Formation of the Solar System What theory best explains the 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) Kant and Laplace proposed the nebular hypothesis over two centuries ago A large amount of evidence now supports this idea Predictions of this theory have been accurate Galactic Recycling The universe shortly after the Big Bang consisted of hydrogen and helium Heavier elements were made in stars and then recycled through interstellar space when stars died When the solar system formed 4.6 billion years ago, about 2% of the original hydrogen and helium in the universe had been converted to other elements. The solar system was born from a cloud of gas (solar nebula) consisting of 98% hydrogen and helium and 2% of other elements 1
Evidence for the Nebular Theory We can see stars forming in other interstellar gas clouds, lending support to the nebular theory The Birth of the Solar System The solar nebula was initially a cold low density cloud of gas of diameter 100-200 AU with 2-3 times the mass of the sun Collapse of the gas may have been triggered by a cosmic event such as the shock wave of a supernova 2
Spinning cloud Initial slow rotation speed of the cloud increased as the cloud contracted (similar to ice skater spinning faster as she pulls her arms in) The cloud heated up as it contracted The sun formed at the centre where temperatures and densities were highest. Flattening Collisions between gas particles in cloud gradually reduce random motions Collisions between gas particles also reduce up and down motions Spinning cloud flattens as it shrinks 3
Disks around other stars Observation of disks around other stars support the nebular hypothesis Motion in the Solar System The spinning disk explains the uniform motions observed in the Solar System today: Planets all orbit in the same direction of spin of the disk they were formed from Planets orbit in the same plane because of the flattening of the disk they were formed from. 4
Why are there two types of planets? Why are there two types of planets? Inner parts of disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice. Inside the ice line (or frost line): Too hot for hydrogen compounds to form ices. Only rocks and metals can be solid Outside the ice line: Cold enough for ices to form. 5
How did terrestrial planets form? Small particles of rock and metal (seeds) were present inside the ice line Planetesimals of rock and metal built up as these particles collided. This process is called accretion Gravity eventually assembled these planetesimals into terrestrial planets over a few million years. Heat caused melting and differentiation inside the planets Differentiation: separation by density of material inside a planet Accretion of Planetesimals Many smaller objects collected into just a few large ones Computer simulations support this process Meteorites provide evidence of early condensation 6
How did jovian planets form? Ice could also form planetisimals outside the frost line. Larger planets may have been able to form from H and He gases by direct gravitational collapse without an accretion phase. Planets were large enough to draw in surrounding gas to form moons and rings What ended the era of planet formation? Outflowing matter from the Sun -- the solar wind -- blew away the leftover gases 7
Where did asteroids and comets come from? Asteroids and Comets Leftovers from the accretion process Rocky asteroids inside frost line Icy comets outside frost line beyond orbit of Pluto (Kuiper belt and Oort Cloud) 8
Asteroid Orbits Most asteroids orbit in a belt between Mars and Jupiter Trojan asteroids follow Jupiter s orbit Rocky planetesimals between Mars and Jupiter did not accrete into a planet. Jupiter s gravity stirred up asteroid orbits and prevented their accretion into a planet. primitive about 4.6 billion years old accreted in the Solar nebula processed younger than 4.6 billion years matter has differentiated fragments of a larger object which processed the original Solar nebula material Origin of Meteorites For a movie of a meteorite that hit the earth in 1992 check out http://csep10.phys.utk.edu/astr161/lect/meteors/fireball.mpg 9
Origin of Comets We can tell where comets originate by measuring their orbits as they visit the Sun. Most approach from random directions and do not orbit in the same sense as the planets. they come from the Oort cloud Others orbit along the ecliptic plane in the same sense as the planets. they come from the Kuiper belt Pluto: Dwarf Planet or Kuiper Belt Comet? By far the smallest planet. Not a gas giant like other outer planets. Has an icy composition like a comet. Has a very elliptical, inclined orbit. Pluto has more in common with comets than with the eight major planets Pluto is currently classified as a dwarf planet. Other dwarf planets: Ceres, Eris 10
