orbits the Earth, slowly increasing its distance from the planet. The Fission Theory does explain the similarity of the Moon's rocks to that of Earth's crust and mantle. It also explains the Moon's low-mass, iron-poor core. However, the Moon lacks "fossil evidence" of a rapid spin, and it does not orbit the Earth following the equatorial plane. Furthermore, in order for the Fission Theory to be true, the Earth would have had to be spinning so fast that the probability of this happening is extremely remote.

Moon formation theories

The Fission Theory: The Moon was once part of the Earth and somehow separated from the Earth early in the history of the solar system. The present Pacific Ocean basin is the most popular site for the part of the Earth from which the Moon came. However, as we now know, the Pacific Ocean did not exist at the time of the Moon's formation. Also, the appearance and location of the continents have changed dramatically over time due to plate tectonics (which was unknown in the early 20th century when this theory was put forth.) The Capture Theory: The Moon was formed somewhere else and was later captured by Earth's gravitational field. First, if the Moon were formed at some distance from the Earth, then the Moon should differ quite considerably in composition. That the Moon's composition is very similar to that of Earth—particularly in the ratio of oxygen isotopes present—strongly suggests that the both bodies formed closed to each other. Second, both theoretical models and computer simulations show that it is extremely unlikely of a capture scenario for a body of the mass of the young Earth (perhaps 2/3 of its modern value).The main theoretical obstacle is the high encounter velocity (at least a few kilometers per second) and the required dissipation of the satellite's kinetic energy. The Condensation (or Co-Accretion) Theory: The Moon and the Earth condensed together from the original nebula that formed the solar system. This theory is disapproved because if the Earth and Moon had been condensed together, then they would share similar properties, such as gravity force, densities, and inner cores. The Moon, however, is less dense than Earth with a small core and little iron. The Colliding Planetesimals Theory: The interaction of Earth-orbiting and Sun-orbiting planetesimals (very large chunks of rocks like asteroids) early in the history of the solar system led to their breakup. The Moon condensed from this debris. The Moon's composition would differ considerably from location to location. The Moon exhibits a differentiated structure that would not be evident if similarly sized planetesimals had collided and stuck together. The Giant Impact (or Collision or Ejected Ring) Theory: A planetesimal the size of Mars struck the Earth, ejecting large volumes of matter. A disk of orbiting material was formed, and this matter eventually condensed to form the Moon in orbit around the Earth. The giant impact theory is the theory currently favored by planetary scientists that best explains the formation of the Moon.

Kuiper Belt Objects

The Kuiper belt, sometimes called the Edgeworth-Kuiper belt (after the astronomers Kenneth Edgeworth and Gerard Kuiper), is a region of the solar system beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. Though the outer regions of the solar system are lower in density, some of the Kuiper belt comets could continue their accretion, growing into moon-sized objects like Pluto (and its moon Charon) and Eris (both now called dwarf planets), which is just slightly larger than Pluto. From the nebular theory, a few astronomers predicted the existence of the Oort cloud and Kuiper belt beginning in the mid-1940s to the early1950s. It took until 1992 to verify these predictions. Astronomer Gerard Kuiper (1905-1973), after whom the Kuiper belt is named. In 1951, Kuiper speculated on a disc having formed early in the solar system's evolution; however, he did not believe that such a belt still existed today. Were Kuiper's hypothesis correct, there would not be a Kuiper belt today.

The Oort Cloud

The Oort Cloud is estimated to be between 50,000 AU (0.775 l-y) and 100,000 AU (1.55 l-y). (1 light-year = 63,240 AU) in diameter. Because the Oort cloud is so distant, no Oort cloud objects have ever been observed. The most distant known trans-Neptunian object, Sedna, travels only to 928 AU—only two percent of the distance to the hypothesized inner edge of the Oort cloud. The Oort cloud (named after the Dutch astronomer Jan Oort) is a spherical cloud of predominantly icy planetesimals that is believed to surround the Sun at a distance between 50,000 AU and 100,000 AU. The Oort cloud is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane. The Oort Cloud contains an estimated trillion (1012) comets, whose orbits are randomly tilted with large eccentricities. Each panel shows the relative size of the solar system from a progressively bigger volume of space. The orbit of Sedna, the most distant TNO discovered, has a perihelion of 76 AU and an aphelion of 937 AU. Even Sedna's huge elliptical orbit is tiny compared to the inner edge of the hypothesized spherical Oort cloud. Sedna's presence suggests that this previously speculated inner region of the cloud does exist.

