Solar System Formation

Solar System Formation Question: How did our solar system and other planetary systems form? Comparative planetology has helped us understand Compare the differences and similarities among the planets, moons, asteroids, and comets of our solar system Figure out what physical processes could have led to them Then construct a model of how our solar system formed based on this ------- This model must explain the characteristics of our own solar system, but it might or might not explain other planetary systems If not, then what to do? Then modify the model to accommodate discrepancies That is the scientific process Let s look at the solar system characteristics comparative planetology has to work with

Solar System Formation -- Characteristics of Our Solar System 1. Large bodies in the solar system have orderly motions and are isolated from each other All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane The Sun and most of the planets rotate in this same direction as well

Solar System Formation -- Characteristics of Our Solar System 2. Planets fall into two main categories Small, rocky terrestrial planets near the Sun Large, hydrogen-rich jovian planets far from the Sun

Solar System Formation -- Characteristics of Our Solar System 2. Planets fall into two main categories

Solar System Formation -- Characteristics of Our Solar System 3. Swarms of asteroids and comets populate the solar system Asteroids are concentrated in the asteroid belt

Solar System Formation -- Characteristics of Our Solar System 3. Swarms of asteroids and comets populate the solar system Asteroids are concentrated in the asteroid belt Comets populate the regions known as the Kuiper belt and the Oort cloud

Solar System Formation -- Characteristics of Our Solar System 4. Several notable exceptions to these general trends stand out Planets with unusual axis tilts Surprisingly large moons Moons with unusual orbits

Solar System Formation -- Characteristics of Our Solar System which any successful theory must account for 1. Large bodies in the solar system have orderly motions and are isolated from each other All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane The Sun and most of the planets rotate in this same direction as well Most moons orbit their planet in the direction it rotates 2. Planets fall into two main categories Small, rocky terrestrial planets near the Sun Large, hydrogen-rich jovian planets farther out The jovian planets have many moons and rings of rock and ice 3. Swarms of asteroids and comets populate the solar system Asteroids are concentrated in the asteroid belt Comets populate the regions known as the Kuiper belt and the Oort cloud 4. Several notable exceptions to these general trends stand out Planets with unusual axis tilts Surprisingly large moons Moons with unusual orbits

Solar System Formation The Nebular Theory The nebular theory is the best current explanation of our solar system It is associated with some well-known 18 th -century philosophers: Emanuel Swedenborg Immanuel Kant Like all scientific theories, it is still being refined and improved

Solar System Formation The Nebular Theory It starts with cold interstellar clouds of gas and dust... These clouds are mostly hydrogen and helium from the Big Bang But they contain heavier elements that were not formed in the Big Bang Astronomers call these metals (even though they re not necessarily classified as such) Where did these heavier elements come from? They came from stars!

Solar System Formation The Nebular Theory Stars make heavier elements from lighter ones through nuclear fusion

Solar System Formation The Nebular Theory Stars make heavier elements from lighter ones through nuclear fusion The heavy elements (the metals ) mix into the interstellar medium when the stars die

Solar System Formation The Nebular Theory Stars make heavier elements from lighter ones through nuclear fusion The heavy elements (the metals ) mix into the interstellar medium when the stars die New stars form from the enriched gas and dust, and the cycle continues

Solar System Formation The Nebular Theory A large cloud -- a nebula perhaps 1 light year across -- floats in space

Solar System Formation The Nebular Theory A large cloud -- a nebula perhaps 1 light year across -- floats in space The cloud begins to collapse WHY?... Local density increase

Solar System Formation The Nebular Theory A large cloud -- a nebula perhaps 1 light year across -- floats in space The cloud begins to collapse Conservation of angular As it collapses it begins to spin faster WHY?... momentum

Solar System Formation The Nebular Theory A large cloud -- a nebula perhaps 1 light year across -- floats in space The cloud begins to collapse As it collapses it begins to spin faster And as it spins faster, it flattens out At the same time, it begins to heat up in the center WHY?... Conversion of gravitational potential energy into thermal energy

Solar System Formation The Nebular Theory A large cloud -- a nebula perhaps 1 light year across -- floats in space The cloud begins to collapse As it collapses it begins to spin faster And as it spins faster, it flattens out At the same time, it begins to heat up in the center When it gets hot enough, a star forms in the center And in the disk around the forming star, planets can form What type of planets can form depends on what the cloud is made of

Solar System Formation The Nebular Theory This is what our own cloud the solar nebula was made of But how do we know this?

