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Comets and Kuiper Belt Objects PTYS/ASTR 206 Comets 4/24/07 Announcements • Reading Assignment -- Chapter 30 • quiz today • In-class activity and course evaluations on Thursday • Public lecture tonight – 7:30PM, this room – Prof. Alfed McEwan, Mars HiRISE • Last Lecture – a week from today – Extrasolar planets – Final exam review PTYS/ASTR 206 Comets 4/24/07 • Most distant “planet” – Most of the time, but not all of the time! Its orbit can bring it inside of Neptune’s (as it did from 1979-1999) • Only “planet” not visited by a spacecraft – New Horizons, launched in Jan. 2006, will reach Pluto in 2015. • Pluto can be seen with an amateur telescope, but it is not easy! – About 14th magnitude • 6th magnitude is the limit for the naked eye • Pluto is about 1600 times dimmer than this PTYS/ASTR 206 Comets 4/24/07 Pluto Pluto: Basic Facts • Avg. distance to the Sun – 39.5 AU • Orbital Period – About 250 years • Retrograde rotation – Like Venus and Uranus • Eccentricity – 0.25 – larger than all of the planets PTYS/ASTR 206 Comets 4/24/07 • Avg. density – About 1900 kg/m3 – Rock and ice Pluto: Basic Facts • Diameter – 0.18 Earth Diameters – Not that well known (+/1%) • Mass – 0.0021 Earth Masses – Also not well known, although the combined mass of Pluto and Charon is well known • Tenuous atmosphere (but not as we have defined in in class) of N2 that is probably not in equilibrium PTYS/ASTR 206 Comets 4/24/07 Neptune and Pluto are in a 3:2 orbital resonance • Pluto’s orbital period is 1.5 times Neptune’s – Neptune is about 164 years – Pluto is about 249 years • This is a stable configuration and they will never crash into each other • Pluto is like other objects in this sense – comprising a class of objects called “Plutino’s” PTYS/ASTR 206 Comets 4/24/07 • Discovered in 1978 by Jim Christy. – Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. Charon: Pluto’s Moon • Charon is the largest moon with respect to its primary “planet” in the Solar System (a distinction once held by Earth's Moon). • Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon. PTYS/ASTR 206 Comets 4/24/07 • Pluto and Charon are in an unusual resonance. • Charon has an orbital period equal to both its rotational period AND Pluto’s orbital period • From one side of Pluto, Charon just sits in the same place in the sky (never sets, never rises)! PTYS/ASTR 206 Comets 4/24/07 Is Pluto a planet ? • Since the discovery of thousands of objects orbiting the Sun, called Edgeworth-Kuiper objects (or Kuiper-belt objects, KBOs), it has been suggested that Pluto is simply one of these and is not a planet • Many KBO’s are in orbital resonances with Neptune (3:2 resonance “Plutino”) • This is further complicated by the fact that the recently discovered Eris is larger than Pluto. • In 2006, Pluto was downgraded to a new class of objects called “dwarf planets” – But the debate rages on PTYS/ASTR 206 Comets 4/24/07 Kuiper Belt Red: All objects that are in a resonance with Neptune (Neptune Trojans – 1:1 ; Plutinos – 2:3 ; Twotinos 1:2 ) PTYS/ASTR 206 Blue: “Classical” objects Comets 4/24/07 The IAU’s decision in August 2006 • The IAU...resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way: (1) A planet is a celestial body that (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 neighbourhood around its orbit. • (2) A "dwarf planet" is a celestial body that (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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. • (3) All other objects, except satellites, orbiting the Sun shall be referred PTYS/ASTR to 206 collectively as "Small CometsSolar System Bodies". 4/24/07 Eris PTYS/ASTR 206 Comets 4/24/07 History of planets • Ancients recognized 7 planets – Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn • Earth was not a planet! • 1543, changed from 7 to 6 planets – Definition changed to an object orbiting the Sun • Sun and Moon removed, added Earth • 1852, changed from 23 to 8 planets – Asteroids were demoted since they did not have a resolvable disc • Uranus and Neptune had been discovered by now • 2006, changed from 9 to 8 planets – Pluto demoted PTYS/ASTR 206 Comets 4/24/07 • Asteroids – Small rocky bodies in orbit about the Sun Semantics • Comets – Small bodies that orbit the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail • Meteoroids – Small asteroids • Meteorites – the debris collected on Earth • Meteors – A brief flash of light (i.e. a shooting star) PTYS/ASTR 206 Comets 4/24/07 Asteroids and Comets • Asteroids – Mostly nearly-circular orbits – Mostly confined to the asteroid belt • Close to the ecliptic plane – Relatively short orbital periods • Comets – Highly elliptical orbits – Random inclinations • i.e. not-confined to the ecliptic – Very long orbital periods PTYS/ASTR 206 Comets 4/24/07 Comets: Harbinger’s of Doom • Rome: marked the assassination of Julius Ceaser • England: blamed for bringing the Black Death • Incan Empire: foreshadowed the brutal conquering by Francisco Pizarro PTYS/ASTR 206 Comets 4/24/07 PTYS/ASTR 206 Comets 4/24/07 Comet Types (based on orbital characteristics) • Short-Period Comets – Comet with an orbital period of less than 200 years – Shortest lived • Breakup due to gravitational forces • Life expectancy of about 12,000 years • Long-Period Comets – Comets with orbital periods greater than 200 years (though typically around millions of years) • Other Definitions – Jupiter Family Comets (JFC) • Orbital period less than 20 years – Intermediate-Period Comets Comets PTYS/ASTR 206 • Orbital period between 204/24/07 and 200 years • Kuiper Belt – Jupiter family and intermediate period comets – These comets have low inclinations compared to longperiod comets • Oort Cloud – Spherical distribution – Frequent observations of longand intermediate-period comets indicate that the reservoir must be huge – Cannot be seen directly because it is so far away – Perturbed by passing stars or galactic tides PTYS/ASTR 206 Comets 4/24/07 Comet Origin Comet orbits are often highly inclined • Long-period comets, originating from the roughly spherically distributed Oort cloud have orbits with random inclinations • Intermediate and Jupiter-family comets tend to have orbits more confined to the ecliptic • Asteroids seldom have highly inclined orbits PTYS/ASTR 206 Comets 4/24/07 Appearance of Comets • When comets are far from the Sun, they are dark and hard to see – Low albedos, far away • Because of their large eccentricities, they occasionally come very close to the Sun • The body outgases as it heats and releases dust and cometary atoms • This produces a long visible tail – Directed away from the Sun PTYS/ASTR 206 Comets 4/24/07