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Our Solar System Origins of the Solar System Astronomy 12 Learning Outcomes (Students will…) -Explain the theories for the origin of the solar system -Distinguish between questions that can be answered by science and those that cannot, and between problems that can be solved by technology and those that cannot with regards to solar system formation. -Estimate quantities of distances in parsec. Estimate the age of the solar system. -Describe and apply classification systems and nomenclature used in the sciences. Classify planets as terrestrial vs. Jovian, inner vs. outer, etc. Classify satellites. Classify meteoroid, asteroid, dwarf planet, planet. Classify comets as long period vs. short period. etc -Formulate operational definitions of major variables. Given data such as diameter and density describe the properties that divide the planets and moons into groups. -Tools and methods used to observe and measure the inner and the outer planets and the minor members of the solar system Our Solar System Our solar system is made up of: Sun Eight planets Their moons Asteroids & Meteroids Comets Inner Planets The inner four rocky planets at the center of the solar system are: Mercury Venus Earth Mars Mercury Planet nearest the sun Second smallest planet Covered with craters Has no moons or rings About size of Earth’s moon Venus Sister planet to Earth Has no moons or rings Hot, thick atmosphere Brightest object in sky besides sun and moon (looks like bright star) Covered with craters, volcanoes, and mountains Earth Third planet from sun Only planet known to have life and liquid water Atmosphere composed of Nitrogen (78%), Oxygen (21%), and other gases (1%). Mars Fourth planet from sun Appears as bright reddish color in the night sky Surface features volcanoes and huge dust storms Has 2 moons: Phobos and Deimos Outer Planets The outer planets composed of gas are : Jupiter Saturn Uranus Neptune Jupiter Largest planet in solar system Brightest planet in sky 60+ moons, 5 visible from Earth Strong magnetic field Giant red spot Rings have 3 parts: Halo Ring, Main Ring, Gossamer Ring Saturn 6th planet from sun Beautiful set of rings 31 moons Largest moon, Titan, Easily visible in the night sky Voyager explored Saturn and its rings. Uranus 7th planet from sun Has a faint ring system 27 known moons Covered with clouds Uranus sits on its side with the north and south poles sticking out the sides. Neptune 8th planet from sun Discovered through math 7 known moons Triton largest moon Great Dark Spot thought to be a hole, similar to the hole in the ozone layer on Earth A Dwarf Planet Pluto is a small solid icy planet is smaller than the Earth's Moon. Pluto Never visited by spacecraft Orbits very slowly Charon, its moon, is very close to Pluto and about the same size Asteroids Small bodies Believed to be left over from the beginning of the solar system billions of years ago 100,000 asteroids lie in belt between Mars and Jupiter Largest asteroids have been given names Comets Small icy bodies Travel past the Sun Give off gas and dust as they pass by SOLAR SYSTEM FORMATION Bode’s Law Also known as the Titius-Bode Law Titius: German astronomer who introduced the idea (1766) that the planets in the Solar System were spaced apart in a mathematical sequence Bode: German astronomer who popularized Titius’s idea (1772) Bode’s Law: hypothesis that the bodies in some orbital systems, including the Sun's, orbit at semi-major axes in an exponential function of planetary sequence. Bode’s Law Step 1: Create a sequence of 10 numbers that follow the pattern 0, 3, 6, 12, … Pattern 0 3 6 12 Bode’s Law Step 2: Add 4 to each number in the sequence. Pattern 0 3 6 Add 4 4 7 10 12 24 48 96 192 384 768 Bode’s Law Step 3: Divide by 10. Pattern 0 3 6 12 24 48 96 192 384 768 Add 4 4 7 10 16 28 52 100 196 388 772 Divide by 10 0.4 0.7 1.0 Bode’s Law Step 4: Compare! Pattern 0 3 6 12 24 48 96 192 384 768 Add 4 4 7 10 16 28 52 100 196 388 772 Divide by 10 0.4 0.7 19.6 38.8 77.2 19.2 39.44 1.0 1.6 2.8 5.2 10 Distanc 0.39 0.72 1.0 1.52 2.77 5.20 9.54 e in AU Planet Mer Ven cury us Eart Mar h s Ast Jupi Satur Uran eroi ter n us d Belt Cer es 30.06 Neptu Pluto ne Why is Bode’s Law Important? Bode’s Law correctly determined where the planets were at the time of its introduction (Mercury to Saturn, with a missing planet in between Mars and Jupiter) Bode’s Law helped to predict where “missing” planets (Ceres, Uranus) would be found! Bode’s Law can be used for moons around planets as well Moons of Uranus What are the shortcomings of Bode’s Law? Bode’s Law is a hypothesis – there is no evidence that this is nothing more than coincidence (scientifically speaking) Bode’s Law did not determine Neptune’s orbit accurately Bode’s Law cannot be tested properly on other planetary systems because only 1 system has enough planets (55 Cancri) Questions from Bode’s Law 1) What does Bode’s Law describe? 2) Why is Bode’s Law criticized? 3) What has Bode’s Law determined? 