Solar System Formation The Nebular Theory This is what our own cloud the solar nebula was made of But how do we know this? We know from the absorption line spectrum of the Sun It tells us the composition of the gas on the surface of the Sun

Solar System Formation The Nebular Theory This is the composition of the Sun s surface gas We also think this was the composition of the solar nebula the Sun and planets formed from But is it reasonable to say that the gas on the surface of the Sun has the same composition as the solar nebula?

Solar System Formation The Nebular Theory After all, the collapse of the solar nebula that is supposed to have formed the planets and the Sun happened 4.6 billion years ago The Sun s been making new, larger atoms from smaller ones (fusion) ever since So if new atoms are being made, why would the outer layers of today s Sun have the same composition as the solar nebula?... The answer has to do with where the new atoms are being made

Solar System Formation The Nebular Theory The new atoms are helium atoms from hydrogen fusion reactions (which generate the energy that gives us sunlight) Now the critical question: Where are these fusion reactions taking place? The answer: In its core And that s in the Sun s center, far from the surface So the surface layers should be essentially unchanged And their composition should be very similar to the solar nebula

Solar System Formation The Nebular Theory So it is reasonable to say that the composition of the surface layers of the Sun is the same as the composition of the solar nebula

Solar System Formation The Nebular Theory The key to the nebular theory is the condensation temperature of these materials, at which they will condense into solid form The nebula was initially very cold, so everything except H and He was in solid form But it heated up as it collapsed And the temperature was different at different distances from the center

Solar System Formation The Nebular Theory This graph shows a modeled temperature profile of the solar nebula The temperature was hottest in the center, and went down away from the center There was a mixture of metals, rocks, and hydrogen compounds throughout the nebula These could only be solid where the temperature was below their condensation temperature So different chemical components of the nebula condensed at different distances A mixture of solid rock and metal existed out to about 4.5 AU from the center At 4.5 AU, the temperature dropped low enough for hydrogen compounds to condense, too The boundary between where they could and could not condense is called the frost line

Solar System Formation The Nebular Theory The frost line was located between the present-day orbits of Mars and Jupiter

Solar System Formation The Nebular Theory Once materials condense into solid form they can stick together This is called accretion And it launches the next step in planet formation Core accretion

Solar System Formation The Nebular Theory Small clumps grow like snowballs until they become planetesimals the size of moons The planetesimals collide and coalesce until planets are born This suffices to explain terrestrial planet formation, but jovian planets require adding an extra layer to the process...literally

Solar System Formation The Nebular Theory Jovian planets also begin by core accretion But this happens in the outer solar system, beyond the frost line, where there is 3x more solid material available So the cores get much bigger (10-15 times the mass of Earth)

Solar System Formation The Nebular Theory Unlike terrestrials, the jovian cores gather gas from the nebula and retain it This is because: They are more massive stronger gravity It is colder lower escape speeds for gas The result is a gas giant -- a jovian planet

Solar System Formation The Nebular Theory There is an alternative to the core accretion model disk-instability" Cool gas beyond the frost line collapses directly into jovian planets This takes much less time than the "core-accretion model" And this makes it consistent with claims that some jovians form faster than would be possible by core-accretion

Solar System Formation The Nebular Theory It is not known for certain whether jovian planets form by core accretion or disk instability Perhaps they form one way in some circumstances and the other way in others The main difference is in the way the process begins Once it starts, the nebular gas swirls in an accretion disk around the growing jovian planet In that accretion disk, moons would form around the jovian planet like planets formed in the solar nebula around the Sun

Solar System Formation The Nebular Theory The process of jovian and terrestrial planet formation was finalized by the infant Sun As the Sun became a star, a strong solar wind blew out from it This cleared the remaining nebular gas away And this halted the growth of the planets from the solar nebula`

A successful theory must explain our solar system So how does this one do?

How Does the Nebular Theory Do? Large bodies in the solar system have orderly motions and are isolated from each other : All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane The Sun and most of the planets rotate in this same direction as well Most moons orbit their planet in the direction it rotates Planets fall into two main categories: Small, rocky terrestrial planets near the Sun No rings and few, if any, moons Large, hydrogen-rich jovian planets farther out Rings of rock and ice and many moons Swarms of asteroids and comets populate the solar system: Asteroids are concentrated in the asteroid belt Comets in the Kuiper belt and the Oort cloud Several notable exceptions to these general trends stand out: Planets with unusual axis tilts Surprisingly large moons Moons with unusual orbits

How Does the Nebular Theory Do? Large bodies in the solar system have orderly motions and are isolated from each other: All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane The planets and moons orbit in the direction that the solar nebula was spinning The Sun and most of the planets rotate in this same direction as well Conservation of angular momentum Most moons orbit their planet in the direction it rotates Conservation of angular momentum

