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Trans-Neptunian Objects and Pluto Astronomy 311 Professor Lee Carkner Lecture 21 Gas Giant Moons Kinetic energy of launch equals potential energy at peak PE = KE mgh = ½mv2 h = ½mv2/mg = ½v2/g Gravity on Io g = GM/R2 g = [(6.67X10-11)(8.94X1022)]/(1.82X106)2 g= 1.8 m/s2 Final height h = [(½)(6002)]/1.8 = 100000 m = 100 km Pluto -- God of the Underworld Pluto is the God of the Dead in Roman mythology Pluto was discovered at Lowell Observatory and its first 2 letters commemorate Percival Lowell The Discovery of Pluto In the late 1800’s it was believed that Neptune’s orbit was being perturbed by a 9th planet Many astronomers tried to determine its position, including Percival Lowell The position turned out to be a coincidence, Pluto is too small to effect Neptune’s orbit The Discovery of Pluto No spacecraft has ever visited it But will not get to Pluto until 2015 The best information comes from HST Pluto Facts Size: 2300 km Smaller than the 7 largest moons Orbit: 39.5 AU Description: Very small, very cold, very distant Composition of Pluto Pluto has a density of 2000 kg/m3 Pluto is probably composed of ice and rock Spectra of Pluto reveal the presence of methane, nitrogen and carbon monoxide ice The temperature on Pluto is only ~50 K so the atmosphere can’t escape Spectra of Pluto Showing Methane Ice Pluto’s Moons Pluto’s largest moon Charon was discovered as a small bulge in a high resolution image (1978) Their sizes are closer than any planet and moon They have very similar densities, masses and sizes Two other smaller moons Nix and Hydra were discovered by HST in 2005 Each is about 50 km in diameter Is Pluto a Planet? Pro Spherical Tradition Con Eccentric orbit Not largest TNO Pluto’s Orbit Pluto’s orbit is much more eccentric and much more inclined than any planet Eccentricity = Most other planets e<0.1 Inclination = Pluto’s orbit carries it inside the orbit of Neptune Pluto is tipped on its side like Uranus Small, Icy Bodies Small icy bodies in the outer solar system (beyond Jupiter) have no good name Lets call all of them “Trans-Neptunian Objects” or “TNOs” They are all similar to Pluto (but usually much smaller) Most are only recently discovered and not well characterized or organized Discovering TNOs Around 1950 Kuiper and Edgeworth proposed a belt of comets out beyond Neptune In 1992 the first (besides Pluto) TNO was discovered (QB1) Discovered via long exposures with large telescopes (including HST) Total population of large TNOs may be 70000 (larger than 100 km) Discovering TNOs Centaur: Resonant: in an orbital resonance with Neptune Classical Kuiper Belt: Scattered Disk: large distances and eccentricities Classical Kuiper Belt Most of the objects have nearly circular orbits, low inclinations and are not effected by Neptune’s gravity Probably formed in place from the leftover material at the edge of the solar nebula Resonant Objects TNOs tend to collect on these orbits Examples: Pluto is in this group so they are called Plutinos Marks the edge of the classical Kuiper Belt, few TNOs beyond this point Theory: Neptune formed closer to the Sun and then migrated outwards Swept up TNOs into resonances as it moved out TNOs and Resonance Scattered Disk Objects Some TNOs have very irregular orbits These objects are thought to have been scattered by gravitational interaction with a gas giant (mostly Neptune) Can be hard to find due to their odd orbits Eris The largest TNO currently known is called Eris Larger than Pluto Semi-major axis of 68 AU, but is currently at 97 AU due to high eccentricity Part of the scattered disk Has a small moon, Dysnomia formerly called “Gabrielle” Large KBO Size Comparisons Centaurs Some TNOs are inside the orbit of Neptune Called Centaurs Have a wide range of orbital parameters Centaurs are thought to be former Kuiper belt objects that have been ejected inward into the gas giant region Will eventually collide with something or be ejected from the solar system altogether The Oort Cloud Spherical shell of comets surrounding the solar system at about 50,000 AU They are too far away to see, so we only have indirect methods of studying them There may be as many as 1 trillion comets in the Oort cloud Diagram of the Oort Cloud Tentative Origin of the TNOs The gas giants and TNOs gravitationally interact with each other Some TNOs are flung very far out and form the Oort cloud Some TNOs are swept up in Neptune’s resonances as Neptune migrates out and form the Resonant TNOs Some TNOs form between 40-50 AU and are not much affected by gravitational interaction and form the Kuiper belt Next Time Read Chapter 14.2 Quiz 3 Monday Oct 24 Summary: Pluto Description: small, cold , distant Pluto resembles a large TNO more than a planet Has a closely orbiting large moon Charon Properties Thin atmosphere Very cold (~50 K) Bright surface features possibly composed of fresher ice Summary: TNOs Past the orbit of Neptune the solar system is made up of many small icy bodies About 1000 found in the last 15 years Are organized into many different classes based on orbits Theories on their origin and evolution still under development