9. Formation of the Solar System

Transcription

1 9. Formation of the Solar System The evolution of the world may be compared to a display of fireworks that has just ended: some few red wisps, ashes, and smoke. Standing on a cool cinder, we see the slow fading of the suns, and we try to recall the vanished brilliance of the origin of the worlds. George Lemaître ( ) Astronomer and Catholic Priest

2 The Layout of the Solar System Planets fall into two main categories Terrestrial (i.e. Earth-like) Jovian (i.e. Jupiter-like or gaseous)

3 Comet Neptune Oort cloud Kuiper belt Asteroid

4 Origin of the Solar System Our theory must explain the data 1. Large bodies in the Solar System have orderly motions. 2. There are two types of planets. small, rocky terrestrial planets large, hydrogen-rich Jovian planets 3. Asteroids & comets exist in certain regions of the Solar System 4. There are exceptions to these patterns.

5 Origin of the Solar System Nebular Theory our Solar System formed from a giant, swirling cloud of gas & dust. As cloud collapsed: Energy of Gravity => heat Conservation of angular momentum => Spinup

6 Solar Nebula Cloud of gas that formed our own Solar System Observe stars in process of forming today. Found within interstellar clouds of gas. newly born stars in Orion Nebula

7 Gravitational Collapse Solar nebular initially spherical & few l.y. across. very cold rotating slightly Compressed, perhaps by nearby supernova Increased gravity accelerated collapse. Gravitational potential energy converted to heat. Conservation of Energy As radius decreased, rotation speed increased Conservation of Angular Momentum

8 Flattening of the Solar Nebula As nebula collapses, clumps of gas collide & merge. Random velocities average out into nebula s rotation. Spinning nebula assumes shape of a disk.

9 As nebula collapses: - heats up - spins faster - flattens.

10 Collapse of the Solar Nebula

11 Orderly Motions in Solar System Sun formed in center of nebula. temperature & density high enough for nuclear fusion planets formed in surround disk explains why: all planets lie in one (ecliptic) plane (in disk) all planets orbit in one direction (spin direction of disk) Sun rotates in same direction planets tend to rotate in same direction most moons orbit in this direction most planetary orbits are near circular (collisions in disk)

12 More Support for the Nebular Theory We have observed disks around other stars. These could be new planetary systems in formation. beta Pictoris AB Aurigae

13 Building the Planets Condensation elements & compounds began to condense (i.e. solidify) out of the nebula. depending on temperature!

14 Building the Planets and temperature in the Solar nebula depended on distance from the Sun!

15 Building the Planets So only rocks & metals condensed within 3.5 AU of the Sun the so-called frost line. Hydrogen compounds (ices) condensed beyond the frost line.

16 Building the Planets accretion -- small grains stick to one another via electromagnetic force until they are massive enough to attract via gravity to form...

17 Building the Planets planetesimals which 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

18 Building the Planets Each gas (Jovian) planet formed its own miniature solar nebula. Moons formed out of the disk.

19 Building the Planets solar wind --- charged particles streaming out from Sun cleared away leftover gas

20 Origin of the Asteroids Solar wind cleared leftover gas, but not planetesimals. Whatever didn't go into planets formed asteroids. Most inhabit asteroid belt between Mars & Jupiter. Jupiter s gravity prevented a planet from forming there.

21 Origin of the Comets Leftover icy planetesimals are the present-day comets. Those among Jovian planets, if not captured, were gravitationally flung into Oort cloud. Those beyond Neptune s orbit remained near ecliptic plane in what we call Kuiper belt. Nebular theory predicted existence of Kuiper belt 40 years before it was discovered! Pluto now understood to be "Kuiper belt object"

22 Exceptions to the Rules So how does nebular theory deal with exceptions? Many more leftover planetesimals than we see today. Most collided with newly-formed planets & moons during the first few 10 8 years of the Solar System. We call this the heavy bombardment period.

23 Exceptions to the Rules Close encounters with and impacts by planetesimals could explain: 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) Why some planets rotate more quickly than others impacts spin them up Why Earth is only terrestrial planet with a large Moon giant impact

24 Earth was struck by a Marssized planetesimal Part of Earth s mantle ejected Coalesced into Moon. orbits in same direction as Earth rotates lower density than Earth Earth was spun up Formation of the Moon: Giant Impact Theory

25 Radiometric Dating Unstable isotopes are radioactive. Change into another isotope through radioactive decay. half life is time for half to decay Measuring relative amounts of two isotopes & knowing half life of radioactive isotope gives age of rock.

26 Radiometric Dating

27 Sample question A meteor is found to contain 3 times as many Argon-40 atoms as Potassium-40 atoms. Given that Potassium-40 has a half-life of 1.25 B.Y., about what is the age since the meteor was last melted?

28 The Age of our Solar System Radiometric dating measures age since rock was solid. On Earth geology causes rock to melt and resolidify. Þ Earth rocks can t tell us Solar System s age. Use rocks that have not melted or vaporized since they condensed from the Solar nebula. meteorites imply an age of 4.6 billion years for Solar System Radioactive isotopes are formed in stars & supernovae suggests that Solar System formation was triggered by supernova short half lives suggest the supernova was nearby

29 Extrasolar Planets Shouldn t other stars have them as well? Planets orbiting other stars called extrasolar planets. Long assumed to exist (e.g. in sci fi) But direct evidence first came in mid 90 s Planet discovered to orbit star 51 Pegasi. Now have detected over 200 extrasolar planets.

30 Detecting Extrasolar Planets Can t actually make images of extrasolar planets. Angle between star and its planets too small to resolve Planets only reflect light, so too much glare from star.

31 Detecting Extrasolar Planets So we detect planets indirectly by observing the star. Planet gravitationally tugs the star, causing it to wobble. This periodic wobble measured from Doppler Shift of star s spectrum.

32 Use Doppler shift to detect (tiny) wobble of star due to planet Plot of the radial velocity vs. time forms a wave. Amp. & period => size & period of planet s orbit. From Kepler's laws, can infer mass planet.

33 Measuring the Properties of Extrasolar Planets Doppler technique gives planet mass and orbit. If Planet transits star, can also infer its size planet from amount of starlight it blocks. To see transit, must view along plane of planet s orbit transits are relatively rare Allow us to calculate density of planet. most extrasolar planets detected have Jovian-like densities.

34 Properties of Other Planetary Systems planets appear to be Jovian more massive than our system closer to their stars

35 Orbits are closer and more elliptical

36 Implications for the Nebular Theory Extrasolar systems have Jovian planets orbiting close to their stars. Theory predicts Jovian planets form in cold, outer regions. Many extrasolar planets have highly eccentric orbits. Theory predicts planets should have nearly circular orbits. Is the nebular theory wrong? Not necessarily; it may just be incomplete. Perhaps planets form far from star and migrate towards it. Doppler technique biased towards finding close Jovian planets Are they the exception or the rule? Migrating Jovians could prevent terrestrials from forming Is our Solar Solar System rare??