Exploring Our Solar System and Its Origin

Transcription

1 Exploring Our Solar System and Its Origin Sun Over 99.9% of solar system s mass Made mostly of H/He gas (plasma) Converts 4 million tons of mass into energy each second Earth and Moon to scale Mercury made of metal and rock; large iron core desolate, cratered; long, tall, steep cliffs very hot and very cold: 425 C (day), 170 C (night) Venus nearly identical in size to Earth; surface hidden by thick clouds hellish conditions due to an extreme greenhouse effect: even hotter than Mercury: 470 C, both day and night atmospheric pressure equiv. to pressure 1 km deep in oceans no oxygen, no water, perhaps more than any other planet, makes us ask: how did it end up so different from Earth? Earth and Moon to scale Earth An oasis of life The only surface liquid water in the solar system; about 3/4 of surface covered by water A surprisingly large moon Mars Looks almost Earth-like, but don t go without a spacesuit! Giant volcanoes, a huge canyon, polar caps, more Water flowed in the distant past; could there have been life? Jupiter Much farther from Sun than inner 4 planets (more than twice Mars distance) Also very different in composition: mostly H/He; no solid surface. Gigantic for a planet: 300 Earth mass; >1,000 Earth volume. Many moons, rings SATURN Giant and gaseous like Jupiter most spectacular rings of the 4 jovian planets many moons, including cloudcovered Titan currently under study by the Cassini spacecraft Great Red Spot

2 Uranus Neptune much smaller than Jupiter or Saturn, but still much larger than Earth made of H/He gas, and hydrogen compounds (H 2 O, NH 3, CH 4 ) extreme axis tilt nearly tipped on its side makes extreme seasons during its 84-year orbit. moons also tipped in their orbits Wispy white clouds are thought to be crystals of methane. Very similar to Uranus (but much smaller axis tilt) Many moons, including unusual Triton: orbits backward ; and is larger than Pluto. Pluto A misfit among the planets: far from Sun like large jovian planets, but much smaller than any terrestrial planet. Comet-like composition (ices, rock) and orbit (eccentric, inclined to ecliptic plane, long years). Its moon Charon is half Pluto s size in diameter Best current photo above; New Horizons mission launch Jan 2006, arrival at Pluto in 2015 Asteroids *100,000+ rocky objects within the orbit of Jupiter *Also called minor planets *The largest, Ceres, has a diameter of about 900 km or ~ (560 mi) *Orbit the Sun in the same direction as the planets *Most orbit the Sun at distances of 2 to 3.5 AU, in the asteroid belt TNOs - Trans-Neptunian Objects *1,000+ small bodies orbiting beyond the orbit of Neptune *The largest of these are known as dwarf planets *Include Pluto, Eris, Charon, Makemake, etc. *Orbit the Sun in the same direction as the planets *Most orbit within the Kuiper belt at 30 AU to 50 AU Comets Objects that result when Kuiper belt objects collide Fragments a few kilometers across, diverted into new and elongated orbits The Sun s radiation vaporizes ices, producing tails of gas and dust particles Astronomers deduce composition by studying the spectra of these tails created by reflected sunlight Oort cloud comets orbit out to 50,000 AU Clues to the Formation of Our Solar System Our Goals for Learning What features of our solar system provide clues to how it formed? What theory best explains the features of our solar system? Common Properties of Planet Orbits in Our Solar System As viewed from above, all of the planets orbit the Sun in a counter-clockwise direction. The planets orbit in nearly the same plane. All planets except Pluto have an orbital inclination of less than 7.

3 Rocky asteroids between Mars & Jupiter Icy comets in vicinity of Neptune and beyond A successful theory of solar system formation must allow for exceptions to general rules Summary: Four Major Features of our Solar System Asteroids and comets far outnumber the planets and their moons Terrestrial Jovians Classifying the Planets The planets (except Pluto) fit into two groups: The Terrestrials or Inner Planets: Mercury Venus Earth Mars The Jovians or Outer Planets: Jupiter Saturn Uranus Neptune Smaller Mass and size higher density made of rock and metal Have solid surfaces few moons no rings Closer to Sun and closer together Larger mass and size low density mostly H, He, & hydrocarbon compounds No solid surface many moons rings Farther from sun and farther apart Size, Mass, and Density The Jovian planets have much bigger diameters and even larger masses than the terrestrial planets. 10 Though less massive than the Jovians, Terrestrial planets are much more dense.