Predictions for Mar's Moons

The existence of two fictional Martian moons was described in Jonathan Swift's satirical novel Gulliver's Travels, published in 1726, 150 years before their discovery: They [the Laputan astronomers] have likewise discovered two lesser stars, or "satellites," which revolve about Mars, whereof the innermost is distant from the centre of the primary planet exactly three of his diameters, and the outermost five; the former revolves in the space of ten hours, and the latter in twenty-one and a half; so that the squares of their periodical times are very near in the same proportion with the cubes of their distance from the centre of Mars, which evidently shows them to be governed by the same law of gravitation, that influences the other heavenly bodies.... Phobos and Deimos are in fact about 1.4 and 3.5 diameters from Mars's center, and their periods are 7.7 and 30.3 hours, respectively. A similar "discovery" was described by Voltaire in his interplanetary romance Micromegas, published in 1752. In recognition of these "predictions," two craters on Deimos are named Swift and Voltaire. Gulliver discovers Laputa, the flying island

The frost line

The frost line occurs between the orbits of Mars and Jupiter where temperatures remain < 150 K. Existence of the frost line explains the existence of rocky and metal-rich inner planets and gaseous outer planets and icy moons and comets in the outer regions of the solar system. The frost line (sometimes called the "snow line" or "ice line") is defined by a temperature of 150 K and is found between the orbits of Mars and Jupiter at about 5 AU from the Sun. Note that 98% of the original solar nebula was gaseous hydrogen and helium that did not condense at any location. Within the frost line, only refractory materials (metals and rocks) condense, but hydrogen compounds remain gaseous Beyond the frost line, refractory materials and hydrogen compounds (H20, CH4, NH3) condense; hydrogen and helium remain gaseous

Planetary Systems form in Orion

The insets show several planetary systems in formation in the Orion Nebula. The bottom left insert shows the relative size of our own solar system. The Orion Nebula exhibits many stellar nurseries that contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Much of the filamentary structure visible in this image are actually shock waves—i.e., fronts where fast moving material encounters slow moving gas. Some shock waves are visible near one of the bright stars in the lower left of the picture. The Orion Nebula is located in the same spiral arm of our galaxy as is our Sun and is about 1,350 l-y (0.4 kpc) distant.

Formation of Jovian Moons

The large planetesimals also caused gas to form flattened disks around them, leading to the accretion of many small planetesimals that ultimately became the numerous moons of the gas giants. This model is referred to as the accretion and gas capture model.

Earth's 2 Moons

The mountainous region on the far side of the Moon, known as the lunar farside highlands, may be the solid remains of a collision with a smaller companion moon, according to a new study by planetary scientists at the University of California, Santa Cruz. The striking differences between the near and far sides of the Moon have been a longstanding puzzle. The near side is relatively low and flat, while the topography of the far side is high and mountainous, with a much thicker crust. The new study, builds on the "giant impact" model for the origin of the Moon. The study suggests that this giant impact also created another, smaller body, initially sharing an orbit with the Moon, that eventually fell back onto the Moon and coated one side with an extra layer of solid crust tens of kilometers thick. Frames from a computer simulation depicting Earth's "second" moon colliding with the Moon we know following their creation by a giant impactor. A low-speed collision would create the differences in lunar topography we see today.