Other Icy Bodies There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune. The largest of these, Eris (previously called Planet X) discovered in summer 2005, is even larger than Pluto How do we explain exceptions to the rules? 11
Heavy Bombardment Leftover planetesimals bombarded other objects in the late stages of solar system formation Origin of Earth s Water Water may have come to Earth by way of icy planetesimals from outer solar system 12
Odd Rotation Giant impacts might also explain the different rotation axes of some planets Meteorite Impacts and Mass Extinctions We know that larger objects have impacted Earth Meteor Crater in Arizona caused by a 50 m asteroid impact occurred 50,000 yrs ago 65 million years ago, many species, including dinosaurs, disappeared from earth Sedimentary rock layer from that time shows: Iridium, Osmium, Platinum grains of shocked quartz spherical rock droplets soot from forest fires 13
Impacts and Mass Extinctions on Earth Elements like Iridium, rare on Earth, are found in meteorites. Shocked quartz, found at Meteor Crater, forms in impacts. Rock droplets would form from molten rock rain. Forest fires would ensue from this hot rain. All this evidence would imply that Earth was struck by an asteroid 65 million years ago. In 1991, a 65 million year old impact crater was found on the coast of Mexico. 200 km in diameter implies an asteroid size of about 10 km across called the Chicxulub crater Iridium: Evidence of an Impact Iridium is very rare in Earth surface rocks but often found in meteorites. Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact. Dinosaur fossils all lie below this layer 14
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Impacts and Mass Extinctions on Earth We have a plausible scenario of how the impact led to mass extinction. debris in atmosphere blocks sunlight; plant die animals starve poisonous gases form in atmosphere Could it happen again? The odds of a large impact are small but not zero! Spaceguard programs and amateur astronomers around the world watch the skies to detect any possible dangers early enough 17
First observation of two bodies colliding Comet Shoemaker-Levy colliding with Jupiter How can we tell the age of the Solar System? 18
Radioactive Decay Some (parent) isotopes in a rock decay into other (daughter) nuclei Example: Potassium-40 decays into Argon-40 A half-life is the time for half the nuclei in a substance to decay The amount of parent isotopes and daughter isotopes in a rock allows us to calculate the age of a rock. Radioactive Decay Example: The halflife of Potassium-40 is 1.25 billion yrs. Thus a rock with equal amounts of Potassium-40 and Argon-40 is 1.25 billion years old. 19
Radioactive Decay After 2 half-lives (2.5 billion years) half of the remaining Potassium will be converted to Argon Thus after 2.5 billion years the amount of Argon will be three times the amount of Potassium A rock with 3:1 ratio of Argon-40 to Potassium-40 is 2.5 billion yrs old. When did the planets form? Radiometric dating tells us that oldest moon rocks are 4.48 billion years old Oldest meteorites are 4.56 billion years old Planets probably formed 4.6 billion years ago 20
Detecting planets around other stars A Sun-like star is about a billion times brighter than the sunlight reflected from its planets Like being in Vancouver and trying to see a pinhead 15 meters from a grapefruit in Waterloo. Planet Detection Direct: Pictures or spectra of the planets themselves Indirect: Measurements of stellar properties revealing the effects of orbiting planets 21
Indirect Detection: Astrometric Technique We can detect planets by measuring the change in a star s position in sky due to gravitational tug of planets (wobble) However, these tiny motions are very difficult to measure (~0.001 arcsecond) Indirect Detection: Doppler Technique Measuring a star s Doppler shift can tell us its motion toward and away from us due to the gravitational tug of a planet Current techniques can measure motions as small as 1 m/s (walking speed!) 22
First Extrasolar Planet Doppler shifts of star 51 Pegasi indirectly reveal a planet with 4- day orbital period Planet around 51 Pegasi has a mass similar to Jupiter s, despite its small orbital distance Short period means small orbital distance First extrasolar planet to be discovered (1995) First Earth-Like Planet Found In April 2007 scientists discovered the most Earth-like planet since the first discovery of extrasolar planets It is orbiting the star Gliese 581 about 20.5 ly away Roughly 1.5 times size of earth Lies in the habitable zone: may be a water planet! 23
