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ASTRONOMY 161 Introduction to Solar System Astronomy Class 23 Uranus, Neptune, Pluto, Eris, & Beyond Wednesday, March 7 Uranus: Basic characteristics Mass = 8.683×1025 kg (14.54 Earths) Diameter = 51,118 km (4.00 Earths) Density = 1,318 kg/m³ Sidereal rotation period = -0.718 d (17 h 14 min) Albedo = 0.51 (Earth = 0.39) Average distance from Sun = 19.19 A.U. Neptune: Basic characteristics Mass = 1.02×1026 kg (17.15 Earths) Diameter = 49,528 km (3.83 Earths) Density = 1,638 kg/m³ Sidereal rotation period = 0.671 d (16 h 6 min) Albedo = 0.41 (Earth = 0.39) Average distance from Sun = 30.07 A.U. Pluto: Basic characteristics Mass = 1.305×1022 kg (0.0021 Earths) Diameter = 2,306 km (0.18 Earths) Density = 2,000 kg/m³ Sidereal rotation period = -6.38d d (6 d 9 hrs) Albedo = 0.49-0.66 (varies) (Earth = 0.39) Average distance from Sun = 39.48 A.U. Eris: Basic characteristics Mass = ? (? Earths) Diameter = 2,400 km (0.19 Earths) Density = ? kg/m³ Sidereal rotation period = ? Albedo = 0.86 (Earth = 0.39) Average distance from Sun = 67.668 A.U. Jupiter and Neptune: Key Concepts (1) Uranus and Neptune are nearly identical in their internal structure. (2) The rotation axis of Uranus is tilted by about 90 degrees, causing extreme seasons. (3) Neptune has surprisingly strong storms, driven by internal heat. (4) Triton, the giant moon of Neptune, is a cold world with nitrogen geysers. Pluto ... and Beyond: Beyond Key Concepts (1) Pluto and its moon Charon are icy worlds that resemble Triton. (2) Eris, the troublemaker (Greek goddess of strife). (3) The Kuiper belt, beyond Neptune, contains small, icy, Pluto-like objects. (4) The icy Kuiper Belt Objects are leftover planetesimals. (5) Studies of the Outer Solar System continue. URANUS Discovered in 1781 by William Herschel. NEPTUNE Discovered in 1846, using Newton’s Laws. Uranus and Neptune are quite large planets. Radius of Uranus = 4.0 x radius of Earth Radius of Neptune = 3.8 x radius of Earth. Uranus: Revolution and Rotation Orbital period = 84 years Rotation period = 17 hours Uranus is somewhat oblate due to its rotation. Its equatorial diameter is 2.3% greater than its polar diameter. Neptune: Revolution and Rotation Orbital period = 165 years Rotation period = 16 hours Neptune is somewhat oblate. Its equatorial diameter is 1.7% greater than its polar diameter. (1) Uranus and Neptune are nearly identical in their internal structure. Uranus and Neptune are planetary “twins”, like Earth and Venus. Uranus is 3% larger in radius but 15% smaller in mass than Neptune. 1) 2) 3) 4) Interiors of Uranus and Neptune Gaseous atmosphere: hydrogen, helium, methane Liquid outer layer: hydrogen, helium Liquid or slushy mantle: water, ammonia Solid core: rock, metal (2) The rotation axis of Uranus is tilted by about 90o, causing extreme seasons. Axis tilts: Jupiter = 3o Saturn = 27o, Neptune = 30o Uranus = 98o The seasons of Uranus Winter solstice (AD 1985): north – perpetual dark, south – perpetual sun. Vernal equinox (2006): sun rises once every 17 hours. And so forth … Near the winter solstice (1986), the atmosphere of Uranus was boring: (left: visible light) (right: ultraviolet) No storms; very faint belts & zones. Heat flow from summery south to wintery north smeared out storms. As the vernal equinox approaches, Uranus is becoming stormy: Heating of northern hemisphere produces storms, made visible by clouds of methane. Uranus has 27 known moons. 22 are small (< 200 km across) and irregular. 5 are mid-sized (400 => 1600 km in diameter) and spherical. No giant moons. (3) Neptune has surprisingly strong storms, driven by internal heat. Temperature at Neptune’s cloud level is 55 Kelvin (-360o F). This is as warm as Uranus, although Neptune is much further from the Sun. Neptune’s atmosphere is being warmed from below by internal heat. This extra heat drives large circular storms, like the “Great Dark Spot” seen above. 1985: Rings of Neptune discovered during a stellar occultation. Image obtained by the Voyager 2 spacecraft in 1989. Neptune has 13 known moons. 12 are small (< 500 km across) and irregular. One is the giant moon Triton. Proteus Triton Nereid (4) Triton: Neptune’s Frosty Moon Surface temperature = 38 Kelvin. Covered with “frost” at poles: frozen methane, frozen nitrogen. Triton has craterfree plains; perhaps the frozen calderas of extinct ice volcanoes? Density of Triton = 2100 km/m3; ice mantle over rocky core. Triton has geysers 5 miles high driven by vaporized nitrogen. Plumes are swept downwind in the very tenuous nitrogen atmosphere. Triton is on a tilted retrograde orbit around Neptune. It has most likely been captured by Neptune. Tidal forces are moving Triton toward Neptune. In 100 million years, Triton will enter the Roche limit. Spectacular rings! A farewell to the Solar System: Voyager 2 looks back at Neptune and Triton. PLUTO Jan. 23, 1930 Jan 29. 