Nebular theory best explains features of our solar system

The nebular theory states that our solar system formed from the gravitational collapse of a giant interstellar gas cloud—the solar nebula. (Nebula is the Latin word for cloud.) In 1755, Immanuel Kant proposed the solar system formed from the gravitational collapse of an interstellar cloud of gas. Pierre-Simon Laplace proposed the same idea 40 years later. Nebular theory is now supported by a large amount of evidence.

Process of Planet Formation

The process of planet formation took between 0.5% and 1% of the current age of the solar system: ≈25-50 million years. 1% of 5 x 10<9 years is 5 x 10<7 or 50 million years A solar system forms out of a dusty disk.

Venus rotates backwards

The sense of Venus's rotation is opposite to the sense of its revolution around the Sun (such motion is called retrograde rotation). Also, the rotational period is very long, about 243 terrestrial days. (Note that because of this a Venus Solar day—about 117 Earth days—is shorter than the Venus rotation period). The best current idea for why Venus rotates so slowly is that Venus suffered a very large impact, fairly late in the epoch of planet formation, and the angular momentum imparted by that impact essentially stopped the planet's rotation. The relationship between Venus' rotation and revolution (orbit of the Sun) is complicated by its retrograde rotation.

Where Did theAngular Momentum Go?

The strong magnetic field and solar wind of the young Sun transferred significant angular momentum via charged particles from the Sun into space, leaving the Sun with much less angular momentum and a much slower rotation. This theory of angular momentum transfer is supported by observations of young star systems. The Sun contains about 1% of the solar system's angular momentum. Jupiter contains the most at 61%; followed by Saturn at 24%; Neptune at 8%; Uranus at 5%; and Earth, Venus, Mars, and Mercury combining for less than 1%. Active variable stars transfer large amounts of plasma and angular momentum through high-power stellar winds.

Radiation and matter

This drawing shows the different kinds of ionizing radiation and their ability to penetrate matter. Alpha radiation (α) consists of helium nuclei and is readily stopped by a sheet of paper. Beta radiation (β), consisting of electrons or positrons, is halted by an aluminum plate. Gamma radiation (γ) is dampened by lead. Gamma rays can be stopped by 4 meters (13.1 feet) of lead. Tungsten and tungsten alloys can stop gamma radiation with much less mass than lead.

Rotation of Earth and Venus

Though Earth and Venus are very close in mass and diameter, other aspects of these two planets are widely divergent. While Earth is wet with mild surface temperatures, Venus is dry and hotter than any other planet. Earth rotates counterclockwise once every 24 hours, while Venus rotates slowly clockwise every 243 days.

Axis Tilts and Rotation Direction of the Planets

Though all of the planets revolve or orbit the Sun counterclockwise, not all of them rotate on their respective axes counterclockwise. Seven of the eight planets rotate counterclockwise (though Uranus is doing this on its side). Venus rotates clockwise (or retrograde), and this "backward" rotation may be the result of a large impact early in its history.

Main Belt Comets & Earth's Water

Three icy comets orbiting among the rocky asteroids in the main asteroid belt between Mars and Jupiter may hold clues to the origin of Earth's oceans. The newly discovered group of comets, dubbed "main-belt comets" by U. Hawaii astronomers Hsieh and Jewitt, has asteroid-like orbits and, unlike other comets, appears to have formed in the warm inner solar system inside the orbit of Jupiter rather than in the cold outer solar system beyond Neptune. The existence of these main-belt comets suggests that asteroids and comets are much more closely related than previously thought and supports the idea that icy objects from the main asteroid belt could be a major source of Earth's present-day water

Exceptions to the rule (with explanations)

Uranus and Pluto rotate nearly on their sides. Venus rotates "backward" (clockwise rather than counterclockwise). Neptune's large moon Triton orbits in the opposite direction. Earth has one of the largest moons in the solar system, which is unusual considering the size of the planet our Moon orbits. Water on the Earth probably came from the impact of icy planetesimals originating beyond the orbit of Mars. Moons with "backward rotation" (like Triton) are probably captured moons. In the case of Triton, it was probably a Kuiper Belt body that wandered too close to Neptune. Our Moon probably formed as the result of a giant impact from a Mars-sized object during the late stages of Earth's accretion. The Moon's composition is similar to that of the Earth's outer layers—crust and mantle. Venus's backward rotation may be the result of a giant impact (though a giant impact hypothesis is difficult to prove). Formation of planets in the solar nebula seems inevitable, but the details of individual planets could have been different.

