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 = [(6.67X )(8.94X10 22 )]/(1.82X10 6 ) 2  g= 1.8 m/s 2  Final height  h = [(½)(600 2 )]/1.8 = 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/m 3   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

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

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 (QB 1 )   Discovered via long exposures with large telescopes (including HST)   Total population of large TNOs may be (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  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

Diagram of the Oort Cloud

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

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 AU and are not much affected by gravitational interaction and form the Kuiper belt 

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

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