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

What is required for a gas giant moon to have a lot of internal heat? a)Nearness to the sun b)A large number of collisions c) A close elliptical orbit d)A strong magnetic field e)A 3:2 resonance with a near-by moon

Why does Titan have an atmosphere? a)Because its orbit is very close to Saturn b)Because it is highly oblate c)Because it outgases a lot of nitrogen d)Because it collects molecules from the solar wind e)Because it is very cold

As you move away from Jupiter, the moons a)Get larger b)Have fewer craters c)Have less ice d)Have less differentiation e)Have more volcanoes

Which gas giant moon has the least amount of ice? a)Io b)Europa c)Ganymede d)Callisto e)Titan

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