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 objects in 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 our own solar system… • …but might or might not explain other planetary systems • If not, modify the model to accommodate discrepancies • In other words, carry out 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 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 Solar System Formation -- Characteristics of Our Solar System

1. Large bodies 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

1. Large bodies 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 – And most moons orbit their planet in the direction it rotates 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 Summary of 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 – 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 any successful theory of solar system formation must account for these Solar System Formation – The Nebular Theory

• The nebular theory is the best current explanation of our solar system • It is not a new idea… …the philosophers Emanuel Swedenborg and Immanuel Kant suggested it in the 1700s • And 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 metallic elements) • 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 • And then new stars form from the enriched gas and dust • And the cycle continues Solar System Formation – The Nebular Theory

• And at the same time stars are forming… …planetary systems can form • Here’s how it works… 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 would this happen?... 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 -- local density increase • As it collapses it begins to spin faster …WHY would this happen?... Conservation of angular 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 -- local density increase • As it collapses it begins to spin faster -- conservation of angular momentum • And as it spins faster, it flattens out …WHY would this happen?... Collisions and conservation of angular 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 -- local density increase • As it collapses it begins to spin faster -- conservation of angular momentum • And as it spins faster, it flattens out – collisions and conservation of angular momentum • At the same time, it begins to heat up in the center …WHY would this happen?... 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 -- local density increase • As it collapses it begins to spin faster -- conservation of angular momentum • And as it spins faster, it flattens out -- collisions • At the same time, it begins to heat up in the center -- conversion of potential to thermal energy • And 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? This is how… • …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 – its atmosphere • We think the solar nebula had the same composition • But a skeptic might say, is it reasonable to say this? Solar System Formation – The Nebular Theory

• After all, the solar nebula collapsed 4.6 billion years ago • The Sun’s been making new atoms with nuclear fusion ever since • That’s how it generates the energy that gives us sunlight • Wouldn’t this change the composition of the Sun’s atmosphere? • The answer has to do with where the new atoms are being made… Solar System Formation – The Nebular Theory

• The sunlight-generating fusion reactions happen in the Sun’s core • The core is in the Sun’s center, far from the surface, and held in by intense gravity • So the surface layers should be essentially unchanged • And their composition should be very similar to the solar nebula the Sun formed from Solar System Formation – The Nebular Theory

• So it seems reasonable that the Sun’s atmosphere is similar to the nebula it formed from Solar System Formation – The Nebular Theory

• The key to the nebular theory is the condensation temperature of these materials • That’s the temperature at which they 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 image shows a graph of a modeled temperature profile of the solar nebula… …along with an artist’s rendition of the 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”, “snow line”, or “ice 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 • But also 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" • In it, cool gas beyond the frost line collapses directly into jovian planets… …much like the solar nebula collapsed to form the solar system • 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 forms an accretion disk • This disk swirls around the growing jovian planet in the same direction that the planet orbits the Sun due to conservation of angular momentum • And in that accretion disk, moons 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… …and cleared the remaining nebular gas away… …thus halting 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… …because they formed in the disk of the solar nebula, which was spinning in that direction – The Sun and most of the planets rotate in this same direction as well… …because of conservation of angular momentum – Most moons orbit their planet in the direction it rotates… …also because of conservation of angular momentum How Does the Nebular Theory Do?

– Small, rocky terrestrial planets near the Sun with no rings and few, if any, moons 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 → 3x more material… …enabling cores to grow large enough to capture gas from the nebula Moons made of rock, metal, 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 Less than 1/3 as much material led to small, rocky cores… …which together with higher temperatures prevented gas capture… …which in turn prevented moon and ring formation How Does the Nebular Theory Do?

• 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?

• 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?

• Those that formed beyond Neptune were relatively unaffected, and make up the Kuiper Belt How Does the Nebular Theory Do?

