10Conservation of Energy As gravity causes the cloud to contract, it heats up.
11Inner parts of the disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice.Temperature Distribution of the Disk and the Frost Line
12Fig 9.5Inside the frost line: Too hot for hydrogen compounds to form icesOutside the frost line: Cold enough for ices to form
13Formation of Terrestrial Planets Small particles of rock and metal were present inside the frost line.Planetesimals of rock and metal built up as these particles collided.Gravity eventually assembled these planetesimals into terrestrial planets.
14Tiny solid particles stick to form planetesimals. Summary of the Condensates in the Protoplanetary Disk
15Gravity draws planetesimals together to form planets. This process of assemblyis called accretion.Same as previousSummary of the Condensates in the Protoplanetary Disk
16Accretion of Planetesimals Many smaller objects collected into just a few large ones.
17Formation of Jovian Planets Ice could also form small particles outside the frost line.Larger planetesimals and planets were able to form from ices as well as metalsThe gravity of these larger planets was able to draw in surrounding H and He gases.
18Moons of jovian planets form in miniature disks.
19Radiation and outflowing matter from the Sun — the solar wind — blew away the leftover gases.
29Thought QuestionHow would the solar system be different if the solar nebula had only been half as hot?Jovian planets would have formed closer to the Sun.There would be no asteroids.There would be no comets.Terrestrial planets would be larger.
30When did the planets form? We cannot find the age of a planet, but we can find the ages of the rocks that make it up.We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it.
31Radioactive Decay Some isotopes decay into other nuclei. A half-life is the time for half the nuclei in a substance to decay.
32Thought QuestionSuppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form?1.25 billion years ago2.5 billion years ago3.75 billion years ago5 billion years ago
33Dating the Solar System Age dating of meteorites that are unchanged since they condensed and accreted tells us that the solar system is about 4.6 billion years old.
34Dating the Solar System Radiometric dating tells us that the oldest moon rocks are 4.4 billion years old.The oldest meteorites are 4.55 billion years old.Planets probably formed 4.5 billion years ago.
36Planet Detection Direct: Pictures or spectra of the planets themselves Indirect: Measurements of stellar properties revealing the effects of orbiting planetsFor more check out this website:
37Direct DetectionSpecial techniques for concentrating or eliminating bright starlight are enabling the direct detection of a very few extrasolar planets.
38Indirect detection : Gravitational Tugs The Sun and Jupiter orbit around their common center of mass.The Sun therefore wobbles around that center of mass with the same period as Jupiter.
39Gravitational TugsSun’s motion around solar system’s center of mass depends on tugs from all the planets.Astronomers who measured this motion around other stars could determine masses and orbits of all the planets.
40Astrometric Technique We can detect planets by measuring the change in a star’s position in the sky.However, these tiny motions are very difficult to measure (~0.001 arcsecond).
41Doppler TechniqueMeasuring a star’s Doppler shift can tell us its motion toward and away from us.Current techniques can measure motions as small as 1 m/s (walking speed!).
42First Extrasolar Planet Detected Doppler shifts of star 51 Pegasi indirectly reveal planet with 4-day orbital periodShort period means small orbital distanceFirst extrasolar planet to be discovered (1995)
43First Extrasolar Planet Detected The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance.
44Thought QuestionSuppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude?It has a planet orbiting at less than 1 AU.It has a planet orbiting at greater than 1 AU.It has a planet orbiting at exactly 1 AU.It has a planet, but we do not have enough information to know its orbital distance.Rocky material condensed everywhere.B. The students often get confused by the differentiation ideas they’ve justheard about in planet formation.D. If a passing star ripped off asteroids, it would have stripped away cometsand planets too. (That may be one explanation for the truncation of theKuiper belt…)
45Transits and EclipsesA transit is when a planet crosses in front of a star.The resulting eclipse reduces the star’s apparent brightness and tells us the planet’s radius.When there is no orbital tilt, an accurate measurement of planet mass can be obtained.
46How do extrasolar planets compare with those in our solar system?
47Measurable Properties Orbital period, distance, and shapePlanet mass, size, and densityComposition
48Orbits of Extrasolar Planets Most of the detected planets have orbits smaller than Jupiter’s.Planets at greater distances are harder to detect with the Doppler technique.
49Orbits of Extrasolar Planets Most of the detected planets have greater mass than Jupiter.Planets with smaller masses are harder to detect with the Doppler technique.
50Planets: Common or Rare? Observations indicate ~2-5% of sun-like stars have Jupiter size planetsThe others may still have smaller (Earth-sized) planets that cannot be detected using current techniques.Current estimations range between 20 and 50% of sun-like stars having planets.