Transit Missions NASA s Kepler mission began looking for transiting planets in 2009 It can measure the 0.008% decline in brightness when an Earth-mass planet eclipses a Sun-like star https://www.youtube.com/watch?v=8v4srfmotuu First Direct Picture of Extrasolar Planets Team led by Canadian astronomer Christian Marois ~7 Jupiter-mass planet orbiting at about 70 AU, ~10 Jupiter-mass planet orbiting the star at about 40 AU. In total almost 2000 planets have been found including numerous Earth-like planets. http://phl.upr.edu/hec 24
Summary What does the solar system look like? Planets orbit Sun in the same direction and in nearly the same plane. What can we learn by comparing the planets to one another? Comparative planetology looks for patterns among the planets. Those patterns give us insight into the general processes that govern planets Studying other worlds in this way tells us about our own Earth Summary What are the major features of the Sun and planets? Sun: Over 99.9% of the mass Mercury: A hot rock Venus: Same size as Earth but much hotter Earth: Only planet with liquid water on surface Mars: Could have had liquid water in past Jupiter: A gaseous giant Saturn: Gaseous with spectacular rings Uranus: A gas giant with a highly tilted axis Neptune: Similar to Uranus but with normal axis Pluto: An icy misfit more like a comet than a planet 25
Summary What are the three major groups of small bodies in the Solar System? Asteroids, comets of the Kuiper belt, comets of the Oort cloud. The groups are distinguished by their orbits and composition What are asteroids like? Most asteroids are small, potato-shaped, and orbit in the asteroid belt. Summary Where do meteorites come from? meteor: a trail of light caused by a small particle entering our atmosphere. meteorite: a rock that survives the plunge from space to reach the ground. Primitive meteorites are remnants from solar nebula Processed meteorites are fragments of larger bodies than underwent differentiation Did a meteorite impact kill the dinosaurs? Iridium layer just above dinosaur fossils suggests that an impact caused mass extinction 65 million years ago. A large crater of that age has been found in Mexico The probability of a major impact in our lifetimes is very low, but not zero. The threat is still being assessed. 26
Summary What are comets like? Comets are like dirty snowballs Most are far from Sun and do not have tails Tails grow when comet nears Sun and nucleus heats up Comets in plane of solar system come from Kuiper Belt The Kuiper Belt contains objects as large as Pluto. Comets on random orbits come from Oort cloud Summary What features of the solar system provide clues to how it formed? Motions of large bodies: All in same direction and plane Two main planet types: Terrestrial and jovian Swarms of small bodies: Asteroids and comets Notable exceptions: Rotation of Uranus, Earth s large moon, etc. 27
Summary What properties of our solar system must a formation theory explain? Motions of large bodies Two types of planets Asteroids and comets Notable exceptions like Earth s moon What theory best explains the features of our solar system? Nebular theory states that solar system formed from a large interstellar gas cloud. Summary What caused the orderly patterns of motion in our solar system? 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 We have observed such disks around newly forming stars 28
Summary Why are there two types of planets? Only rock and metals condensed inside the frost line Rock, metals, and ices condensed outside the frost line How did the terrestrial planets form? Rock and metals collected into planetsimals Planetesimals then accreted into planets How did the jovian planets form? Additional ice particles outside frost line made planets there more massive Gravity of these massive planets drew in H, He gases Summary What ended the era of planet formation? Solar wind blew away remaining gases Magnetic fields in early solar wind helped reduce Sun s rotation rate Where did asteroids and comets come from? They are leftover planetesimals, according to the nebular theory How do we explain exceptions to the rules? Bombardment of newly formed planets by planetesimals may explain the exceptions How do we explain the existence of Earth s moon? Material torn from Earth s crust by a giant impact formed the Moon 29
Summary How does radioactivity reveal an object s age? Some isotopes decay with a well-known half-life Comparing the proportions of those isotopes with their decay products tells us age of object When did the planets form? Radiometric dating indicates that planets formed 4.5 billion years ago 30