Origin of the Solar System The Solar Nebula
Our theory must explain the data • The nebular theory holds that our Solar System formed out 1. Large bodies in the Solar System have of a nebula which collapsed orderly motions. under its own gravity. 2. There are two types of planets. • observational evidence – small, rocky terrestrial planets – We observe stars in the process of forming today. – large, hydrogen-rich Jovian planets – The are always found within 3. Asteroids & comets exist in certain regions interstellar clouds of gas. of the Solar System – This image shows the Orion solar nebula – name given to Nebula..newborn solar the cloud of gas from which our 4. There are exceptions to these patterns. systems within! own Solar System formed
Gravitational Collapse Flattening of the Solar Nebula • As the nebula collapses, clumps of gas collide & merge. • The solar nebular was initially somewhat spherical and a few light years in diameter. • Their random velocities average out into the nebula’s –very cold direction of rotation. – rotating slightly • The spinning nebula assumes the shape of a disk. • It was given a “push” by some event. – perhaps the shock wave from a nearby supernova • As the nebula shrank, gravity increased, causing collapse. • As the nebula “falls” inward, gravitational potential energy is converted to heat. – Conservation of Energy • As the nebula’s radius decreases, it rotates faster – Conservation of Angular Momentum – See: http://www.youtube.com/watch?v=AQLtcEAG9v0 for an example of how this works!
The Coriolis effect Why does the cloud spin? • Air moves fastest at the equator due to the Earths Spin • Its part of a spinning galaxy—lots of angular momentum avaialble • Particles closer to the center of galaxy move more rapily than particles farther away..speed decreases with radial distance. • As a shock wave and then gravity compress the cloud, angular momentum must be conserved, so toques transfer some angular momentum from the galactic disk to the interstellar cloud. • This is similar to the formation of cyclones and • As temp differences causes air currents to move hurricanes in our atmosphere. North south, the result is a whilpool of air
1 Rotating disk with collapsing cloud Rotating disk with collapsing cloud Centripital force (gravity) makes particles orbit center Imagine disk of Galaxy seen from above of cloud, while cloud orbits center of galaxy. Note: Particles on outside rotate more slowly about galactic Speed decreases With radial center distance Stars on inside rotate more rapidly Collisions occur Faster particles will win! Disk begins to swirl!
Stars in Galaxy orbit about center of galaxy
Collapse of the Solar Nebula Orderly Motions in the Solar System
• The Sun formed in the very center of the nebula. – temperature & density were high enough for nuclear fusion reactions to begin • The planets formed in the rest of the disk. • This would explain the following: – all planets lie along one plane (in the disk) – all planets orbit in one direction (the spin direction of the disk) – the Sun rotates in the same direction – the planets would tend to rotate in this same direction – most moons orbit in this direction – most planetary orbits are near circular (collisions in the disk)
More Support for the Nebular Theory Building the Planets • We have observed disks around other stars. Condensation – elements & compounds began to • These could be new planetary systems in formation. condense (i.e. solidify) out of the nebula…. depending on temperature! Imagine atoms and molecules coming together atom by β Pictoris atom—electric force and ionization is main mechanism
AB Aurigae Proto-Solar systems in the Orion Nebula!
2 Building the Planets …and temperature in the Solar nebula The Young Sun emitted pulses of depended on distance from the Sun! UV and X-rays • These photons ionized some atoms in the solar nebula and deposited charge on others • These made the atoms and molecules “sticky” • Try to slide down a plastic slide on a dry, sunny day. Your hair will stand up!
Building the Planets Building the Planets So only rocks & metals condensed within 3.5 AU accretion -- larger bits stick to one another. Since gravity of the Sun… the so-called frost line. is weak,...acretion is mostly a gravitational effect between Hydrogen compounds (ices) condensed beyond the objects much larger than dust frost line. grains…basically..macroscopic objects attracting one another via law of gravity.
Building the Planets Acretion continues today! …After condensation produces macrosopic objects These will: • combine near the Sun to form rocky planets • combine beyond the frostline to form icy planetesimals which… • capture H/He far from Sun to form gas planets
3 Building the Planets Building the Planets • Each gas (Jovian) planet formed its own “miniature” solar wind --- charged particles streaming out from solar nebula. the Sun cleared away the leftover gas—this might be • Moons formed out of the disk. a relatively rare event! Other solar systems show evidence that their nebulae weren’t cleared out!
Origin of the Asteroids Origin of the Comets • The Solar wind cleared the leftover gas, but not the • The leftover icy leftover planetesimals. planetesimals are the • Those leftover rocky planetesimals which did not present-day comets. accrete onto a planet are the present-day asteroids. • Those which were located between the • Most inhabit the asteroid belt between Mars & Jupiter. Jovian planets, if not – Jupiter’s gravity prevented a planet from forming there. captured, were gravitationally flung in all directions into the Oort cloud. • Those beyond Neptune’s orbit remained in the ecliptic The nebular theory predicted the existence plane in what we call of the Kuiper belt 40 years before it was the Kuiper belt. discovered!
Exceptions to the Rules Exceptions to the Rules So how does the nebular theory deal with exceptions, Close encounters with and impacts by planetesimals could explain: i.e. data which do not fit the model’s predictions? • Why some moons orbit opposite their planet’s rotation – captured moons (e.g. Triton) • Why rotation axes of some planets are tilted – impacts “knock them over” (extreme example: Uranus) • There were many more leftover planetesimals than we • Why some planets rotate more quickly than others see today. – impacts “spin them up” • Most of them collided with the newly-formed planets • Why Earth is the only terrestrial planet with a large & moons during the first few 108 years of the Solar Moon System. – giant impact • We call this the heavy bombardment period.
4 Formation of Moon Formation of the Moon (Giant Impact Theory)
• The Earth was struck by a Mars-sized planetesimal • A part of Earth’s mantle was ejected • This coalesced in the Moon. – it orbits in same direction as Earth rotates – lower density than Earth See: move at: – Earth was “spun up” http://www.youtube.com/watch?v=xvc7Wf1xj4c& (illustration from Joe Tucciarone feature=fvw And also: http://www.youtube.com/watch?v=ibV4MdN5wo 0&feature=related
Radiometric Dating How Old is the Solar System? • Isotopes which are unstable are said to be radioactive. • They spontaneously change in to another isotope in a process • How do we measure the age of a rock? called radioactive decay. – protons convert to neutrons • How old is the Solar System and how do we – neutrons convert to protons know? • The time it takes half the amount of a radioactive isotope to decay is called its half life. • By knowing rock chemistry, we chose a stable isotope which does not form with the rock…its presence is due solely to decay. • Measuring the relative amounts of the two isotopes and knowing the half life of the radioactive isotope tells us the age of the rock.
The Age of our Solar System • Radiometric dating can only measure the age of a rock since it solidified. • Geologic processes on Earth cause rock to melt and resolidify. ⇒ Earth rocks can’t be used to measure the Solar System’s age. • We must find rocks which have not melted or vaporized since the condensed from the Solar nebula. – meteorites imply an age of 4.6 billion years for Solar System – This value is consistent with the age of the oldest moon rocks (4.4 billion years) • Radioactive isotopes are formed in stars & supernovae – suggests that Solar System formation was triggered by supernova – short half lives suggest the supernova was nearby
5