Trans-Neptunian Objects and Pluto

1. Trans-Neptunian Objects and Pluto Astronomy 311 Professor Lee Carkner Lecture 21

2. 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

3. 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

4. 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

5. The Discovery of Pluto

6. No spacecraft has ever visited it • But will not get to Pluto until 2015 • The best information comes from HST

7. Pluto Facts • Size: 2300 km • Smaller than the 7 largest moons • Orbit: 39.5 AU • Description: Very small, very cold, very distant

8. 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

9. Spectra of Pluto Showing Methane Ice

10. 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

11. 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

12. Pro Spherical Tradition Con Eccentric orbit Not largest TNO Is Pluto a Planet?

13. 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

14. 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

15. 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)

16. Discovering TNOs

17. Centaur: • Resonant: in an orbital resonance with Neptune • Classical Kuiper Belt: • Scattered Disk: large distances and eccentricities

18. 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

19. 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

20. TNOs and Resonance

21. 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

22. 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”

23. Large KBO Size Comparisons

24. 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

25. 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

26. Diagram of the Oort Cloud

27. 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

28. 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

29. Next Time • Read Chapter 14.2 • Quiz 3 next Monday • Final exam the Monday after (November 3) at 3 pm

30. 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

31. 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