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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 = ½mv 2 h = ½mv 2 /mg = ½v 2 /g Gravity on Io g = GM/R 2 g = [(6.67X10 -11 )(8.94X10 22 )]/(1.82X10 6 ) 2

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Trans-Neptunian Objects and Pluto

Astronomy 311

Professor Lee Carkner

Lecture 21

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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

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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

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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

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  • The best information comes from HST

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    Pluto Facts

    • Size: 2300 km

      • Smaller than the 7 largest moons

    • Orbit: 39.5 AU

    • Description: Very small, very cold, very distant

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    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

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    Features of Pluto

    • HST can see regions of different brightness on Pluto’s surface

    • The other bright regions may be areas where impacts have gouged out fresh ice

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    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

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    Eccentric orbit

    Not largest TNO

    Is Pluto a Planet?

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    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

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    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

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    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)

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    • Centaur:

    • Resonant: in an orbital resonance with Neptune

    • Classical Kuiper Belt:

    • Scattered Disk: large distances and eccentricities

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    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

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    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

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    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

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    • 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”

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    • 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

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    The Oort Cloud

    • In 1950 Dutch astronomer Jan Oort postulated a spherical shell of comets surrounding the solar system at about 50,000 AU

    • He computed the orbits of long period comets and found:

      • They should spend most of their time far from the Sun

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    Population of the Oort Cloud

    • There may be as many as 1 trillion comets in the Oort cloud

    • These bodies probably formed in the gas giant region and were ejected out to the Oort cloud by a close encounter with a large planet

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    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

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    Next Time

    • Read Chapter 14.2

    • Quiz 3 next Monday

    • Final exam the Monday after (November 3) at 3 pm

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    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

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    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