Origin of Earth's Water

Water may have come to Earth by way of icy planetesimals from the outer solar system. Recent research points to icy asteroids rather then comets.

Radiometric Dating of the Solar System

We can measure the age of rock through radiometric dating that uses the principle of radioactive decay in certain isotopes. We measure the half-life of the radioactive isotope of the sample. The oldest Earth rocks are about 3.8 billion years old. The oldest lunar rocks are about 4.4 billion years old. Radiometric dating of meteorites shows that the oldest ones formed 4.55-4.60 billion years ago, indicating the start of accretion in the solar nebula. Alpha decay is one example type of radioactive decay, in which an atomic nucleus emits an alpha particle (i.e., a helium nucleus), and thereby transforms (or "decays") into an atom with a mass number 4 less and atomic number 2 less. Many other types of decays are possible.

Comparison of Relative Abundances of Elements: Whole Earth vs. Earth's Crust

When it first formed, the Earth was molten. Heavy elements (such as iron and nickel) sunk to the center, leaving lighter elements near or on the surface. (slide 29)

The oldest Earth rock

Whole-rock samples from the Sand River Gneisses in the Limpopo Valley, South Africa, have been dated at 3.79 ± 0.06 billion years. The Amitsoq Gneisses in western Greenland, for example, have been dated by five different methods; within the analytical uncertainties, the ages are the same and indicate that these rocks are about 3.7 billion years old. The oldest whole rocks in North America, found in Minnesota, are dated at 3.56 billion years old. The Acasta Gneiss is a rock outcrop of Archaean tonalite gneiss in the Slave craton in Northwest Territories, Canada. It is the oldest known crustal rock outcrop in the world, and so it is important in establishing the history of the continental crust. The Acasta Gneiss is named for the nearby Acasta River east of Great Slave Lake some 350 km north of Yellowknife. The rock exposed in the outcrop formed just over four billion years ago. This age is based on radiometric dating of zircon crystals at 4.03 b-y, which are the oldest rocks in the world so far. Acasta Gneiss formation has been dated at 4.031-3.58 billion years old and was formed in the Basin Groups, an unofficial period of the Hadean eon.

Proplyds in the Orion Nebula

Within the Orion nebula, researchers have identified two different types of protoplanetary disks, or "proplyds," that might evolve on to agglomerate planets. These discs around young and forming stars are (1) those that lie close to the brightest star in the cluster and (2) those farther away from it. The bright star heats up the gas in the nearby discs, causing them to shine brightly. The discs that are farther away do not receive enough of the radiation from the star to excite the gas molecules into glowing; thus, they can only be detected as a dark silhouette against the background of the bright nebula, as the dust that surrounds these discs absorbs background visible light. In these silhouetted discs, astronomers are better able to study the properties of the dust grains that are thought to bind together and possibly form planets like our own. A Hubble Space Telescope view of a small portion of the Orion Nebula reveals five young stars. Four of the stars are surrounded by gas and dust trapped as the stars formed, but were left in orbit about the star. These are possibly protoplanetary disks, or "proplyds," that might evolve on to agglomerate planets. The proplyds which are closest to the hottest stars of the parent star cluster are seen as bright objects, while the object farthest from the hottest stars is seen as a dark object. The field of view is only 0.14 l-y (8,848 AU) across.

End of the Solar Nebula

Young stars rotate much faster than mature stars, leading to very strong solar winds that clear gas and debris fairly quickly from planetary disks. Young stars also produce strong jets of gas that carry material away from them. Numerous jets have been discovered associated with newly formed stars.

Earth's Timeline

slide 110