4 Again, with the exception of ODD BALL Pluto, the rotation rates of Jovian planets on their axes are much faster than the Terrestrial planets. Despite these fast rotation rates, the diameters of the Jovian planets are tremendously larger than those of the Terrestrial Planets. What theory best explains the features of our solar system? According to the nebular theory our solar system formed from a giant cloud of interstellar gas (nebula = cloud) The lightest and simplest elements, hydrogen and helium, are abundant in the universe. Heavier elements, such as iron and silicon, are created by thermonuclear reactions in the interiors of stars, and then ejected into space by those stars. Ejection of Matter from Stars Great clouds of gas and dust ejected from old stars are gathered into regions from which new stars can be made. Summary of the Nebular Model for formation of the solar system. LARGE STAR NEAR THE END OF ITS LIFE FORMATION OF PLANETARY NEBULA SUPERNOVA EXPLOSIONS This region in the constellation of Orion shows new stars still surrounded by the nebula from which they were formed.

5 Inner parts of disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice. Fig 9.5 Inside the frost line: too hot for hydrogen compounds to form ices. Outside the frost line: cold enough for ices to form. Why are there two types of planets? 1. Outer planets get bigger because abundant hydrogen compounds condense to form ICES. 2. Outer planets accrete and keep H & He gas because they re bigger. 3. Inner planets too hot, gases evaporate Other Star Systems Forming We can look at young star systems developing today. The planets orbiting these stars are formed from the surrounding disks of gas and dust, called protoplanetary disks or proplyds. PLANET FORMATION Within the disk that surrounds the protosun, solid grains collide and clump together into planetesimals. The terrestrial planets are built up by collisions and the accretion of planetesimals by gravitational attraction. The Jovian-like planets are formed by gas accretion. Four Unexplained Features of our Solar System Why do large bodies in our solar system have orderly motions? Why are there two types of planets? --> 3) Where did the comets and asteroids come from? 4) How can we explain the exceptions the the rules above? Comets and asteroids are leftover planetesimals. Asteroids are rocky because they formed inside the frostline. Comets are icy because they formed outside the frostline Four Unexplained Features of our Solar System Why do large bodies in our solar system have orderly motions? Why are there two types of planets? Where did the comets and asteroids come from? --> 4) How do we explain the existence of our Moon and other exceptions to the rules?

6 Remember! Early in history of solar system, such impacts far more common Earth s moon was probably created when a big planetesimal slammed into the newly forming Earth Other large impacts may be responsible for other exceptions like rotation of Venus and Uranus Review of nebular theory Fig 6.27 Four Features of our Solar System - Explained Why do large bodies in our solar system have orderly motions? Why are there two types of planets? Where did the comets and asteroids come from? How do we explain the existence of our Moon and other exceptions to the rules? When 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 The decay of radioactive elements into other elements is a key tool in finding the ages of rocks Age dating of meteorites that are unchanged since they condensed and accreted tell us that the solar system is about 4.6 billion years old. Since 2008, the oldest rock on earth has been discovered by McGill University in the Nuvvuagittuq greenstone belt on the coast of Hudson Bay, in northern Quebec, and is dated from 3.8 to 4.28 billion years old, based on isotopes of neodymium and samarium Other Planetary Systems Our Goals for Learning How do we detect planets around other stars? What have other planetary systems taught us about our own? Extrasolar planets are either too dim or too close to the stars they orbit to observe directly. However, we can detect the effect they have on the spectra from their star to confirm their existence. Kepler mission The gravitational fields of a star and its planet will cause passing light to change direction. The focusing of light by gravity is called microlensing. Most common