How Does the Nebular Theory Do? Large bodies in the solar system have orderly motions and are isolated from each other Planets fall into two main categories: Large, hydrogen-rich jovian planets far from the Sun, with rings of rock and ice and many moons Outside the frost line, lower temperatures led to condensation of hydrogen compounds (ices) along with metals and rocks Cores large enough to capture gas could form Moons made of rock and ice formed in the swirling jovian nebula around each growing jovian planet Rings appear when some of those moons get torn apart by tidal forces Small, rocky terrestrial planets near the Sun with no rings and few, if any, moons Inside the frost line, higher temperatures meant that only metals and rocks could condense, providing less than 1/3 as much material and leading to small, rocky cores The smaller cores and higher temperatures prevented gas capture, and moon and ring formation

How Does the Nebular Theory Do? Large bodies in the solar system have orderly motions and are isolated from each other Planets fall into two main categories Swarms of asteroids and comets populate the solar system: Asteroids mainly in the asteroid belt The asteroids in the asteroid belt are a frustrated planet The Trojan asteroids are planetesimals that became locked in gravitational "wells" caused by the gravity of Jupiter and the Sun

How Does the Nebular Theory Do? Large bodies in the solar system have orderly motions and are isolated from each other Planets fall into two main categories Swarms of asteroids and comets populate the solar system: Asteroids mainly in the asteroid belt Comets in the Kuiper belt and the Oort cloud The icy planetesimals that formed beyond the frost line near Jupiter and Saturn were thrown in random orbits, forming the Oort Cloud

How Does the Nebular Theory Do? Large bodies in the solar system have orderly motions and are isolated from each other Planets fall into two main categories Swarms of asteroids and comets populate the solar system: Asteroids mainly in the asteroid belt Comets in the Kuiper belt and the Oort cloud Those that formed beyond Neptune were relatively unaffected, and make up the Kuiper Belt

How Does the Nebular Theory Do? Large bodies in the solar system have orderly motions and are isolated from each other Planets fall into two main categories Swarms of asteroids and comets populate the solar system: Asteroids mainly in the asteroid belt Comets in the Kuiper belt and the Oort cloud Those that formed near Uranus and Neptune were flung into the inner solar system, and some provided water for Earth and other terrestrial planets

How Does the Nebular Theory Do? Large bodies in the solar system have orderly motions and are isolated from each other Planets fall into two main categories Swarms of asteroids and comets populate the solar system: Several notable exceptions to these general trends stand out: Moons with unusual orbits Unusual (backward) orbits indicate captured objects Planets with unusual axis tilts The unusual axis tilts can be explained by giant impacts during the Era of Heavy Bombardment Surprisingly large moons The surprisingly large moon is our own It is unlikely that it formed at the same time as Earth because its density is lower But Earth is too small to have captured it It too can be explained by a giant impact

Summary of Nebular Theory

Summary of Nebular Theory There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds)

Summary of Nebular Theory There was a huge nebula of gas (H and He) and dust (metal, rock, and hydrogen compounds) Initially the nebula was very cold, and all of the dust was in the form of solid particles The nebula began to contract, spin faster and faster, flatten out, and heat up As it heated, the dust particles vaporized The nebula was hottest in the center The farther away from the center, the cooler it got Different types of dust resolidified at different distances from the center depending on their condensation temperatures Close to the center only rock and metal dust was able to condense Far from the center, beyond the frost line, hydrogen compounds could also condense The solid particles stuck together ( accreted ), forming bigger and bigger clumps until they were the size of planets Inside the frost line, where only rock and metal could condense, small terrestrial planets formed Beyond the frost line, where hydrogen compounds as well as rock and metal could condense, much larger jovian planet cores could form The jovian cores were massive enough, and the temperatures were cold enough, that they could attract and retain gas from the surrounding nebula, becoming our gas giant planets

When did all this happen, and how do we know? It was 4.6 billion years ago that our solar system formed But how do we know this?... From radiometric dating, using radioactive isotopes Every element exists as a mixture of isotopes Some isotopes, like 14 C, are radioactive Every radioactive isotope has its own half-life If a sample has a certain amount of radioactivity, after one half-life it will have half as much With radiometric dating, you estimate the initial amount of radioactivity in a sample, and determine its age from the amount that s left

When did all this happen? Carbon-14 ( 14 C) provides a familiar example of radiometric dating It s used to date mummies, archaeological artifacts, and the like The diagram shows how it works 14 C is useful for dating things up to ~60,000 years old But its half-life of ~5700 years is too short to be useful in measuring the age of our solar system