51Surprising Characteristics Some extrasolar planets have highly elliptical orbits.Some massive planets orbit very close to their stars: “Hot Jupiters.”
53Do we need to modify our theory of solar system formation?
54Revisiting the Nebular Theory Nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU).The discovery of “hot Jupiters” has forced a reexamination of nebular theory.“Planetary migration” or gravitational encounters may explain “hot Jupiters.”
55Planetary MigrationA young planet’s motion can create waves in a planet-forming disk.Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.
56Gravitational Encounters Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit.Multiple close encounters with smaller planetesimals can also cause inward migration.
57Modifying the Nebular Theory Observations of extrasolar planets have shown that the nebular theory was incomplete.Effects like planet migration and gravitational encounters might be more important than previously thought.
58Chapter 12 Asteroids, Comets, and Dwarf Planets Their Nature, Orbits, and ImpactsFun clip: the trailer from the movie Armageddon, if you can find it. The file was called armageddon.mov in Feb 2004 but it’s scarce now for some reason.See also for Phil Plait’s review
59Asteroid Facts Asteroids are rocky leftovers of planet formation. The largest is Ceres, diameter ~1,000 km.There are 150,000 in catalogs, and probably over a million with diameter >1 km.Small asteroids are more common than large asteroids.All the asteroids in the solar system wouldn’t add up to even a small terrestrial planet.
60Asteroid OrbitsMost asteroids orbit in a belt between Mars and Jupiter.Trojan asteroids follow Jupiter’s orbit.Orbits of near-Earth asteroids cross Earth’s orbit.
61Origin of Asteroid Belt 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.
62Origin of MeteoritesMost meteorites are pieces of asteroids.
63Meteor TerminologyMeteoroid: A small piece of rock or dust in space. Not as big as an asteroid.Meteor: A meteoroid that is falling through the Earth’s atmosphere. (shooting stars)Meteorite: A rock from space that falls through Earth’s atmosphere and reaches Earth..
64Comet FactsFormed beyond the frost line, comets are icy counterparts to asteroids.The nucleus of a comet is like a “dirty snowball.”Most comets do not have tails.Most comets remain perpetually frozen in the outer solar system.Only comets that enter the inner solar system grow tails.
65Anatomy of a Comet Coma is atmosphere that comes from heated nucleus. Plasma tail is gas escaping from coma, pushed by solar wind.Dust tail is pushed by photons.
67Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit.
68Meteors in a shower appear to emanate from the same area of sky because of Earth’s motion through space.
69Only a tiny number of comets enter the inner solar system; most stay far from the Sun. Oort cloud:On random orbits extending to about 50,000 AUKuiper belt:On orderly orbits from 30–100 AU in disk of solar system
70How did they get there?Kuiper belt objects are believed to have formed in the their current location along with the formation of the rest of the solar system. They orbit in a flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets.Oort cloud objects were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction.
71Is Pluto a Planet? Much smaller than the eight major planets Not a gas giant like the outer planetsHas an icy composition like a cometHas a very elliptical, inclined orbitPluto has more in common with comets than with the eight major planets.
72Discovering Large Iceballs In summer 2005, astronomers discovered Eris, an iceball even larger than Pluto.Eris even has a moon: Dysnomia.
73Dwarf PlanetsThere are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune.The largest ones are comparable in size to Earth’s Moon.If the object is massive enough for its own gravity to make it spherical(ish) then it is called a dwarf planet.
74Kuiper Belt ObjectsThese large, icy objects have orbits similar to the smaller objects in the Kuiper Belt that become short period comets.So are they very large comets or very small planets?
769.4 Cosmic Collisions: Small Bodies Versus the Planets Our goals for learning:Have we ever witnessed a major impact?Did an impact kill the dinosaurs?Is the impact threat a real danger or just media hype?How do other planets affect impact rates and life on Earth?
77Mass ExtinctionsFossil record shows occasional large dips in the diversity of species: mass extinctions.The most recent was 65 million years ago, ending the reign of the dinosaurs.
78Iridium: Evidence of an Impact Iridium is very rare in Earth surface rocks but is 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.
79Iridium Layer No dinosaur fossils in upper rock layers Thin layer containing the rare element iridiumDinosaur fossils in lower rock layers
80Likely Impact SiteGeologists found a large subsurface crater about 65 million years old in Mexico.
81Frequency of Impacts Small impacts happen almost daily. Impacts large enough to cause mass extinctions are many millions of years apart.