7 We detect planets around other stars by looking for a periodic motion of the stars they orbit. We measure the motion through the Doppler shift of the star s spectrum very small shifts ~ nm Earth mass The size of the wobble tells us the planet s mass The period of the wobble tells us the radius of its orbit (Kepler s 3rd law) We can also detect planets if they eclipse their star Fraction of starlight blocked tells us planet s size These are only a few of the many ways in which our planet is special and perhaps unique 1. Orbits in habitable zone (liquid water exists) 2. Has a large, fairly close moon 3. Orbits right type right time 4. Solar system is in right region of the galaxy 5. Planet is right size, not too big or too small 6. Has plate tectonics 7. Solar system has a Jupiter size planet, not too close 8. Stable, nearly circular orbits 9. Etc... We do know today that there are planets around other stars, called extrasolar planets. As of May there are 1791 such planets in 1111 systems. Let s see how many of these are even remotely earthlike. We will observe first of all that the Earth s orbit and mass are quite unusual List of all of the known 1791 extrasolar planets found in 1111 planetary systems Data complete as of May 14, 2014 Source : planets in 424 systems by astrometry/radial velocity 1133 planets in 615 systems by transiting planets 29 planets in 27 systems by microlensing 48 planets in 44 systems by imaging 14 planets in 11 systems by timing (pulsar planets)

8 All known extrasolar planets with planetary masses from.001 M J to 0.01 M J. Earth would be M J HABITABLE ZONES AROUND MAIN-SEQUENCE STARS: NEW ESTIMATES, The Astrophysical Journal Volume 765 Number 2 (march 10,2013) :0.99 to 1.70 AU All known extrasolar planets with orbits between 0.99 AU and 1.7 Au and masses between 0 and 0.14 M J ( M J ) and at least 44 Earth masses (.14 M J ). Plot of 113 extra-solar planets in the latest HZ definition 0.99 to 1.7 AU* Of the 113 planets, list every planet with mass >.001M J and <.03 M J (10X M earth ). The 4 smallest are: name HD g Gj 163 d HD c HD b mass radius period axis *HABITABLE ZONES AROUND MAIN-SEQUENCE STARS: NEW ESTIMATES, The Astrophysical Journal Volume 765 Number 2 (March 10,2013) p l a n e t n a m e semi-major axis HD b MOA HD 142 b HD b HD b Kepler-34(AB) ome Ser b GJ 317 b HD b HD c HD 4313 b HR 228 b HD b HD b HD b HIP b 11 Com b HD 1690 b HD b 24 Sex b HD b HD b HD b HD b HD b 11 UMi b HD b 16 Cyg B b HD 4208 b The smallest is HD g, mass = M J, period= days with semi-major axis = AU Mass of earth = M J HD 10180g is 21 times as massive as Earth All other things equal, I would weigh 3234 pounds It also has an orbital eccentricity of 0.2 same as Mercury The 10 least massive planets yet discovered Name mass radius period axis PSR b 7.00E KOI-1843 b KOI-55 c Kepler-42 d alf Cen B b KOI-142 b Kepler-42 c Gl 581 e Kepler-11 f HD c HD b Typical Extrasolar system compared with our solar system Solar system masses in terms of Jupiter mass Mercury a = Venus b = Earth c = Mars d = Jupiter e = x M E I would weigh more than 900 pounds KOI-55 c orbits a star with temp of 27,700 K Our solar system a b c d e Based on the 1781 known extrasolar planets as of May 2014, what can we conclude? First, most are more massive than Jupiter and closer to their star than Earth is to Sun. Our solar system is unusual. Revisions to the nebular theory are necessary! Planets can apparently migrate inward from their birthplaces. Highly eccentric orbits are the norm Note also, the sun is not an average star, it ranks in the top 10% of all stars in size. The average star is in our galaxy a small, very cool M class star. The sun is also high in metal content. Stars with low metal content will not have rocky planets. The sun is also unusually stable for a main sequence star, whose luminosity (brightness) has increased only a few % over the last 2 billion years, providing a very long term stable environment for life to flourish and develop Survey of stars in the solar neighborhood Very few have masses greater than the sun Mass weighted avg. of the 319 stars is 0.45 M/M o most common stars are 0.1 to 0.2 M/M 0` heavier Our Sun lighter

9 No Earth-like planets found yet. Data aren t good enough to tell if they are common or rare Older methods can only detect BIG planets. Kepler mission is providing more data on Earth size planets. Earth probably IS unusual Is Earth Unusual? conclusion Based on the multiple lines of evidence from a variety of scientific areas ( and I have only presented a very small sample of a large body of evidence) what is one to conclude? Based on almost any reasonable criteria that one could devise, the existence of intelligent life on Earth and perhaps in the Universe as well is an ENIGMA... without GOD