4) What else could Bode’s Law be used to determine? How old is the Solar System? Approximately 4.5 to 4.6 billion years old Planetary Nebula or Close Encounter? Historically, two hypothesis were put forward to explain the formation of the solar system…. #1 – Nebular Theory or Gravitational Collapse of Planetary Nebula Solar system formed form gravitational collapse of an interstellar cloud of gas #2 - Close Encounter (of the Sun with another star) Planets are formed from debris pulled out of the Sun during a close encounter with another star. But, it cannot account for Hot gas expands so planets should not form Probability for such encounter is small in our neighborhood… Astronomers favour Hypothesis #1 THE NEBULAR THEORY 6- Steps to Form a Solar System Step 1: Solar Nebula Huge cloud of cold gas and dust forms. This cloud spins slowly. The solar nebula is made up of 98% gas (H and He) but also contains 1.4% hydrogen compounds (water, methane, ammonia), 0.4% silicates and 0.2% iron/nickel/aluminum Step 2: Protosun The solar nebula (cloud) condenses into a dense central region and a lessdense outer region. The protosun begins to spin faster and flatten into a disk. Step 3: Rings and Planetesimals Instabilities in the rotating disk caused regions within it to condense into rings. Planetesimals formed in these rings. Planetesimals are a few cm to a few km in size Planetesimals are attracted to each other by gravity They collide to build planets This process explains the orderly motion of most of the solar system objects! Step 4: Gas Giants In the outer part of the disk, the planetesimals are made of rock and ice. When big enough, they will attract large amounts of gas around them. Step 5: Rocky Planets Near the protosun only rocky material and metals can withstand the heat. Therefore any planets formed here will be rocky. Step 6: Remaining Debris Blown Away Radiation from the Sun blows away most of the remaining gas and loose material. Some of the leftover planetesimals form the Oort cloud. Nebular Hypothesis Formation of the Solar System – Nebular Theory Planetesimals forming planets Where else has there been evidence for this theory??? Beta Pectoris dust disk Possible planetary system! Too young to tell yet! Evidence for Ongoing Planet Formation Many young stars in the Orion Nebula are surrounded by dust disks: Probably sites of planet formation right now! Dust Disks around Forming Stars Dust disks around some T Tauri stars can be imaged directly (HST). Planets form from the same cloud as the star. Planet formation sites observed today as dust disks of T Tauri stars. T Tauri stars are YOUNG stars with less mass that are very bright due to their large radii. Encounter Hypothesis Problems with Close Encounter Theory 1) Does not explain angular momentum 2) Hot gas expands so planets should not form 3) Probability for such encounter is small in our neighborhood…(main problem!) So…How WAS the Solar System Formed? A viable theory for the formation of the solar system must be: • based on physical principles (angular momentum, the law of gravity, the law of motions) • able to explain other planetary systems • able to explain all (or at least most) of the observable facts with reasonable accuracy How was the Solar System Formed? A viable theory for the formation of the solar system must account for 4 characteristics or observable facts: 1. 2. 3. 4. Patterns of motion Two types of planets Asteroids & comets Exceptions to patterns Patterns of Motion • • All the planets orbit the Sun in the same direction The rotation axis of most of the planets and the Sun are roughly aligned with the rotation axis of their orbits (fairly vertical axis). • Orientation of Venus, Uranus, and Pluto’s spin axes are not Why do they spin in similar to that of the Sun and other planets. (axial tilts >> 30’) roughly the same orientation? Why are they different? Patterns of Motion The rotation axis of most of the planets and the Sun are roughly aligned with the rotation axis of their orbits (fairly vertical axis). Orientation of Venus, Uranus, and Pluto’s spin axes are not similar to that of the Sun and other planets. Their axial tilts are much greater than 30’. Questions 1) Explain how all the planets orbiting the Sun in the same direction makes sense using the Solar Nebula theory. 2) What problems are still evident in this theory in terms of patterns of motion? Two Types of Planets: Terrestrial and Jovian Why are there 2 types of planets according to the Solar Nebular theory? The Story of Planet Building Planets formed from the same protostellar material as the Sun, still found in the Sun’s atmosphere. Rocky planet material formed from clumping together of dust grains in the protostellar cloud. Mass of less than~15 Earth masses: Planets cannot grow by gravitational collapse. These planets are made of metal and rock and can withstand the Sun’s heat. Earthlike planets Mass of more than~15 Earth masses: Planets are large enough to grow by gravitationally attracting material from the dust/gas cloud Jovian planets (gas giants) What about Pluto??? Exceptions to Patterns •Uranus has different axial tilt •Some moons larger than others •Some moons have unusual orbits So… 1) Uranus’ tilt must have occurred due to an impact after it formed. 2) Moons are formed either by collisions of planetesimals or by disks of gas/dust around the planets (similar to how the planets formed). 3) Unusual orbits… The Jovian Problem Two problems for the theory of planet formation: 1) Observations of extrasolar planets indicate that Jovian planets are common. 