1930 Discovered in 1930, by sheer luck. Orbital period = 248 years Rotation period = 6.4 days ERIS (a.k.a. “Xena”) Oct. 21, 2003 Discovered in 2005, systematic search. Orbital period = 557 years Rotation period = ?? Orbits of Neptune, Pluto and Eris Do not worry – they will not collide. The orbits are not in the same plane. Pluto’s moon, CHARON, was discovered in 1978. From the ground, Charon looked like a “lump” on Pluto’s side. Hubble Space Telescope (1) Pluto and its moon Charon are icy worlds that resemble Triton. Diameter of Pluto = 2300 km Diameter of Charon = 1200 km Separation = 20,000 km Pluto and Charon are tidally locked into “double synchronous” rotation. Rotation = revolution = 6.4 days. Very little is known about the surface of Pluto and Charon. Hubble Space Telescope images suggest white icecaps are present, as well as dark patches (dusty?) and light patches (clean ice?). Albedo varies. Pluto and Charon have many properties in common with Neptune’s moon Triton. • Cold surfaces (about 40 Kelvin) • Icy mantles and rocky cores (about 2000 kg/m3) • Pluto has a thin atmosphere (like Triton); Charon has none. At last, a mission to Pluto: NEW HORIZONS Launch: January 2006 Jupiter flyby: February 2007 Pluto flyby: July 2015 Then onward into the Kuiper belt. (2) Eris (“Xena”), the troublemaker. Discovered in 2005 by Mike Brown and collaborators. It has a moon. It is BIGGER than Pluto! Trouble! Eris and Pluto: Spectra Q: Are Pluto and Eris planets? A: That depends on how you define “planet”. As of August 2006, new category of “dwarf planet”. The International Astronomical Union defines "planet" as a celestial body that, within the Solar System, (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and (c) has cleared the neighborhood Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune The International Astronomical Union defines a "dwarf planet" as a celestial body that, within the Solar System, (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape; (c) has NOT cleared the neighborhood around its orbit; and (d) is not a satellite Dwarf Planets (so far): Ceres, Pluto, Eris (3) The Kuiper belt, beyond Neptune, contains small, icy, Pluto-like objects. The Kuiper belt lies close to the ecliptic plane, and stretches from 30 A.U. to 50 A.U. from the Sun. Named after Gerard Kuiper, who predicted it should be full of planetesimals. Over 800 Kuiper belt objects (a subset of transNeptunian objects) have been discovered in the belt since 1992. Kuiper belt Searches continue to reveal many objects orbiting in the Kuiper belt. Estimate: As many as 100,000 objects over 100 kilometers across. Kuiper Belt Objects are a subset of the Trans-Neptunian Objects. (4) The icy Kuiper Belt Objects are leftover planetesimals. Kuiper Belt Objects have colors (and spectra) consistent with them being icy. They are probably planetesimals that formed within the Kuiper belt. The Kuiper belt Objects are scattered too thinly for them to have accreted into a larger body. (5) Studies of the Outer Solar System continue. More objects like Pluto and Eris (or even bigger) might exist. They are hard to find: ● Very dim; ● Very slow moving. Perhaps more easily detected from their gravitational influence? From studies of Neptune’s orbit: No more Jovian (massive) planets within 200 A.U. of the Sun. TransNeptunian Objects searches are incomplete. More Erislike objects are likely to be discovered. Sedna: Even further away! Discovered in 2003 by Brown et al.: Eccentricity: 0.855 Semi-major axis: 525.606 A.U. Perihelion: 76.156 A.U. Aphelion: 975.056 A.U. Orbital period: 12050.32 years! Might be added as a “dwarf planet”. Sedna: the most distant object known in the Solar System. Sedna Case of Sedna's Missing Moon Solved Few closing questions: 1) Name the major differences and similarities between Uranus and the Earth. 2) Name the major differences between Uranus and Neptune. 3) Is there magnetic field around Uranus? 4) Are there volcanoes on Neptune? 5) Are there quakes on Neptune? 6) Is there a ring around Uranus? Few closing questions: 1) Does Pluto have an atmosphere? 2) Name the major differences between Pluto and Eris. 3) What is Pluto made of? 4) Is there a ring around Pluto? 5) Does Pluto have a moon? 6) Can there be another large planet out there? (planet “X”)