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THE EARTH AND OTHER PLANETS • The Solar System – The Planets – Kuiper Belt – Oort Cloud – Origin of the Solar System • Extrasolar Planets • Earth-Moon System The planets known to the ancients included Mercury through Saturn, including the moon and the sun, making 7 celestial bodies other than the earth. We divide the planets of our solar system into two major groups: – The Terrestrial Planets • • • • Mercury Venus Earth Mars – The Gas Giants • • • • Jupiter Saturn Uranus Neptune To Escape Earth • Wernher von Braun’s dream • Space Race – Soviets first into space and set many initial records: sputnik, Gagarin, Tereshkova, Korolev – USA behind but caught up after Mercury (e.g. Shepard & Glenn) and Gemini. Apollo 1 disaster followed mostly by success until moon landing (e.g. Armstrong and Aldrin) – Probes to the planets Mercury☿ • Diameter =4.9X103km • ~0.39 AU from the sun, but very eccentric • Orbit ~0.24 earth years • 1 day = 176 earth days • 1 mercurial day for each 2 orbits (as seen from Mercury) • No moons • Very dense (second only to earth) • Large liquid core • Probes: Mariner 10 (1974-75) and Global MESSENGER (2008-present) Mercury from a mosaic of photographs taken by the Mercury Global MESSENGER Venus • • • • • • • • Diameter =12.1X103km ~0.72 AU from the sun Orbit ~0.62 earth years 1 day = 243 earth days, but the rotation is retrograde relative to the other planets No moons Not as dense as the earth or Mercury Seems to have a liquid core and active vulcanism Probes from the USA, USSR, EU from 1961 to present Radar image by the probe Magellan 1990-1994 Mars • • • • • Diameter =6.7X103km ~1.5 AU from the sun Orbit =1.88 earth years 1 day = 0.98 earth days 2 moons (likely captured asteroids) • Density lowest of the terrestrial planets (3.9g/ml) • Evidence of past geological activity • Many active probes: flyby, orbitals and landers Mars image mosaic from images taken by Mars Global Surveyor (1999-2004). Shows northern water ice cap and Valles Marinaris. Olympus Mons shrouded in clouds. Asteroids and Bode’s Law • A numerical sequence defined by Titus and Bode successfully defined the relative positions of the planets Mercury through Saturn • a=4+n where n = 0,3,6,12,24,48 • Gap in the sequence between Mars and Jupiter • Search initiated for missing planet. Franz von Zach and Guiseppe Piazzi found Ceres at the right distance Johann Elert Bode (1747-1826), Germany Johann Daniel Titus (1729-1796), Germany Ceres, the largest asteroid (950km) and only dwarf planet in the inner solar system. Image by HST Jupiter • • • • • Diameter =142.8X103km ~5.2AU from the sun Orbit =11.9 earth years 1 day =0.41 earth days ~61 moons; Ganymede is larger than Mercury; thin ring • Density =1.3g/ml • Gas giant planet mainly of Composite image of Jupiter by the Cassini probe. The black dot is the hydrogen and helium. shadow of Europa. Atmosphere banded with giant red spot. Core with layers of metallic hydrogen • Many probes to study the Galilean moons (L-R): Io, Europa, Ganymede, planet and its moons and Callisto Saturn • • • • • Diameter = 120X103km ~9.8 AU from the sun Orbit =29.5 earth years 1 day =0.43 earth days ~30 moons and large system of rings; Titan is larger than Mercury and has an atmosphere • Density =0.7g/ml • Gas giant with a core surrounded by metallic hydrogen with outer layers of liquid hydrogen and helium • Many probes to study the planet, its rings and moons Saturn photographed by Cassini in 2004 Composite image of Saturn and its largest moons taken by Voyager 1 in 1980 • • • • • Uranus Diameter =51.8X103km ~19.2 AU from the sun Orbit =84 earth years 1 day =0.72 earth days More than 30 moons and a ring system • Density =1.3g/ml • Gas giant with an axial tilt of 97 degrees. A solid core surrounded by a mantle of ices (water, ammonia, methane), an atmosphere (hydrogen, helium & methane), and an outer atmosphere • Studied up close by Voyager 2 (1986) Uranus taken with HTS; north is on the right Discovery of Uranus • Minor deviations by Saturn suggested the influence of another planet • It had been observed earlier but not recognized as a planet • William Herschel (March 1781) initially thought it was a comet • The orbit was determined to be circular by Russian astronomer Anders Johan Lexell and Bode said it must be a planet • Herschel, recognized as its discoverer held naming rights and wanted to call it Georgian Star –not popular • Named Uranus (the only planet with a Greek, rather than Roman name). Ouranos was the Greek god of the sky and father of Cronos (Saturn), who was father of Zeus (Jupiter) Frederick William Herschel (1738-1822); Britain; astronomer and composer Neptune • • • • • • • Diameter =49.5X103km ~30.1 AU from the sun Orbit =164.9 earth years 1 day =0.67 earth days More than 11 moons Density =1.8g/ml Gas giant similar to Uranus. Solid core; mantle (water, methane & ammonia ices); atmosphere (hydrogen, helium, & methane) • Visited by a few probes including Voyager 2 in 1988 Image of Neptune by Voyager 2. Note banded appearance of the atmosphere and 2 dark spots Discovery of Neptune • Observed by Galileo but not recognized as a planet • Independently, John Couch Adams and Urbain Le Verrier noted that Uranus moved as though it were perturbed by a large outer planet • Did not conform to Bode’s Law • Adams and Le Verrier given joint credit for the discovery John Couch Adams (18191892), Britain Urbain Jean Le Verrier (18111877), France Discovery of Planet X • Percival Lowell convinced of the existence of a ninth planet, founded the Lowell Observatory in Flagstaff, AZ in 1894 • Searched until 1916 when he died • Legal battle with Lowell’s widow delayed search until 1929 • Clyde Tombaugh began the search and found Planet X in 1930 • Called Pluto PL Percival Lawrence Lowell (1855-1916), USA Clyde William Tombaugh (19061997), USA Pluto • • • • • Diameter =2,306km 29.7-48.9 AU from sun Orbit =247.7 earth years 1 day =366.7 earth days Up to 5 moons, Charon is the largest • Density =2.1g/ml • New Horizons probe HST image New Horizons image International Astronomical Union’s Definition of a Planet • Must be in orbit around the sun • Sufficient mass to assume a spherical shape • Must have cleared out its orbital neighborhood Current classifications of natural objects in the solar system 12 candidate planets under the old designation The Fall of Pluto • Designation changed to a dwarf planet in 2006 because: • Its orbit is strongly elliptical • Its orbit strongly inclines relative to the ecliptic • It does not seem to have cleared out its orbital path (mass only .07% of other objects in its orbital path Kuiper Belt • A belt from 30-50AU from the sun • Collectively called Trans-Neptunian Objects (TNO), many of which are dwarf planets (aka plutinos) • Mainly in the ecliptic Oort Cloud Origin of the Solar System • A nebular hypothesis proposed by Descartes • Developed by Kant and Laplace to explain the ecliptic and orbital intervals Immanual Kant (17241804), Prussia Pierre-Simon, Marquis de Laplace (1749-1827), France EXTRASOLAR PLANETS • Campbell, B., G. A. H. Walker, and S. Yang. 1988. A search for substellar companions to solar-type stars. The Astrophysical Journal. 331: 902. • Kepler mission launched March 6, 2009 • As of February 22, 2017, 3,453 confirmed planets have been found Gamma Cephei Ab, the first exoplanet discovered. 45 light-years distant Size Distribution of Exoplanets Origin of the Earth-Moon System • Centrifugal Origin by George Darwin • Formed in place as a double planet • The Capture Theory – resurrected as a capture from Venus (2013) • Giant Impact Hypothesis Giant Impact Hypothesis • Moon rocks brought back by Apollo demonstrate the same isotopic signature as the earth • Moon much less dense than the earth • Defined in 1974 by William Ward & Al Cameron (on collision from Harvard) together with William Hartman & Donald Davis (on Great Bombardment from Arizona) Tides and Phases First considered by Galileo but studied and explained by George Darwin George Howard Darwin (1845-1912), Britain Earth • Diameter =12,756km • Density =5.5g/ml • Third planet from the sun • Most of the surface covered by water • Relatively little CO2 in the atmosphere • Inclined 23o • Tectonically active The earth photographed by Apollo 17