2) Protoplanetary disks tend to be evaporated quickly (typically within ~ 100,000 years) by the radiation of nearby massive stars. Too short for Jovian planets to grow! Solution: Computer simulations show that Jovian planets can grow by direct gas accretion without forming rocky planetesimals. Clearing the Nebula Remains of the protostellar nebula were cleared away by: • Radiation pressure of the sun • Sweeping-up of space debris by planets • Solar wind • Ejection by close encounters with planets Surfaces of the moon and Mercury show evidence for heavy bombardment by asteroids. What does the solar system look like from far away? • Sun, a star, at the center • Inner (rocky) Planets (Mercury, Venus, Earth, Mars) ~ 1 AU • Asteroid Belt ~ 3 AU • Outer (gaseous) Planets (Jupiter, Saturn, Neptune, Uranus) ~ 5-40 AU • Kuiper Belt ~ 30 to 50 AU -includes Pluto • Oort Cloud ~ 50,000 AU Extrasolar Planets An extrasolar planet, or exoplanet, is a planet beyond our solar system, orbiting a star other than our Sun. Information obtained primarily from wikipedia.org Types of Extrasolar Planets Terrestrial Planets Small, rocky planets that orbit close to the star Types of Extrasolar Planets Gas Giant A type of extrasolar planet with similar mass to Jupiter and composed of gases Example 1: Corot-9b This gas giant was discovered March 17, 2010 over 1500 light years away! Example 2: 79 Ceti b Types of Extrasolar Planets Super Earth A super-Earth has a MASS between that of Earth and the Solar System's gas giants. This term does not imply anything about the surface conditions or habitability of the planet! Example 1: Corot-7b Example 2: GJ 1214b Neptune Earth Corot-7b Types of Extrasolar Planets Hot Jupiter A type of extrasolar planet whose mass is close to or exceeds that of Jupiter (1.9 × 1027 kg), but unlike in the Solar System, where Jupiter orbits at 5 AU, hot Jupiters orbit within approximately 0.05 AU of their parent stars (about one eighth the distance that Mercury orbits the Sun) Example: 51 Pegasi b Found orbiting a star (51 Pegasi) in the constellation Pegasus about 50 light years away… Types of Extrasolar Planets Eccentric Jupiter A gas giant that orbits the star in a highly eccentric path (like a comet) Examples: 16 Cygni Bb and HD 96167 b Types of Extrasolar Planets Pulsar Planet A type of extrasolar planet that is found orbiting pulsars, or rapidly rotating neutron stars Example: PSR B1257+12 in the constellation Virgo Types of Extrasolar Planets Theoretical Extrasolar Planets… 1) Ocean Planet 2) Hot Neptune Key Terms – Types of Extrasolar Planets 1) Terrestrial Planet – small, rocky 2) Gas Giant – large, gaseous 3) Super Earth – size is up to 10 Earths (not as large as the gas giants) 4) Hot Jupiter Planet: mass is close to Jupiter (1.9 x 1027 kg) and orbit is within 0.5 AU of star 5) Eccentric Jupiter Planet: mass is close to Jupiter but orbit is highly elliptical 6) Pulsar Planet: orbits pulsars (a pulsars is a neutron star which is a remnant of a gravitationally collapsed massive star) Key Terms – Types of Extrasolar Planets Theoretical Planets 1) Ocean Planet 2) Hot Neptune http://channel.nationalgeographic.com/cha nnel/known-universe-interactive Methods of Detecting Extrasolar Planets Transit Method •If a planet crosses ( or transits) in front of its parent star's disk, then the observed visual brightness of the star drops a small amount. •The amount the star dims depends on the relative sizes of the star and the planet. Methods of Detecting Extrasolar Planets Astrometry •This method consists of precisely measuring a star's position in the sky and observing how that position changes over time. •If the star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit. •If the star is large enough, a ‘wobble’ will be detected. Methods of Detecting Extrasolar Planets Doppler Shift (Radial Velocity) •A star with a planet will move in its own small orbit in response to the planet's gravity. The goal now is to measure variations in the speed with which the star moves toward or away from Earth. •In other words, the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's spectral lines (think ROYGBIV) due to the Doppler effect. •A red shift means the star is moving away from Earth •A blue shift means the star is moving towards Earth Methods of Detecting Extrasolar Planets Pulsar Timing •Pulsars emit radio waves extremely regularly as they rotate. Because the rotation of a pulsar is so regular, slight changes in the timing of its observed radio pulses can be used to track the pulsar's motion. •Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulsetiming observations can then reveal the geometry of that orbit Methods of Detecting Extrasolar Planets Gravitational Microlensing •The gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned. •If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Methods of Detecting Extrasolar Planets Direct Imaging •Planets are extremely faint light sources compared to stars and what little light comes from them tends to be lost in the glare from their parent star. •It is very difficult to detect them directly. In certain cases, however, current telescopes may be capable of directly imaging planets.