When did all this happen? One isotope whose half-life is long enough is potassium-40 ( 40 K) 40 K decays to argon-40 ( 40 Ar) with a half-life of 1.25 billion years 40 K is found in rock along with 40 Ar from its decay If the rock is melted, the 40 Ar escapes as a gas When the rock cools and resolidifies, it contains 40 K, but no 40 Ar simulation So by measuring the ratio of 40 Ar to 40 K in a piece of rock, you can determine how long it s been since the rock solidified

When did all this happen? How can 40 K be used to date the formation of the solar system? The solar system formed from the solar nebula, a vast cloud of gas and (solid) dust The solid (cold) dust particles initially contained both 40 K and 40 Ar But as the nebula contracted and heated, the dust vaporized, and the 40 Ar was released When the dust condensed to solid form again, it contained 40 K, but not 40 Ar If rocks accreted from this dust could be found unchanged, their age would be the age of the solar system This is a type of meteorite called a chondrite Chondrites have not melted since they accreted from the nebular dust when the solar system formed So whatever 40 Ar they contain has appeared since then

Is ours the only solar system? Observation of other stars reveals many of them surrounded by disks of dust and gas These protoplanetary disks are exactly what the nebular theory predicts But until the 1990s, there was no convincing evidence for planets around other stars, now called extrasolar planets or exoplanets As of today, more than 1800 exoplanets have been confirmed

Detecting Extrasolar Planets by Radial Velocity Most confirmed extrasolar planets have been found by the radial velocity technique This technique depends on the gravitational effect of a planet on its star This image shows what would happen if Jupiter and the Sun were the only objects in our solar system They both would orbit around their common center of mass

Detecting Extrasolar Planets by Radial Velocity In a system with more than one planet, the star s movement can be complicated This image shows the path of the Sun around the solar system s center of mass The motion is mainly due to the effects of Jupiter and Saturn, because they are so massive Other stars are affected similarly by their planets

Detecting Extrasolar Planets by Radial Velocity This back-and-forth motion of the star along the line of sight from Earth causes Doppler-shifting of its light And this can be detected in a light curve

Detecting Extrasolar Planets by Radial Velocity After recording the light curve, computer modeling is used to determine how many and what type of planets are there This light curve led to the discovery of the first planet orbiting a Sun-like star 51 Pegasi It is fairly simple, and is consistent with a single planet The period of the wobbling gives you the orbital period and distance (~0.05AU how?) The magnitude gives you the minimum mass of the planet (~.5M Jupiter how?)

Detecting Extrasolar Planets by Radial Velocity This light curve is more complicated

Detecting Extrasolar Planets by Radial Velocity This light curve is more complicated It is consistent with the triple-planet system at right

Detecting Extrasolar Planets by Transit In the transit method (used by the Kepler SpaceTelescope), astronomers look for a periodic decrease in the light from a star The decrease indicates that a planet is transiting the star, blocking some of the starlight How often and how much the light decreases gives information about the planet s orbit and size Combining this info with radial velocity info can give the density of the planet

Detecting Extrasolar Planets by Imaging Planets do not emit their own light, and so are hard to see in telescopes, but a small number of extrasolar planets have been found this way The red object in the image above is the first of them It is orbiting a brown dwarf (the brighter object)

Detecting Extrasolar Planets A few exoplanets have been found by gravitational microlensing In this method, the light from a distant star is bent by the gravity of an intervening star If the intervening star has a planet, the planet s gravity adds to the effect in a recognizable way A statistical analysis of planets detected by this technique led to the prediction that each star in the Milky Way has ~1.6 planets You can see a list of all the known extrasolar planets and more at The Extrasolar Planets Encyclopedia

Detecting Extrasolar Planets At one time, most confirmed exoplanets were very large and very close to their star This was not because extrasolar systems more like ours do not exist (they do) It was simply a reflection of the methods that are used They tend to be more sensitive to large planets close to their star

Detecting Extrasolar Planets But the existence of hot Jupiters jovian planets very close to their star is not consistent with the nebular theory we have discussed Following the scientific method, we need to see if there is some way the nebular theory can be modified to account for this And there is

Detecting Extrasolar Planets It s a matter of timing In our own solar system, the waking Sun expelled all the nebular gas and dust The strong solar wind produced when fusion was about to start blew it all away But if that hadn t happened, the planets and the nebular disk would interact

Detecting Extrasolar Planets and the planets would migrate inward The star still blows the nebula away when it finally comes alive But a jovian planet that formed beyond the frost line might find itself, after migration, closer to its star than Mercury is to our Sun And the nebular theory lives to fight another day