• 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 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 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 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 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 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 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 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 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 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 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, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores could form 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, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores could form • The jovian cores were massive enough, and the temperatures cold enough, to attract and retain gas from the surrounding nebula, becoming our “gas giant” planets 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, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores could form • The jovian cores were massive enough, and the temperatures cold enough, to attract and retain gas from the surrounding nebula, becoming our “gas giant” planets • When the Sun matured into a star, the solar wind blew out the remaining gas and arrested the development of the planets 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, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores could form • The jovian cores were massive enough, and the temperatures cold enough, to attract and retain gas from the surrounding nebula, becoming our “gas giant” planets • When the Sun matured into a star, the solar wind blew out the remaining gas and arrested the development of the planets • Planetesimals still remained, and these collected into the asteroid belt, Kuiper belt, or Oort cloud—or were captured by planets as moons—or collided with the planets, in some cases altering their axis tilts 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, hydrogen compounds as well as rock and metal could condense, and much larger jovian planet cores could form • The jovian cores were massive enough, and the temperatures cold enough, to attract and retain gas from the surrounding nebula, becoming our “gas giant” planets • When the Sun matured into a star, it emitted a strong solar wind that blew out the remaining gas and arrested the development of the planets • Planetesimals still remained, and these collected into the asteroid belt, Kuiper belt, or Oort cloud—or were captured by planets as moons—or collided with the planets, in some cases altering their axis tilts 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 (14C is a familiar example) • Every element exists as a mixture of isotopes

• Some of these isotopes, like 14C, are radioactive • Each 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?

• Here’s how radiometric dating with 14C works → → → • The 14C half-life is ~5700 years… …useful for dating things up to ~60,000 years old… …plenty long enough to date mummies and the like… …but too short to date the age of our solar system • So what can we do? When did all this happen?

http://archserve.id.ucsb.edu/courses/anth/fagan/anth3/courseware/Chronology/movies/Melting.html When did all this happen?

• We use a different isotope! • One isotope whose half-life is long enough is potassium-40 (40K) • 40K decays to argon-40 (40Ar) with a half-life of 1.25 billion years • 40K is found in rock along with 40Ar from its decay • If the rock is melted, the 40Ar escapes as a gas • When the rock cools and resolidifies, it contains 40K, but no 40Ar • So by measuring the ratio of 40Ar to 40K in a piece of rock, you can determine how long it’s been since the rock solidified When did all this happen? • So how can 40K 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 40K and 40Ar • But as the nebula contracted and heated, the dust vaporized, and the 40Ar was released • When the dust condensed to solid form again, it contained 40K, but not 40Ar • 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 40Ar they contain has appeared since then When did all this happen? • How can 40K 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 40K and 40Ar • But as the nebula contracted and heated, the dust vaporized, and the 40Ar was released • When the dust condensed to solid form again, it contained 40K, but not 40Ar • 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 40Ar they contain has appeared since then • Radiometric dating using 40Ar/40K shows that chondrites formed 4.6 billion years ago • Other isotopes give similar results, giving us confidence that the age is correct 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 • As of today, more than 4000 “extrasolar planets” or “exoplanets” have been confirmed • And there are over 4500 candidate planets NASA Exoplanet Archive The Extrasolar Planet Encyclopaedia Detecting Extrasolar Planets by Radial Velocity

• The first extrasolar planets were 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 (on the surface of the Sun) Detecting Extrasolar Planets by Radial Velocity

• This image shows the actual path of the Sun around our solar system’s center of mass • In a system with more than one planet, the star’s movement can be quite complicated • 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 – which earned its discoverers the 2019 Nobel Prize in Physics • It is fairly simple, and is consistent with a single planet • The period of the wobbling gives you the orbital period and therefore the distance (~0.05AU) – What could you use to do this? • The magnitude gives you the minimum mass of the planet

(~.5MJupiter) – How, and why minimum? 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

• Some 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 NASA Exoplanet Archive 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

Exoplanets in the Habitable Zone

• Nearly 361 of the ~3900 confirmed exoplanets are in the HZ • Only a few are Earth-sized, but the moons of Jovian planets could harbor LAWKI (think Avatar) Exoplanets in the Habitable Zone

• Nearly 361 of the ~3900 confirmed exoplanets are in the HZ • Only a few are Earth-sized, but the moons of Jovian planets could harbor LAWKI (think Avatar) • Planned telescopes like the James Webb Space Telescope (2021) will look at the atmospheres of transiting exoplanets Spectral Signatures of Life (AWKI)

Venus

Earth oxygen/ozone

Mars TRAPPIST-1 TRAPPIST-1 TRAPPIST-1